JPH09300053A - Production of chromium alloy steel round cast billet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a continuously round cast billet of a hardly workable alloy steel containing >0.5wt.% Cr improved in center porosity, component segregation at the core part and heat cracking by a conventional continuous casting equipment. SOLUTION: In the continuous casting process, light rolling reduction is applied to the part of the round cast billet 3 at the terminal stage of solidification by rolling reduction rolls 4a. Desirably. It is preferable that an electromagnetic stirring treatment is applied to molten steel 2 in a mold 1, and when the solid phase ratio fs in the core part of the cast billet satisfies 0.1<=fs<1, the rolling reduction is applied thereto in the range of 0.1-3% cross-sectional area reduction ratio in the C direction by the rolling reduction rolls having two or more stages. Further, in this process, it is more preferable that two or more sets of the rolling reduction rolls consisting of two pieces of V type caliver rolls in parallel to each other in the axial line and facing each other are arranged and the light rolling reduction is applied to the cast billet so that the rolling reductional direction of each rolling reduction roll is made different from each other. By this method, the Cr alloy steel round cast billet having sound internal quality can be provided with a simple continuous casting process.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a high C for seamless steel pipe.
Regarding a method for producing a round billet material slab such as r alloy steel by continuous casting, in particular, the internal quality is sound, there is no center porosity and segregation, and the rolled steel pipe has no internal flaws.
In addition, the present invention relates to a method for easily casting a round cast slab having a good outer surface shape that does not cause rolling defects in the pipe production.

[0002]

2. Description of the Related Art Generally, a seamless steel pipe is produced by continuously casting a bloom slab with a large cross-section square shape, heating it, and then slabbing, blooming and billeting mills with a round cross-section billet (round billet). Hot rolled into a pipe mill or cast into a round billet with a continuous casting machine and then transported to a pipe mill, and then subjected to Mannesmann or press punching, or hot extrusion to produce hollow elements. The pipe is manufactured, then stretched by a rolling machine such as an elongator, a plug mill or a mandrel mill, and finally the diameter is adjusted by a sizer or a stretch reducer to obtain a product.

A slab, which can be cast relatively easily by continuous casting like general low carbon steel and has good hot workability, is supplied as it is as a raw material for a seamless steel pipe. . However, in continuous casting such as stainless steel, segregation and porosity are likely to occur in the axial center of the slab, and the slab with poor hot workability is often supplied after large processing. . The reason why such a step is performed in the case of a cast piece having poor hot workability is that the material billet is first subjected to severe processing such as Mannesmann drilling, and if such a preceding step is omitted, the porosity of the shaft center part is omitted. And segregation causes flaws on the inner surface of the pipe.
Therefore, it is said that not only the slab called a difficult-to-work material, but also the slab with a large amount of carbon and the slab to which Cr is added must be processed before drilling.

In particular, the internal quality of the slab has a great influence on the pipe production performance in the subsequent rolling process. If the Cr content in the steel increases, segregation and porosity tend to occur in the axial center of the continuous cast slab, which causes scaly surface flaws on the inner surface of the seamless pipe in the pipe manufacturing process. However, the inner surface of the pipe must be cared for to remove the flaw, which not only increases the manufacturing cost but also cannot be used as a product depending on the degree of defects.

Generally, the casting defect at the axial center of the continuous cast slab is caused by the solidification interface shape and solidification shrinkage in the final solidification stage of the slab, such as clogging of the molten steel flow path due to crystals and high viscosity of the molten steel. Because of this, the flow resistance of the molten steel becomes large and occurs. That is, it is said that when the concentrated molten steel between dendrite trees preferentially flows, it segregates, and when the molten steel is not sufficiently supplied to the core of the slab, it becomes porosity.

