JPH11138238A - Production of b-containing austenitic stainless steel cast slab - Google Patents

Abstract

PROBLEM TO BE SOLVED: To restrain the coarsening of eutectic crystal and the development of porosity and local crack as problems at the time of producing a B-containing austenitic stainless steel with continuous casting and to prevent the lowering of hot-workability and cold-workability. SOLUTION: An equiaxed crystal ratio is made to 10-50% to prevent the lowering of the hot-working at the time of producing the B-containing autenitic stainless steel with the continuous casting. The method is executed by controlling the thickness of a cast slab to 130-270 mm and overheating degree of molten steel in a tundish to in the rage of 20-80 deg.C. In order to prevent the lowering of the hot-workability and further to prevent the lowering of the cold- workability, the tapered ratio having >=0.1% is given to the length of >=1 m from the last solidified position to the upstream side and also, the rolling reduction ratio at the non-solidified part is applied at >=0.5% with rolling reduction rolls at the stage which the part having <1 solid phase remains >=5 mm in the thickness direction.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for continuously casting a B-containing austenitic stainless steel slab excellent in hot workability and cold workability.

[0002]

2. Description of the Related Art Conventionally, austenitic stainless steel containing B has been used for containers of spent nuclear fuel and the like because of its high neutron absorption capacity.

[0003] Since this steel type has a eutectic solidification structure, the temperature at which strength is developed is lower than that of ordinary stainless steel and carbon steel, and breakout occurs due to insufficient strength of the solidified shell. It was difficult.

As a countermeasure against the above, the present applicant has disclosed in Japanese Patent Application Laid-Open
No. 9391 proposes a method for controlling the casting speed, the interval between the support rolls, and the solid phase ratio at the final position of the secondary cooling zone.

The above-mentioned invention is an excellent method capable of performing stable continuous casting. However, the steel contains eutectic (eutectic of boride such as (Fe, Cr) B and γ-iron) in the steel and is compared with the base metal. The eutectic structure is fragile and no measures have been taken to address the problem of cracking during hot and cold working.

Japanese Patent Application Laid-Open No. 4-272131 discloses that a soaking treatment is performed before hot working or that Ca,
A method for improving hot workability by adding one or more of Y, La + Ce has been proposed.

The present invention also does not take any measures against the problem of cracking during hot and cold working caused by eutectic peculiar to austenitic stainless steel containing B.

[0008]

DISCLOSURE OF THE INVENTION The present invention suppresses defects caused by eutectic which are problematic when producing a B-containing austenitic stainless steel slab by continuous casting, and provides a castable steel having good hot workability. Providing a method of manufacturing the pieces; and
An object of the present invention is to provide a method for producing a cast piece having good hot and cold workability.

[0009]

Means for Solving the Problems The following findings (A) to (I) have been obtained. (A) As shown in the conceptual diagrams of the macrostructures in FIGS. 1A to 1C, when the equiaxed crystal is not formed and the columnar crystal is grown to the center of the thickness (when the equiaxed crystal ratio is 0%). Example, see FIG. 1 (a))
In addition, cracks occur along the eutectic structure in the central part.
The plate-shaped eutectic at the center of the slab is a cause of linear continuous cracks generated at the center of the plate thickness after hot working, and the center of the slab is made equiaxed (equiaxed). Example of 30% rate, FIG.
(B)), it is possible to prevent the formation of a plate-like eutectic, which is the cause of the cracks.

(B) Increasing the equiaxed crystal ratio (60% equiaxed crystal ratio)
For example, referring to FIG. 1 (c)), a coarse granular equiaxed crystal is generated in the center, and a coarse granular eutectic is generated in this portion.
It was recognized that discontinuous minute cracks occurred.

(C) As shown in FIG. 2, the equiaxed crystal ratio is 30%.
Micro-observation of Example No. 3 further confirmed that vacancies existed in the granular eutectic portion and that local cracks occurred between equiaxed grains.

It has also been found that the above-mentioned vacancies in the eutectic portion and local cracks existing between equiaxed grains cause cold workability to deteriorate.

