JPH05337612A - Method for casting nb-containing ferritic stainless steel by twin roll type continuos casting method - Google Patents

Abstract

PURPOSE:To provide a casting method for preventing the development of surface cracking of a cast slab by preventing the development of cracking at the time of solidifying the cast slab and the enlargement of cracking at the time of cooling the cast slab at the same time, in the method for casting an Nb- containing ferritic stainless steel. CONSTITUTION:In the method for casting the ferritic stainless steel containing 0.001-0.05% C, 0.2-1.0% Si, 0.05-0.8% Mn, <=0.03% P, <=0.01% S, 0.1-0.5% Nb and 0.001-0.05% N by a twin roll type continuous casting method, such sufficient quantity of Ti as to make free [C+N] quantity being capable to combine with Nb in the molten steel supplied to the casting <=0.03% is added. Then the cast slab is cooled at >=10000 deg.C/min cooling rate to 1200 deg.C and at >=1500 deg.C/min cooling rate to 800 deg.C, and thereafter, by water-cooling, the accumulation of C and N into Nb macro segregating part and the precipitation of NbCN at high temp. are prevented.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for casting Nb-containing ferritic stainless steel without surface cracks by twin roll type continuous casting.

[0002]

2. Description of the Related Art In the twin roll type continuous casting method, a molten metal is poured into a molten metal pool portion formed by a pair of cooling rolls arranged in parallel and side dams for sealing both end faces of the cooling rolls. The solidified shells in the shape of a circle, and the closest position of both rotating chill rolls (the so-called "kiss point").
Alternatively, it is a continuous casting method in which the solidified shells are united in the vicinity of the "kissing point" and delivered as an integrated thin strip.

The thin strip-shaped slab cast by the twin roll type continuous casting method has a thickness of several mm (usually about 1 to 6 mm), and is cold-rolled without hot rolling to produce a thin plate product. can do. Therefore, a production method of casting a slab as a slab for hot rolling of several 100 mm square by continuous casting using a vibration mold, hot rolling it, and then cold rolling it (slab slab / hot rolling process ), The production efficiency and cost are significantly improved.

Conventionally, Nb has been produced by the twin roll type continuous casting method.
It was practically impossible to cast a ferritic stainless steel containing steel. This is because, during the cooling of the thin strip-shaped slab sent out from between the cooling rolls, a crack is generated on the surface of the slab, which causes a significant decrease in yield or breakage during cold rolling. As a countermeasure, Nb /
Although methods such as adjusting the (C + N) ratio and rapidly cooling the Laves phase precipitation temperature of 850 to 650 ° C. have been taken, it has not been possible to reduce cracking to the extent that there is no practical problem. ..

[0005]

SUMMARY OF THE INVENTION The present invention is designed to prevent the occurrence of surface cracks in a slab by simultaneously preventing the occurrence of cracks during solidification of the slab and the spread of cracks during cooling of the slab. An object of the present invention is to provide a method for casting Nb-containing ferritic stainless steel by a roll-type continuous casting method.

[0006]

According to the present invention, the above-mentioned objects are, by weight, C: 0.001 to 0.05%, Si: 0.2 to 1.0%, Mn: 0.05. -0.8%, P: 0.03% or less, S: 0.01% or less, Nb: 0.1-5.0%, N: 0.001-0.05% containing ferritic stainless steel 6mm thick by twin roll continuous casting
In the following method of casting into a strip-shaped slab, an amount of T sufficient to set the total amount C + N of free C and free N that can be combined with Nb in the molten steel used for casting to 0.030% or less.
i is added, and the cooling rate of the slab is set to 1000 at least 1200 ° C. after the slab emerges from between the cooling rolls.
0 ° C./min or more, 800 ° C. up to 1500 ° C./min or more, and then water cooling to prevent C and N from accumulating in the Nb macrosegregation portion and at the same time to increase Nb at high temperature.
This is achieved by a method for casting Nb-containing ferritic stainless steel in a twin roll type continuous casting method, which is characterized by preventing precipitation of CN.

[0007]

The present inventor has found that in the twin roll type continuous casting method of Nb-containing ferritic stainless steel, surface cracks of the slab are generated by the following mechanism. That is, Nb micro-segregation occurs first during solidification of a slab, and then surrounding Nb segregation accumulates in the internal crack portion generated by solidification stress, resulting in macro-segregation. As the temperature of the slab lowers, C and N accumulate in the macrosegregated portion and the melting point lowers, so that the high temperature strength (particularly tensile strength) in that portion lowers and cracks occur. When the temperature further decreases, NbC
N precipitates, cracks expand along the NbCN cluster, cause breakage during cold rolling, and remain as flaws on the product sheet.