Particularly in the molten steel containing Cr, the viscosity of the molten steel increases as the Cr content increases, and the Cr content reaches a maximum around 13 wt.% As shown in FIG. Therefore, conventionally, even if porosity was generated in the slab, it was pressure-bonded by slabbing, then processed into a round billet with a billeting mill, and supplied as a material for pipe manufacturing. Furthermore, center segregation was also reduced by the diffusion of the constituent elements that occur in the process of repeating this processing and heating. Further, in order to improve the quality of the slab and prevent the occurrence of defects during pipe manufacturing, the electromagnetic stirrer is installed in the mold of the continuous casting machine to stir the molten steel in the mold to ensure that the final solidification part The equiaxed zone is formed to suppress the center segregation and center porosity of the slab.

As a means for improving the internal quality of the slab, there is a method of subjecting the slab to light reduction in a continuous casting line. For example, Japanese Patent Laid-Open No. 6-63715 discloses a technique for preventing center segregation by rolling down a slab in the final stage of solidification with a roll to suppress the flow of concentrated molten steel in the solidification end region (crater end region) (hereinafter, referred to as “ Prior art 1 ")
Is disclosed. However, in Prior Art 1, a large number of sets of reduction rolls are arranged so that the reduction direction is constant over a considerably long section in which the shaft center of the cast slab is in a solid-liquid coexisting state. It does not disclose the shape of the roll or the required amount of reduction.

"Materials and Processes" vol.7 (1994) No.1.p.19
Nos. 4 to 197 disclose a technique (hereinafter, referred to as "prior art 2") of applying a method of rolling a round bloom slab in two stages in an unsolidified state to stainless steel SUS410. However, in Prior Art 2, some porosity remains in the macro corrosion test results. Moreover, the axial density of the obtained cast piece was 7.7, which did not reach the density 7.8 when porosity was not generated, and the porosity was not completely pressed.

“Materials and Processes” vol. 7 (1994) No.1.p.
Nos. 179 to 182 are the forging method in which the slab in the final stage of solidification is pushed into the mold to continuously reduce the steel in order to improve the center segregation of the continuous casting bloom having a C content of 0.4 to 0.6 wt.%. (Less than,
"Prior Art 3") is disclosed. However, the prior art 3 is characterized in that the pressure is excessively reduced by squeezing the uncoagulated liquid phase in one step at a stretch, and therefore it is excellent in eliminating the center porosity, but the axial center portion of the slab is It exhibits negative segregation.

[0010]

When the process of improving the porosity and the component segregation of the core of the cast slab by the slabbing process, a cost is added. Further, each of the above-mentioned prior arts has the following problems.

Prior art 1 is effective for improving the center segregation of the slab, but since the light reduction rolls are installed in the same reduction direction over a long section of the casting line,
When a flat roll is used, the round slab is deformed into an elliptical shape, and there is a problem in that it cannot be manufactured as it is. In addition, cracks may occur inside depending on the amount of reduction applied to the slab and the direction of reduction.

The prior art 2 is effective in improving the solidification structure of the shaft center portion, but is insufficient for eliminating the center porosity as described above. Further, the prior art 3 is an excellent process for pressure bonding of porosity, but since a negative segregation region is formed in the axial center portion of the slab, the uniformity of the components is strictly demanded and the inner surface corrosion of the pipe is a problem. It cannot be applied as a material for seamless steel pipes.

Therefore, an object of the present invention is to solve the above-mentioned problems, and in particular, as a material for a seamless pipe which does not cause inner surface flaws or defective pipe making, a round billet having an excellent core portion of a cast slab. It is to provide a method for producing a slab by continuous casting.

[0014]

SUMMARY OF THE INVENTION In view of the above-mentioned circumstances, the inventors of the present invention have conducted extensive research into a technique for producing a cast slab having a good internal quality with inexpensive and simple equipment, and have developed the following method. .

In the method for producing a chromium alloy steel round billet slab according to claim 1, the Cr content is more than 0.5 wt.% Cr.
It is characterized in that alloy steel is continuously cast by a round mold having a diameter of 340 mm or less, and light reduction is applied by a reduction roll to the final solidification portion of the obtained round billet slab.