(D) As shown in FIG. 3, there is a correlation between the equiaxed crystal ratio of the B-containing austenitic stainless steel slab and the crack occurrence ratio during hot rolling. 1
By setting the content to 0 to 50%, preferably 20 to 40%, the crack generation rate can be reduced.

Here, the definition of the equiaxed crystal ratio is [(thickness of the equiaxed crystal portion of the slab) / (thickness of the slab)] × 100 (%).

(F) As shown in FIG.
Containing austenitic stainless steel, slab thickness,
It has been found that an appropriate equiaxed crystal ratio can be obtained by optimizing the degree of superheat of molten steel in a tundish, and that it can be controlled by the following (2).

(G) During continuous casting of a slab having a thickness of 130 to 270 mm, the degree of superheat of molten steel in a tundish is 2
By setting the temperature at 0 to 80 ° C., the equiaxed crystal ratio can be controlled at 10 to 50%.

By controlling the equiaxed crystal ratio to 10 to 50%, the hot workability of the slab can be improved, and in order to improve the severe cold workability, the empty space of the slab is improved. It is effective to suppress the occurrence of holes and local cracks, and the following (3) is a method for producing a slab having good hot and cold workability.
It has been found that implementing (4) is effective.

(H) As shown in FIG. 5, at the final solidification position where the solid phase ratio of the slab becomes 1 in the continuous casting and at the upstream side thereof, 0.1% or more with respect to the thickness of the slab. The taper amount (%) is given to a taper portion length (m) of 1 m or more.

Here, the definition of the taper amount is: taper amount (%) = ΔD / L × 100 (%). Here, L is the length (m) of the tapered portion, ΔD is ΔD = Do (upstream slab thickness) −D (downstream slab thickness), and the unit of each slab thickness is (m). is there.

(I) When a portion where the solid phase ratio is less than 1 remains in the slab at a center of 5 mm or more in the thickness direction of the slab, a pinch roll or a reduction roll installed on a roller apron (a reduction amount of mm ) / (Unsolidified thickness mm) The unsolidified portion rolling reduction is 0.5 or more.

The configurations of the present invention obtained from the above findings are shown in the following (1) to (4). (1) B-containing austenitic stainless steel casting excellent in hot workability, characterized in that the equiaxed crystallinity is 10 to 50% when producing a B-containing austenitic stainless steel slab by continuous casting. How to make pieces.

(2) When producing a B-containing austenitic stainless steel slab by continuous casting, the slab thickness is reduced to 13
0 to 270 mm, the degree of superheat of molten steel in the tundish during continuous casting is set to 20 to 80 ° C., and the equiaxed crystal ratio is set to 10
A method for producing a B-containing austenitic stainless steel slab excellent in hot workability, characterized by being controlled to ５０50%.

(3) A taper amount of 0.1% or more with respect to the thickness of the slab at the final solidification position where the solid phase ratio of the slab produced by the continuous casting method becomes 1 and at the upstream side thereof. 1
The method for producing a B-containing austenitic stainless steel slab excellent in hot workability and cold workability according to the above (1) and (2), which is provided over a length of at least m.

(4) In a stage where a portion having a solid phase ratio of less than 1 in the slab remains at least 5 mm in the thickness direction, the unsolidified portion rolling reduction is set to 0.5 or more. ) ~
The method for producing a B-containing austenitic stainless steel slab excellent in hot workability and cold workability according to any of (3).

In the present invention, "B-containing austenitic stainless steel" means that B is 0.5 to 3.0% by weight.
Austenitic stainless steel contained.

B is an element added to secure the neutron absorbing ability, and it is necessary to add 0.5% by weight or more in order to secure the performance. The upper limit is preferably 3% by weight or less because the content increases and the portion other than the center segregation portion becomes brittle.

[0027]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In order to study a method for controlling the equiaxed crystal ratio, the relationship between casting conditions and the equiaxed crystal ratio was investigated.

The continuous casting tester uses a vertical continuous casting facility with a machine length of 4 m and a slab thickness of 100 to 300 m.
m, slab width 400-600mm, casting speed 0.4-
Casting was performed with the range changed to 0.8 m / min.

