JP3058229B2 - Method for producing thin cast chromium stainless steel slab with excellent toughness - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

2. Description of the Related Art In recent years, a technique for directly casting a thin slab having a plate thickness of 10 mm or less from molten steel has been developed, and has already been practiced on an industrial scale. This new technology eliminates the hot-rolling process and is a process that directly cold-rolls thin slabs to produce cold-rolled sheet products. . Hereinafter, the process of the present invention is referred to as an STC process (Strip Cast Process).
The process of casting a slab with a thickness of 100 mm or more by continuous casting, performing hot rolling to a hot-rolled sheet having a thickness of about several mm, and manufacturing a cold-rolled sheet product from the hot-rolled sheet. Called the current hot rolling process. The present invention relates to a method for producing a thin cast slab having good toughness when producing a Cr-based stainless steel thin cast containing Nb by an STC process.

[0002]

2. Description of the Related Art Conventionally, Cr-based stainless steel has been manufactured by a so-called current hot rolling process in which a slab is cast and subjected to hot rolling. In this process, there was a problem that ridging (roping) occurred in the cold-rolled sheet product due to the texture developed during hot rolling. Therefore, an attempt has been made to produce a thin product without ridging by casting a thin slab by the STC process. For example, Japanese Patent Application Laid-Open No. Sho 62-17664
No. 9 describes "a method for producing a ferritic stainless steel sheet strip without roping". However, in the present invention, Cr containing 0.2 to 1.0 wt% of Nb is used.
No mention is made of the phenomenon of toughness degradation occurring in stainless steels of the system type. When a Cr type stainless steel containing 0.2 to 1.0 wt% of Nb is cast, the toughness of the slab deteriorates. Cannot be cold rolled.

Further, in Japanese Patent Application Laid-Open No. 64-4458, "Quenching zone of ferritic stainless steel excellent in toughness", a slab having good toughness can be manufactured by setting the columnar crystal ratio of the slab to 70% or more. However, no technical study has been conducted on the toughness of the cast slab of Cr-based stainless steel containing Nb and the temperature history of the cast slab.

[0004]

The present inventors have conducted a production test of Nb-added Cr-based stainless steel thin slabs by the STC process. Has a problem that the slab has low toughness and cannot be cold rolled. This problem occurs regardless of the structure of the slab (columnar crystal ratio, etc.).
% Only in Cr-based stainless steels containing more than 1%. On the other hand, in the current hot rolling process, there was no problem regarding the toughness of the hot rolled annealed sheet, and it was found that this problem was a problem peculiar to the STC process.

[0005]

Means for Solving the Problems The present invention has the following structure to solve the above-mentioned problems. That is, the present invention provides Cr: 16 to 25 wt%, Nb: 0.2 to 1.0 wt%,
In a method of casting a thin slab having a plate thickness of 10 mm or less from a Cr-based stainless steel containing Ti: 0.01 to 0.5 wt%,
Immediately after casting, in-machine heat treatment in a temperature range of 1250 to 900 ° C, that is, one of slow cooling or heat keeping at 20 ° C / sec or less, or heating at 20 ° C / s or less is performed for 4 seconds or more, and then slabs. Is rolled at a temperature of 650 ° C. or less. Hereinafter, the present invention will be described in detail.

[0006]

[Action] Cr: 16 to 25 wt%, Nb: 0.2 to 1.0
The toughness of a thin slab cast from a Cr-based stainless steel containing 0.1% by weight and 0.01% by weight of Ti is greatly affected by the behavior of precipitates in the steel. FIG. 1 schematically shows the effect of the size and amount of the slab precipitate on the toughness. The precipitates which deteriorate the toughness most are Fe ₃ Nb ₃ C (M ₆ C) and Fe ₂ Nb (L
aves). Since these precipitates precipitate in a large amount and coarsely in steel, the toughness extremely deteriorates. Since these two kinds of precipitates precipitate in a temperature range of 700 to 1000 ° C.,
When the slab is wound in this temperature range, the slab toughness is significantly degraded.

[0007] Next, as precipitates that deteriorate toughness, (N
b, Cr) N. (Nb, Cr) N is Nb during solidification
It precipitates very finely in the segregated part, and the toughness deteriorates because the strength of the base material is increased. Next, there are Nb (C, N) and Ti (C, N) as precipitates which deteriorate toughness. N
b (C, N) and Ti (C, N) are about 120
It precipitates during 0 ° C and is very stable.

Therefore, in order to improve the toughness of the slab, it is necessary to prevent the precipitation of these various precipitates. Therefore, in the present invention, Nb is kept in a temperature range of 1200 to 900 ° C., that is, by performing slow cooling at a cooling rate of 20 ° C./sec or less, or keeping the temperature, or heating at 20 ° C./s or less for 4 seconds or more. Diffuse the segregation of
By winding at a temperature of 700 ° C. or less, Fe ₃ Nb
_It is possible to produce a thin cast piece having good toughness without ₃ C, Fe ₂ Nb or (Nb, Cr) N. However, Nb (C,
N) and Ti (C, N) remain, and the toughness is lower than that of the Nb-free material, but at a level that is almost no problem.