The method for producing a round billet slab of chromium alloy steel according to a second aspect is the method for producing a round billet slab according to the first aspect, further comprising performing electromagnetic stirring treatment on molten steel in a round mold. And the light reduction of the round billet slab by the reduction roll is performed by two or more reduction slabs, and the axial solid phase ratio fs of the round billet slab is 0.1 ≦ fs <1. It is characterized in that a reduction in the C-direction cross-sectional area reduction rate within the range of 0.1 to 3% is applied at the time of filling.

A method for producing a round billet slab of chromium alloy steel according to a third aspect is the method for producing a slab according to the first or second aspect, wherein as the light reduction method, the axes are parallel to each other and the round billet slab is formed. Two or more reduction rolls made up of two V-type caliber rolls facing each other with the middle being the middle are arranged along the casting direction of the round billet slab, and the reduction directions of the round billet slabs by the respective reduction rolls are It is characterized by rolling down round billet slabs in the final stage of solidification in different directions.

[0018]

Embodiments of the present invention will be described below. An embodiment of the invention described in claim 1 will be described.

FIG. 1 is a vertical cross-sectional conceptual view of a continuous casting process for explaining an embodiment of the invention described in claim 1.
1 is a round-shaped mold, 2 is molten steel, 3 is an unsolidified slab, 3'is a fully solidified round billet slab, 4a is a single-stage reduction roll,
5 is a solid-liquid coexisting phase at the final stage of solidification of the slab, 6 is a guide roll group,
And 7 is a drawing roll.

As shown in FIG. 1, molten steel 2 containing Cr: 0.5 wt.% Or more, which was tapped from a converter (not shown),
Continuous casting using a round-shaped mold 1 having a diameter of 340 mm or less, and a light reduction is applied to a final solidification portion of the obtained unsolidified round billet slab 3 by a reduction device 4 consisting of a single stage of reduction roll 4a to form a slab. A round billet cast slab 3'in which the porosity of the core of the cast slab and especially the segregation of the components is improved is produced.

When a round billet slab 3'having a diameter of 340 mm or less is cast by continuous casting, the cooling rate rapidly increases in the vicinity of the final solidification portion of the slab axis, solidification proceeds rapidly, and the Cr alloy In steel, when the Cr content increases, the viscosity of molten steel increases and porosity is likely to occur. According to the test results of the present inventors, when the Cr content exceeds 0.5 wt.%, Center porosity is likely to occur, and segregation of Cr produces a δ ferrite phase. Furthermore, it induces radial cracks due to thermal stress. Due to these factors, the frequency of occurrence of internal flaws in the seamless steel pipe increases. In order to prevent the occurrence of such internal flaws in the seamless steel pipe, during the process of continuously casting the predetermined round billet 3'prior to the piercing and rolling process, the solid-liquid coexisting phase 5 is present in the core of the cast slab, which is not solidified at the end of solidification. By applying a light reduction to the round billet slab 3 with a reduction roll 4a, center segregation and center porosity are eliminated. The reason why the roll is used as the light rolling means is simple. The chromium alloy steel round billet slab obtained by the above-mentioned manufacturing method is transported to a pipe manufacturing factory and manufactured into a seamless steel pipe through a predetermined process.

By using the above-described method of the present invention, a continuous billet having a small cross section is directly produced instead of a round billet having a large cross section continuously cast and hot-rolled as in the conventional method.
Since it can be supplied as a material for the hollow shell, it also contributes to a reduction in manufacturing cost.

An embodiment of the invention described in claim 2 will be described. FIG. 2 is a vertical sectional conceptual view of a continuous casting process for explaining an embodiment of the invention described in claim 2. Reference numeral 4 is a reduction device, which is composed of two-stage reduction rolls 4b and 4c, 8 is an in-mold electromagnetic stirring device, and other symbols are the same as those in FIG.