The degree of superheating of the molten steel was also changed, and the relationship with the equiaxed crystal ratio was investigated. The degree of superheat of the molten steel was measured in a tundish, and the measurement position was on the side wall between the hot water inlet from the ladle and the hot water inlet to the mold, and the molten steel temperature was measured by a thermocouple.

As shown in FIG.
mm, it is impossible to stably generate 10% or more equiaxed crystals. When the slab thickness is in the range of 130 to 270 mm, the equiaxed crystal ratio decreases when the degree of superheat of molten steel exceeds 80 ° C. When the degree of superheat of molten steel is less than 20 ° C., the equiaxed crystal ratio sharply increases. This is because the degree of superheat of molten steel in the tundish is 2
When the temperature becomes 0 ° C or less, the superheat degree becomes 0 ° C or less in the mold,
This is probably because the formation of equiaxed crystals starts rapidly. Also,
If the degree of superheat of molten steel is excessively reduced, there is a risk of nozzle blockage, and stable continuous casting operation becomes difficult.

When the thickness of the slab is 300 mm, it is difficult to reduce the equiaxed crystal ratio to 50% or less. The change of the equiaxed crystal ratio with the casting speed was small. From these experimental results, preferred casting conditions for controlling the equiaxed crystal ratio to 10 to 50% are a slab thickness of 130 to 270 mm and a superheat of molten steel of 20 to 80 ° C.

A more preferred range of the degree of superheat of molten steel is 30.
C. to 60C. As a method for increasing the equiaxed crystal ratio, a method of performing electromagnetic stirring is widely known, but it is needless to say that the method can be applied to the present invention.

As shown in FIG. 2, the large eutectic grains have a diameter of about 2 to 3 mm. The presence of voids and localized cracks in the coarse eutectic and eutectic does not affect hot workability, but corner cold cracks such as bending can cause corner cracks. Cause.

It is considered that the cause of the formation of voids and local cracks is that molten steel is not supplied in accordance with the solidification shrinkage when the center of the slab solidifies.

In the last stage of solidification, if the thickness of the slab is reduced by the amount corresponding to the solidification shrinkage, it is possible to compress the voids and local cracks. Based on the final solidification position of the slab, the state of occurrence of voids and local cracks in the slab obtained by setting various amounts of taper on the roller apron of the test continuous casting facility shown in FIG. 5 was examined.

A 5 cm square at the center of the obtained slab was observed with a loupe, and the number of holes and local cracks was examined. As shown in FIG. 6, when the taper amount is 0.1% or more and the length of the tapered portion is 1 m or more, voids and local cracks are reduced.
It was found that when the length of the tapered portion was 1 m or more at 3% or more, voids and local cracks were eliminated.

When the taper amount is set to 1% or more, the roll reaction force suddenly increases, so equipment having high rigidity is required, and the equipment cost becomes excessive, which is not appropriate. 1%
The above taper amount is an excess taper amount exceeding the compensation amount of solidification shrinkage, and is desirably less than 1%.

The length of the tapered portion is desirably 5 m or less in consideration of equipment costs and effects. It is necessary to provide an appropriate amount of taper between the rolls of the roller apron in order to prevent the formation of voids and local cracks immediately before the final solidification. The zone can be changed to upstream or downstream.

As described above, the cast slab is equiaxed within an appropriate range, and the roller apron is given an appropriate taper amount, so that porosity at the center of the cast slab and local cracking are obtained. Can be prevented.

However, even if the above measures are taken, the particle diameter is 2 mm.
The above eutectic may still remain. In this case, if more severe processing conditions are added, the coarse eutectic may cause cracking.

As a countermeasure, the concentrated molten steel in the vicinity of the final solidification, which is a base solution for generating a eutectic by applying an appropriate unsolidified portion pressure to the slab at the final stage of solidification, is flushed horizontally to increase the thickness of the concentrated molten steel. In view of the fact that the eutectic particle size can be reduced by reducing the amount of eutectic, a test was conducted by changing the amount of reduction of the unsolidified portion.