In the present invention, the macroscopic C and N accumulation in the segregated portion and the precipitation of NbCN are prevented to prevent the occurrence and expansion of cracks. Therefore, by adding Ti that bonds with C and N at a temperature higher than that of Nb and fixing C and N as TiC and TiN, respectively, the accumulation of C and N in the macrosegregation portion is reduced and the high temperature strength is improved. It prevents deterioration and reduces NbCN precipitation to prevent crack expansion. Further, Nb has a strong tendency to segregate in the dendrite intergranular portion of the solidified structure of the slab, but Ti has almost no segregation tendency and therefore does not adversely affect the slab. The present invention utilizes such properties of Ti.

The amount of Ti added is sufficient to suppress the total amount of free C and N capable of binding to Nb to 0.030% or less. However, normally, from the viewpoint of preventing clogging of the pouring nozzle and cost reduction, the Ti addition amount is preferably 0.2% or less. Due to the limitation of the Ti addition amount, the upper limits of the C content and the N content are each 0.0
It was set to 5%. Further, the lower limits of the C content and the N content, respectively, are the minimum amounts that can be refined by the current refining technology.
It was set to 001%.

The cooling rate of the slab is 1 at least up to 1200 ° C. in order to secure the high temperature strength of the Nb macrosegregation portion.
Rapid cooling of 0000 ℃ / min or more, 150 up to 800 ℃
Water cooling is performed at 0 ° C / min or more and thereafter. The means for setting the slab cooling rate after the slab is delivered from the kiss point within the specified range of the present invention is not particularly limited. As one preferable means, the slab can be continuously pressed against the circumferential surface of either one of the cooling rolls from the kiss point by a pressing roll arranged immediately below the pair of cooling rolls. By using one or more pressing rolls, it is possible to cool up to 1000 ° C. at a cooling rate of 10,000 ° C./min or more.

Further, from 1200 ° C. to 800 ° C., 15
In order to cool at a cooling rate of 00 ° C / min or more, if the cast piece thickness is less than 2 mm, it is sufficient to allow cooling (natural cooling by heat dissipation), but if the cast piece thickness is 2 mm or more, It is necessary to supply gas or the like to the surface of the slab for forced cooling. less than,
The present invention will be described in more detail by way of examples with reference to the accompanying drawings.

[0012]

Example Nb-containing ferritic stainless steel having the chemical composition shown in Table 1 was cast into a thin strip-shaped cast piece (thickness: 1.6 to 4 mm) by a twin roll type continuous casting method. Casting N in the table
o-1 to 7 are examples of the present invention, and casting Nos. 8 to 16 are comparative examples.

[0013]

[Table 1]

[0014]

[Table 2] The cooling rate after the kiss point was controlled in two stages. That is, the average cooling rate (C _R ) from the passage of the kiss point to the initial cooling rate end temperature (T _A ) and 80 thereafter.
There were two stages of cooling rate up to 0 ° C., and thereafter water cooling was performed. Among these, the initial cooling rate was realized by continuously pressing the cast slab from the kiss point to the circumferential surface of one cooling roll with one or a plurality of pressing rolls arranged immediately below the cooling roll. Also, after the end of the initial cooling rate, 80
A constant cooling rate of 1500 ° C./min up to 0 ° C. is realized by natural air cooling when the cast piece thickness is less than 2 mm, and an inert gas, preferably nitrogen gas, is cast when the cast piece thickness is 2 mm or more. It was realized by spraying on the surface and controlling the cooling rate.

FIG. 2 shows a structural example of the twin roll type continuous casting apparatus used. The molten metal is injected from the tundish 1 through the nozzle into the molten metal pool 3 formed by the cooling roll 2 and the side dam. Solidified shells 4 are generated on the circumferential surfaces of both cooling rolls 2, and the pair of solidified shells 4 are combined near the closest position of both cooling rolls 2 of the rotating cooling rolls to be delivered as a slab 5. Since the slab 5 has a cooling rate up to 1200 ° C. of 10000 ° C./min, the slab 5 is pressed against one cooling roll 2 by the pressing roll 6 to promote cooling.