In this embodiment, as shown in FIG. 2, the molten steel 2 being continuously cast is electromagnetically stirred by using an in-mold electromagnetic stirring device 8 in addition to the above embodiment of the invention of claim 1. In addition, as a light reduction method for the final solidification portion of the unsolidified round billet slab 3, a two-stage reduction roll 4 is used when the axial solid fraction fs of the slab is 0.1 ≦ fs <1.
The reduction device 4 composed of b and 4c applies reduction in the C-direction cross-sectional area reduction rate of 0.1 to 3%.

By imparting electromagnetic stirring to the molten steel in the mold, it has the effect of producing equiaxed crystals in the axial center of the slab, and is more effective in suppressing V segregation and eliminating center porosity due to subsequent light reduction. To

The axial solid fraction fs of the slab is the most important factor in the application of the present invention under the condition of light reduction to the final solidification portion of the unsolidified round billet slab 3. V segregation occurs and starts to grow at a time of 0.1 ≦ fs ≦ 0.5, and by applying at least the first stage of light reduction during this time, the flow of the concentrated molten steel is suppressed to form a segregation channel. It is desirable to prevent In addition, the axial solid fraction f of the slab
When the reduction is applied only when s is 0.5 or less,
Even after a slight reduction in pressure, porosity is likely to be formed due to subsequent solidification shrinkage, and radial cracks due to thermal stress are likely to occur.

On the other hand, if the axial solid fraction fs of the cast slab is less than 1, that is, when the liquid phase is present without complete solidification, porosity and cracking can be suppressed, or defects that have occurred. Crimping is possible. Therefore, the final stage of light reduction, but in the present embodiment, the second stage of light reduction may be performed at a time when the axial solid fraction fs of the cast slab is less than 1.

Regarding the light reduction method of the cast slab, in addition to the above-mentioned solid phase ratio condition of the cast slab, the reduction rate of the cross-sectional area of the cast slab in the C direction is limited to suppress the V segregation of the shaft center portion and the porosity. It is effective in suppressing the formation and crimping, and suppressing the generation of radial cracks and crimping. When the reduction rate of the sectional area of the cast slab in the C direction is 0.1% or less, the occurrence of V segregation can be suppressed, but porosity and radial cracking cannot be reduced. On the other hand, porosity and thermal stress cracking can be reduced by applying a reduction whose cross-sectional area reduction rate in the C direction exceeds 3%, but thick segregation is formed due to squeezing of the concentrated molten steel, and deformation accompanying the reduction is also possible. This causes cracks inside the slab, which causes defects on the inner surface of the pipe during pipe manufacturing.

An embodiment of the invention described in claim 3 will be described. In the embodiment example of the invention according to claim 1 or 2, when the part at the final stage of solidification of the slab is lightly reduced, this embodiment is a rolling roll whose axes are parallel to each other and the slab is in the middle. At least two sets of two facing V-type caliber rolls, in this case 2 sets
Two sets are arranged along the casting direction, that is, two sets are arranged,
Moreover, the slabs are arranged such that the slabs are pressed in different directions, for example, at right angles, and the slabs are subjected to a predetermined light reduction.

A V-type caliber roll was adopted as the shape of the reduction roll, and two opposing rolls were made into a pair so that the pressure caused by the rolls was efficiently concentrated at the axial center of the cast slab, resulting in a casting defect. This is for surely preventing the occurrence of. Further, the reduction direction by the reduction rolls in each stage is different from each other in the slab C direction cross section, for example, the reduction rolls arranged in two stages apply reductions in the directions perpendicular to each other. This is because uneven deformation is prevented and the outer shape of the cast slab has a shape close to a perfect circle. For example, when flat rolling is applied in a certain direction with a flat roll, the round billet slab is deformed into an elliptical shape, causing core runout during pipe production, resulting in uneven thickness of the pipe, and in some cases it is impossible to roll. Become. Further, the deformed round billet slab can be peeled into a perfect circle, but the cost is increased.

The above-described method of the present invention makes it possible to obtain a round billet slab having a good internal quality by a simpler method than the conventional method, and using this as a raw material, a pipe manufacturing method which is completely the same as the conventional method can be used to further reduce the manufacturing cost. It is possible to manufacture a high-grade seamless steel pipe which has a low pipe temperature, does not cause problems in pipe manufacturing, and has no inner surface defects.