As shown in FIG. 7, when an unsolidified portion of 5 mm or more remained, the maximum eutectic grain size at the center of the slab after rolling with a pinch roll was used as an index.
Coarse eutectic grains are obtained by setting the rolling reduction of the unsolidified portion defined by [rolling amount (mm) / thickness of the unsolidified portion (mm)] to 0.5 or more, preferably 0.8 or more. The diameter can be reduced (less than 2 mm).

The same results were obtained when the above-mentioned pinch rolls were used instead of the rolls and the rolls were mounted on a roll apron.

The reason for the reduction when the unsolidified portion of 5 mm or more remains is that, when the thickness of the molten steel in the unsolidified portion is less than 5 mm, local bridging occurs and the concentration near the final solidification at which the eutectic is formed. This is because there is a possibility that the molten steel may not be sufficiently washed out in the horizontal direction.

The upper limit of the thickness of the molten steel in the unsolidified portion is 50 mm. If the thickness exceeds 50 mm, the effect of reducing the eutectic grain size cannot be expected even if the pressure is reduced. The thickness of the unsolidified portion of the molten steel can be measured in the same manner as the above-described final solidification position by tacking measurement and heat transfer solidification analysis.

Normally, in continuous casting, the reduction of the slab at the end of solidification is performed for the purpose of reducing or eliminating positive segregation at the center of the slab. In the case of B-containing austenitic stainless steel, the eutectic grain size at the center of the slab is controlled by appropriately setting the draft of the unsolidified portion while maintaining the positive segregation at the center of the slab, and the processing conditions Thus, the effect of improving the cold workability, which is severe, can be obtained. In addition,
No problems such as internal cracks caused by the rolling-down process were observed.

[0047]

EXAMPLE A B-containing austenitic stainless steel having the components shown in Table 1 was continuously cast under the conditions shown in Table 2 using a single-point straightening type continuous casting machine having a machine length of 16 m and a bending radius of 8 m. It was rolled to a thickness of 3.5 to 6 mm by cold rolling. Using a 4.5 mm hot-rolled sheet, bending by cold pressing is performed, and 225 mm is drawn by a cold drawing method.
The tube was shaped into a square tube and subjected to a stabilizing heat treatment.

[0048]

[Table 1]

[0049]

[Table 2]

Example 1 The occurrence of cracks in the cross section of the sheet after hot rolling was investigated by changing the equiaxed crystal ratio, the degree of superheat of molten steel, and the thickness of the slab. For the evaluation of cracks, take a cross-section sample of the plate,
The evaluation was based on the ratio of the crack length to the plate width.

Table 3 shows the evaluation criteria for cracks. As shown in Table 4, in Invention Examples 1 and 2, thicknesses 150 and 2
The slabs of 00 mm were heated to 48 and 38 degrees of molten steel superheat, respectively.
When manufactured under the condition of ° C., the equiaxed crystal ratio was 29 to 33%, and there was no or slight cracking after hot rolling.

When the slab thickness of Comparative Example 1 was 100 mm, no equiaxed crystal was formed, and the ratio of the crack length after hot rolling was 85%.
%. When the superheat degree of the molten steel was lowered to increase the equiaxed crystal ratio to 68%, a coarse eutectic was recognized at the center of the plate thickness, and the ratio of the crack length was 40%.

[0053]

[Table 3]

[0054]

[Table 4]

(Example 2) The amount of taper of the roller apron was adjusted by adjusting the roll interval of the segment at a position of 3.7 to 7.1 m from the meniscus of the continuous casting machine.

The obtained slab was hot-rolled to obtain 4.
Using a hot-rolled sheet having a thickness of 5 mm, the pipe was formed into a square tube of 225 mm square and subjected to a stabilizing heat treatment.

Table 5 shows the evaluation criteria for corner cracking.
As shown in Table 6, when the taper amount of 0.15% or 0.38% of each of Examples 3 to 4 of the present invention was given by 1 m or more, the cracks in the bent portion were less than 1 mm and were within the allowable range. Was. With the taper amount of 0.08% in Comparative Example 3, cracks occurred at the corners. Even when the taper amount of Comparative Example 4 was 0.25%, cracks occurred when the length was 0.6 m.