The reason for setting the Cr concentration to 16 wt% or more is 16
If the content is less than wt%, the slab crosses the γ loop during cooling of the slab, so that martensite transformed from γ remains even at room temperature and deteriorates the toughness of the slab. Also, 25wt%
The reason given below is that the toughness deteriorates due to the increase in Cr, so even if the temperature history of the cast slab is controlled,
This is because the slab toughness could not be improved.

The reason why the amount of Nb is set to 0.2 wt% or more is that if the content is less than 0.2 wt%, the toughness does not deteriorate due to such precipitates. This is because if Nb exceeds 0.0 wt%, the slab toughness cannot be improved even if the temperature history of the slab after casting is controlled.

[0011] The reason why the amount of Ti is set to 0.01 wt% or more is that by adding 0.01 wt% or more of Ti, the surface properties of the slab are improved. The reason is that if more than 1.0 wt% of Ti is contained, the toughness of the slab is significantly deteriorated by large Ti-based precipitates (inclusions).

FIG. 2 shows the relationship between the slab winding temperature and the slab toughness. It turns out that the slab toughness is deteriorated by winding at a temperature exceeding 700 ° C. FIG. 3 shows the effect of in-machine heat retention conditions in a temperature range of 1250 to 900 ° C. on slab toughness. It can be seen that the slab toughness is improved if the slab is slowly cooled or kept at 20 ° C./sec or less or heated at 20 ° C./sec or less for 4 seconds or more as the heat retention in the machine. FIG. 4 shows equipment for keeping the temperature in the machine. This is a facility for controlling the temperature history of the cast slab S after casting in a transport line up to winding. In the figure,
1 is a twin-drum casting machine, 2 is an in-machine warming device, 3 is a secondary cooling device, and 4 is a coiler.

[0013]

EXAMPLE Nb based on 18% Cr steel shown in Table 1
10 tons of various Cr-based stainless steels containing 0.15 to 0.64 wt% of the components within the range of the present invention and cast into a thin slab having a thickness of 3 mm by an internal water-cooled twin-drum casting machine. After performing various holding, slow cooling, or heating in a temperature range of 1250 to 900 ° C, winding was performed in a temperature range of 650 ° C or less. As a comparative method, a thin slab is cast from a Cr-based stainless steel having the same composition (Table 1) by the same method, and after the casting, the cooling rate of 1250 to 900 ° C or the winding temperature is out of the range of the present invention. It was cooled as it was. Table 2 shows the toughness of each slab produced by the above method. As shown in this table, the thin slab produced by the method of the present invention was 2 kgfm / 20 ° C.
Although good toughness of not less than ² cm ² was exhibited, the thin slab produced by the comparative method had a toughness at 20 ° C. of ² kgfm / cm ² or less, and was so low that cold rolling was impossible.

[0014]

[Table 1]

[0015]

[Table 2]

[0016]

According to the present invention, an Nb-added Cr-based stainless steel thin slab having good toughness can be manufactured by the STC process. Therefore, the technical effect is extremely large in terms of economy.

[Brief description of the drawings]

FIG. 1 is a view showing the relationship between precipitates of a slab and slab toughness.

FIG. 2 is a diagram showing a relationship between a slab winding temperature and a slab toughness.

FIG. 3 is a diagram showing a relationship between in-machine heat retention conditions of a slab and slab toughness.

FIG. 4 is a schematic diagram showing an equipment image of in-machine heat retention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Twin drum type casting machine 2 ... In-machine heat insulation apparatus 3 ... Secondary cooling apparatus 4 ... Coiler

──────────────────────────────────────────────────続 き Continuation of front page (51) Int.Cl. ⁷ Identification code FI C22C 38/00 302 C22C 38/00 302Z 38/50 38/50 (72) Inventor Katsuhara Kishihara 3434 Shimada, Oaza, Hikari-shi, Yamaguchi Nippon Steel Corporation Hikari Works (72) Inventor Hideki Oka 3434 Shimada, Hikari-shi, Yamaguchi Prefecture Nippon Steel Corporation Hikari Works (56) References JP-A-2-258931 (JP, A JP-A-1-197046 (JP, A) JP-A-5-293595 (JP, A) JP-A-60-2628 (JP, A) JP-A-62-176649 (JP, A) JP-A 64-64 4458 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B22D 11/06 330 B21B 3/02 C21D 6/00 102 C21D 8/02 C21D 9/46 C22C 38/00 302 C22C 38/50

Claims (2)

(57) [Claims] 1. Cr: 16-25 wt%, Nb: 0.2-
In a method of casting a thin slab having a plate thickness of 10 mm or less from a Cr-based stainless steel containing 1.0 wt% and Ti: 0.01 to 0.5 wt%, the in-machine heat treatment is performed in a temperature range of 1250 to 900 ° C. immediately after casting. For 4 seconds or more, and then winding the slab at a temperature of 700 ° C. or less. 2. The method according to claim 1, wherein the in-machine heat-retaining treatment is any one of slow cooling at a cooling rate of 20 ° C./sec or less, heat keeping or heating at 20 ° C./s or less.