[0032]

Next, the present invention will be described in more detail with reference to examples. With the electromagnetic stirrer in the mold 8 shown in FIG. 2 and the continuous casting machine for a curved billet having a radius of 11.5 m, which has the rolling down device 4 including the two-stage rolling down rolls 4b and 4c, the slab cross-section diameter of 170 mm or 330 mm is obtained. A round billet slab was produced by casting a molten Cr alloy steel for a seamless steel pipe having a Cr content of 2.5 wt.% Or 13.8 wt.% In a round mold. The light reduction of the final solidification slab was performed by two-stage reduction by a flat roll, or one-stage or two-stage reduction by a caliber roll. Then, the slab is heated to a predetermined temperature, rolled with a Mannesmann punch to produce a hollow rough tube,
After stretching with an elongator, the diameter was adjusted with a sizer to produce a seamless steel pipe.

3 to 5 are schematic views showing the arrangement and configuration of a reduction roll for imparting a light reduction to the slab in the final stage of solidification, showing the arrangement of the reduction roll consisting of a V-type caliber roll with respect to the slab. 3 is a schematic side view, and FIG. 4 is A- of FIG.
A line sectional view, and FIG. 5 is a BB line sectional view of FIG. As shown in these figures, V contacting the unsolidified slab 3
Two caliber rolls 9a, 9a 'and 9b, 9b' in which the die caliber portion is cut at an angle of θ = 90 ° are combined to form one set of reduction rolls, which are the first-stage reduction roll 4b and the second-stage reduction roll, respectively. The reduction roll 4c was used. The inclination angle between the first-stage reduction roll 4b and the second-stage reduction roll 4c was set to a right angle.

Tables 1 and 2 show Examples 1 to 12 which are manufacturing methods within the scope of the present invention, and Comparative Examples 1 to 3 which are manufacturing methods outside the scope of the present invention.

[0035]

[Table 1]

[0036]

[Table 2]

The cast molten steel having the chemical composition is a steel type which is liable to form segregation and porosity of the slab axis in each of the examples and comparative examples, and which is difficult to work. The pouring temperature was set to a temperature according to a conventional method depending on the chemical composition of the molten steel, and the molten steel was cast in the mold by an installed electromagnetic stirring device while stirring (excluding Example 12). The amount of cooling water for the slab was also an appropriate amount according to the chemical composition of the molten steel and the slab size.

The diameter of the pressing rolls 4b and 4c for imparting a light reduction to the unsolidified slab 3 at the final stage of solidification is 380 mm.
Therefore, the rolling-down method was hydraulic. Each rolling roll 4b, 4c
The installation positions of the molten steel 2 in the mold are 15.5 m and 17.5 m from the meniscus of the molten steel 2, respectively. Even if they differ from each other, the axial solid fraction fs of the slab can be arbitrarily selected by adjusting the casting speed.

After completion of casting, a predetermined test piece was sampled from the round billet slab, and porosity, cracks and V
A cast internal property test such as segregation was performed. Then, after the pipe was manufactured, the state of occurrence of internal flaws in the seamless steel pipe was observed, and the quality of the cast slab was comprehensively evaluated.

[Center porosity test of slab] Three specimens each having a length of 500 mm in a cross section passing through the axis of the slab and having a length of 500 mm were subjected to visual observation under mechanical grinding and a penetrant flaw detection test (PT). The center porosity was evaluated by the ratio of the total length of porosity distribution to the sample length. A porosity index of 1 indicates that a porosity defect could not be detected, and the smaller the index, the better.

[Breaking of slab and V segregation revealing test] One piece of a C-section cross section of the slab and L after carrying out the porosity test
Cracking and V segregation were revealed by a hydrochloric acid macro-corrosion test for three test pieces each having a cross section of 500 mm in length. Then, the diameter and the length of the defect were visually measured, and the crack and the V segregation were evaluated based on the product of the diameter and the length. Crack and V
A segregation index of 1 means that none of these defects could be found, and the smaller the index, the better.