[0058]

[Table 5]

[0059]

[Table 6]

Example 3 At the end of solidification, a test was conducted in which a slab was rolled down with two pinch rolls. The obtained slab was hot-rolled in the same manner as in Example 2, and the resulting hot-rolled sheet having a thickness of 4.5 mm was formed into a 225 mm square tube and subjected to a stabilizing heat treatment. We investigated corner cracks for

The evaluation criterion for corner cracking is the same as that of the second embodiment. As shown in Table 7, when the rolling reduction of the unsolidified portion of Comparative Example 5 was 0.2, the grain size of the eutectic in the central portion was 2 mm or more, pores remained, and large cracks occurred in the bent portion. did.

When the rolling reduction of the unsolidified portion was increased to 0.4 of Comparative Example 6, the pores were compressed, but the eutectic particle size at the center was 2 mm.
As described above, fine cracks occurred at the corners. When the rolling reduction of the unsolidified portion was 0.6 or more in Examples 5 to 7 of the present invention, the grain size of the eutectic at the center was reduced to less than 2 mm, and cracks at the corners could be prevented. With the thickness of the unsolidified portion of 3 mm in Comparative Example 7, the eutectic particle size was 2.5 even when the rolling reduction of the unsolidified portion was 0.9.
The crack length at the corner became 1 mm or more.

[0063]

[Table 7]

[0064]

According to the present invention, it is possible to suppress the eutectic coarsening and the occurrence of voids and local cracks during continuous casting of a B-containing austenitic stainless steel, thereby improving hot workability and cold working. Can be prevented from decreasing.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram of a macrostructure showing a relationship between an equiaxed crystal ratio of a slab and a crack of the slab.

FIG. 2 is a conceptual diagram showing a result of micro observation of an example of a cast slab having an equiaxed crystal ratio of 30%.

FIG. 3 is a graph showing the relationship between the equiaxed crystal ratio of a slab and the incidence of cracking during hot rolling.

FIG. 4 is a graph showing the relationship between the equiaxed crystal ratio of a slab, the slab thickness, and the degree of superheat of molten steel.

FIG. 5 is a conceptual diagram showing a taper amount (%) and a taper portion length (m) of a roller apron.

FIG. 6 is a graph showing the relationship between the amount of taper of a roller apron, the length of the tapered portion, and the occurrence of local cracks and voids in a slab.

FIG. 7 is a graph showing the relationship between the unsolidified portion draft and the eutectic grain size of a slab.

[Brief description of reference numerals]

 1: Slab 2: Roll

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification code FI C22C 38/54 C22C 38/54 G21F 5/002 G21F 5/00 W 5/005

Claims (4)

[Claims] 1. An excellent hot workability B characterized in that, when a B-containing austenitic stainless steel slab is manufactured by continuous casting, the equiaxed crystal ratio of the slab is 10 to 50%. Method for producing cast austenitic stainless steel slabs. 2. When a B-containing austenitic stainless steel slab is produced by continuous casting, the thickness of the slab is set to 130.
To 270 mm, the degree of superheat of molten steel in the tundish during continuous casting is set to 20 to 80 ° C., and the equiaxed crystal ratio is set to 10 to 270 mm.
A method for producing a B-containing austenitic stainless steel slab excellent in hot workability, characterized by being controlled to 50%. 3. A final solidification position at which the solid fraction of a slab produced by the continuous casting method is 1, and an upstream side thereof,
2. The method according to claim 1, wherein a taper amount of 0.1% or more with respect to the thickness of the slab is provided over a length of 1 m or more.
A method for producing a B-containing austenitic stainless steel slab excellent in hot workability and cold workability according to claim 2. 4. The unsolidified portion reduction ratio is set to 0.5 or more at a stage where a portion having a solid phase ratio of less than 1 remains in the slab in a thickness direction of 5 mm or more. B having excellent hot workability and cold workability described in any one of 3.
Method for producing cast austenitic stainless steel slabs.