Since the cooling rate up to 800 ° C. is 1500 ° C./min or more, a plurality of gas blowing pipes 7 are installed in a position directly below the pressing roll 6 in the width direction of the slab 5, and the slab is measured by a radiation thermometer. Controls cooling as it relates to temperature. 8
Cooling below 00 ° C. is performed by the water cooling device 10. Slab 5
Is conveyed by a pinch roll 8 and wound up by a coiler 9.

The slabs obtained by the above castings are pickled.
50% cold rolling-final annealing-pickling-temper rolling was performed to obtain a thin plate product. Table 2 shows the casting conditions, the amount of slab cracking, and the yield of cold-rolled thin plate products. Free [C
+ N] amount and the average cooling rate up to 1200 ° C. are shown in FIG. As shown in FIG. 1, the upper limit of the cooling rate up to 1200 ° C. is 40,000.
C / min is the limit of the cooling capacity of the cooling roll in the apparatus used.

[0018]

[Table 3]

[0019]

[Table 4] As shown in Table 2, in the casting Nos. 1 to 7 according to the present invention, no slab cracking was observed, and the cold rolling yield was 98%.
It was above (indicated by a circle in FIG. 1). On the other hand, molten steel composition (particularly Nb amount), Ti addition amount and / or 800
Casting Nos. 8 to 19, which had an insufficient cooling rate to ℃, had a minimum of 2.0 cm / m ² (casting No. 16) and a maximum of 2
A slab crack of 5.0 cm / m ² (casting No. 8) was generated, and the cold rolling yield was at most 50% (Casting No. 1).
6), the minimum was 15% (casting No. 8), which was extremely low (marked with Δ or × in FIG. 1). In particular, casting Nos. 14, 15, 16, 17 (Fig. 1) with slow cooling rates up to 800 ° C
(Indicated by Δ) indicates that the ferrite grains have become coarse and the toughness has deteriorated, making it impossible to rewind the slab coil. Furthermore, of these, casting No. 15 in which the amount of free [C + N] was reduced by long-term decarburization treatment with AOD without adding Ti,
The cost of 16 is also high because of this processing. Also, 12
Casting No. 13 with slow cooling rate from 00 ℃ to 800 ℃
Similarly, the toughness decreased, and a heating treatment was required when rewinding the slab coil. Casting Nos. 18 and 19 had a Nb content of more than 5%, and thus cracking occurred even under the cooling conditions of the present invention.

[0020]

As described above, according to the present invention,
When a Nb-containing ferritic stainless steel is cast by the twin roll type continuous casting method, it is possible to prevent surface cracking of the slab by simultaneously preventing cracking during solidification of the slab and expansion of cracking during cooling of the slab. And the cold rolling yield can be significantly improved.

[Brief description of drawings]

FIG. 1 is a graph showing a result of quality judgment of a thin plate product with respect to a combination of a free [C + N] amount and an average cooling rate up to 1200 ° C.

FIG. 2 is a cross-sectional view showing the configuration of a twin roll type thin plate continuous casting apparatus.

[Explanation of Codes] 1 ... Tundish 2 ... Cooling drum 3 ... Hot water pool 4 ... Solidification shell 5 ... Thin cast piece 6 ... Pressing roll 7 ... Gas blowing pipe 8 ... Pinch roll 9 ... Coiler 10 ... Water cooling device

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification number Internal reference number FI Technical indication location C22C 38/48 (72) Inventor Yoshiyuki Uejima 20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel Corporation (72) Inventor Mamoru Yamada 3434 Shimada, Hikari City, Yamaguchi Prefecture Nippon Steel Works Hikari Steel Works Ltd.

Claims (1)

[Claims] 1. By weight, C: 0.001 to 0.05%, Si: 0.2 to 1.0%, Mn: 0.05 to 0.8%, P: 0.03% or less, S : 0.01% or less, Nb: 0.1 to 5.0%, N: 0.001 to 0.05% containing ferritic stainless steel thin strip with a thickness of 6 mm or less by twin roll continuous casting method In the method of casting into a slab, Ti is added in an amount sufficient to make the total amount C + N of free C and free N capable of combining with Nb in the molten steel used for casting to 0.030% or less, and The slab cooling rate should be at least 1 after the slab emerges from between the chill rolls.
By increasing the temperature up to 200 ° C. to 10,000 ° C./min or higher, and the temperature up to 800 ° C. to 1500 ° C./min or higher, and then cooling with water, it is possible to prevent the accumulation of C and N in the macrosegregation portion of Nb and at the same time to remove NbCN at high temperature. A method for casting Nb-containing ferritic stainless steel in a twin roll continuous casting method, which is characterized by preventing precipitation.