[Inner surface evaluation test] The cast round billet slab is heated to a predetermined temperature between 1100 and 1300 ° C,
A hollow shell was manufactured by rolling with a Mannesmann piercing machine, stretched with an elongator, and then sized with a sizer to manufacture a seamless steel tube. Then, the occurrence state and degree of inner surface flaws of the steel pipe were inspected, and the inner surface of the pipe was evaluated. It is also an index that can comprehensively evaluate the internal quality of the slab used as the raw material, and the smaller the score, the better the internal quality, with a score of 1 having no defects, and a score of 2 having very few defects and passing. A score of 3 indicates that the defect is slight but requires maintenance, and a score of 4 indicates that the defect is large and is not accepted.

The above test results are also shown in Tables 1 and 2. The following items can be understood from the test results. Comparative Examples 1 and 3 to which light reduction was not applied are inferior in overall porosity index, cracking index, and V segregation index in-cast property, and inferior in pipe inner surface score.

In Comparative Example 2 in which the axial center portion of the cast slab was lightly reduced after complete solidification, since the liquid phase did not exist at the time of applying the reduction, the internal quality of the slab was hardly improved. As a result, it is inferior in the internal rating.

As described above, all of the comparative examples are inferior in at least one of the cast slab internal quality scores. On the other hand, Examples 1 to 12 according to the method of the present invention are excellent in all of the slab internal quality scores, or relatively excellent in all the scores.

Example 12 in which the molten steel was not electromagnetically stirred in the mold but light reduction was applied in the final stage of solidification, and the electromagnetic stirring of molten steel in the mold and light reduction in the final stage of solidification were also performed, but a flat roll was used. In Example 1, which was lightly reduced in Example 2, both the slab internal properties and the tube inner surface rating were slightly improved, as compared with Comparative Examples 1 and 3 in which light reduction was not applied at the final stage of solidification.

With the V-type caliber roll, the axial center solid phase fraction f is obtained by reducing the cross-sectional area in the C direction in the range of 0.1 to 3% at a light reduction.
Even if it is applied when s satisfies 0.1 ≦ fs <1, Example 2 or 3 in which the first-stage reduction roll or the second-stage reduction roll is used to perform the first-stage light reduction is the internal property of the slab. Of 3
Although there is an improvement in any of the indices, there is still room for improvement in the in-pipe score.

Axial solid fraction f by V-type caliber roll
Two-stage light reduction was performed when s satisfied 0.1 ≦ fs <1, but fs at the second-stage reduction was small (fs =
0.44) In both Example 5 and Example 11 in which the C-direction cross-section reduction rate was large (3.32%), there was room for improvement in the 3 index of cast piece internal quality characteristics and the pipe inner surface flaw rating. is there.

In contrast to the above embodiment, the fourth embodiment,
All of 6, 7, 8, 9 and 10 are subjected to the second stage light reduction at the time when the axial solid fraction fs satisfies 0.1 ≦ fs <1 by the V-type caliber roll, and the first stage light reduction is performed. When the axial solid fraction fs is 0.1 ≦ fs ≦ 0.5, the second stage light reduction is performed at 0.5 <fs, and the C-direction cross-sectional area reduction rate is 0.1 to 3%. It was in the range of, and both the 3 index of the slab internal property and the tube inner surface flaw rating were further excellent.

On the basis of the above-mentioned results, when one-stage light reduction or two-stage light reduction is applied to the slab in the final stage of solidification,
FIG. 6 is a conceptual diagram showing the effect of the axial solid fraction fs at the time of applying a light reduction on the porosity index, cracking index and V segregation index. From the figure, the axial solid fraction fs in the case of one-step light pressure
However, when the porosity index is 0.5 ≦ fs <1, the crack index is 0.7 ≦ fs <1, and the V segregation index is 0.1 ≦ fs ≦ 0.5. It was found that the improvement effect is exhibited only when it is added. However, in this case, the improvement effect is only manifested for each index. On the other hand, according to the conditions within the scope of each claim of the present invention, in particular, the conditions within the scope of claim 3 for obtaining a score satisfying the soundness of the inner surface of the pipe, the slab internal quality characteristic is 3 It can be seen that the index can be improved at the same time.

[0051]

EFFECTS OF THE INVENTION Since the present invention is configured as described above, it is possible to provide a slab material of Cr alloy steel having a good internal quality by a simpler process than conventional ones, and it is possible to obtain a hard workability without internal flaws. A Cr alloy steel seamless pipe can be manufactured. That is, the present invention achieves continuous casting of a defect-free billet of a difficult-to-work Cr alloy steel, particularly a high-Cr alloy steel, which has been considered difficult in the past, and as a result, uses a pipe manufacturing method that does not change from the conventional one. Also, there is an industrially useful effect that a seamless steel pipe with a high added value can be manufactured without any inner surface flaws and the manufacturing cost can be reduced.

[Brief description of drawings]

FIG. 1 is a conceptual vertical cross-sectional view of a continuous casting process for explaining an embodiment of the present invention.

FIG. 2 is a vertical sectional conceptual view of a continuous casting process for explaining another embodiment of the present invention.

FIG. 3 is a schematic side view showing a main part of the arrangement of a reduction roll made of a V-type caliber roll with respect to a cast piece in one embodiment of the present invention.

FIG. 4 is a sectional view taken along line AA of FIG. 3;

FIG. 5 is a sectional view taken along line BB of FIG. 3;

FIG. 6 shows the influence of the solid phase fraction fs of the axial center at the time of light reduction on each index of the internal properties of the slab when the first stage light reduction or the second stage light reduction is applied to the slab in the final stage of solidification. it is a conceptual diagram of.

FIG. 7 is a graph showing the relationship between the Cr content in steel and the viscosity of molten steel.

[Explanation of symbols]

 1 Round shape mold 2 Molten steel 3 Unsolidified round billet slab 3'Round billet slab 4 Rolling down device 4a Rolling down roll 4b 1st stage rolling down roll 4c 2nd stage rolling down roll 5 Solid-liquid coexisting phase 6 Guide roll group 7 Drawing roll 8 In-mold electromagnetic stirrer 9a, 9a ', 9b, 9b' V-type caliber roll

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Hiroshi Maeda 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (72) Riyo Nakagome 1-2-1 Marunouchi, Chiyoda-ku, Tokyo Nippon Steel Tube Co., Ltd. (72) Inventor Tatsuro Katsumura 1-2-2 Marunouchi, Chiyoda-ku, Tokyo Nihon Steel Tube Co., Ltd.

Claims (3)

[Claims] 1. A Cr alloy steel having a Cr content of more than 0.5 wt.% Is continuously cast by a round mold having a diameter of 340 mm or less, and the round billet slab obtained is lightly rolled by a rolling roll at the end of solidification. A method for manufacturing a chromium alloy steel round billet slab, which comprises adding 2. The method for manufacturing a round billet slab according to claim 1, further comprising electromagnetic stirring treatment of molten steel in the round mold, and the light reduction by the reduction roll is performed in two or more stages. When the axial solid fraction fs of the round billet slab satisfies 0.1 ≦ fs <1 with the pressing roll, C
The method for producing a chrome alloy steel round billet slab according to claim 1, wherein the reduction in the direction cross-sectional area reduction rate is within the range of 0.1 to 3%. 3. In the light reduction, a reduction roll composed of two V-shaped caliber rolls whose axes are parallel to each other and face each other with the round billet slab in the middle is used in the casting direction of the round billet slab. 2 or more are arranged along each, and the respective round billet slabs for the round billet slabs by the respective drafting rolls are rolled down in different directions from each other, and the round billet slabs at the final stage of solidification are rolled down. 2. The method for producing a round billet slab of chromium alloy steel according to 2.