JPH0698460B2 - Casting method of iron-based alloy containing Cr and Ni by twin roll type continuous casting machine - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention relates to the casting of an iron-based alloy in a twin roll thin plate (strip) continuous casting machine, the chemical composition of the thin strip to be cast, and the solidification of the solidified shell. By controlling the cooling rate, the residual ferrite ratio in the cast ribbon can be made larger than the specified value to prevent defects such as cracks on the surface of the ribbon, fine cracks, and uneven unevenness. Casting technology effective to achieve, namely Cr, N
The present invention relates to a novel casting method of an iron-containing alloy containing i using a twin roll type continuous casting machine.

[Conventional technology]

Conventionally, in the case of a twin roll type strip casting machine as shown in FIG. 1, for example, the production of a strip (strip) is performed on a roll kiss portion between two cooling rolls arranged horizontally or inclined, After forming a molten metal pool, pouring the molten metal into the molten metal pool, and simultaneously cooling it with the cooling roll to generate a solidified shell, and after gradually growing the solidified shell, from the gap between the rolls. The thickened slab is drawn downward to produce a metal ribbon.

However, when an iron-based alloy containing Cr and Ni is cast by a twin roll type ribbon continuous casting machine as shown in Fig. 1, cracks, fine cracks, and uneven irregularities are formed on the surface of the cast ribbon. Such surface defects may occur. This defect occurs at a depth of 0.1 to 0.2 mm from the surface of the ribbon,
It is a serious problem in improving the surface quality of the ribbon.

As a technique for preventing the above-described defects that occur on the surface of the ribbon, for example, in JP-A 1-299745, the residual ferrite ratio in the cast ribbon has a predetermined value, so that a predetermined component in the material Is proposed to increase the nickel equivalent by adding.

However, this conventional technique has not yet completely prevented the defects occurring on the surface of the cast ribbon. That is, in this conventional technique, since only the composition of the cast metal was focused and the effect of the cooling rate at the time of solidification was not taken into consideration, the residual ferrite amount could not be sufficiently increased, and therefore surface defects were caused. It was not possible to prevent the occurrence of.

[Problems to be Solved by the Invention]

As described above, the conventional technique has a difficulty in solving the defects occurring on the surface of the ribbon cast by the twin roll type ribbon continuous casting machine only by adjusting the components of the raw material. That is, according to the study by the present inventors, in the twin roll type continuous casting machine, it was revealed that the residual ferrite ratio of the ribbon after casting strongly influences the cooling rate of the roll surface on the molten steel, and therefore, Simply adjusting the chemical composition of the material as in the past is not sufficient, and it has not been possible to prevent defects that occur on the surface of the ribbon. In addition to that, conventionally, an appropriate residual ferrite ratio itself for preventing defects generated on the surface of the ribbon has not been clarified.

Therefore, an object of the present invention is to overcome such problems of the conventional techniques and to establish a technique for continuously casting a thin strip of Cr, Ni-containing steel having no surface defects.

[Means for Solving the Problems]

As a result of conducting research aiming at the achievement of the above-mentioned object, the inventors of the present invention have convinced that the casting method as described below is a technology to which the invention can be advantageously applied, and have conceived the present invention.

That is, the present invention supplies molten iron-based alloy to the pool between a pair of cooling rolls that are horizontally or inclined to form a solidified shell on the peripheral surface of the roll and gradually thickens the solidified shell. In the method for producing a ribbon of an iron-based alloy by pulling it downward from the gap between the rolls, it is controlled by the following two means so that the residual ferrite rate after casting becomes 2.0% or more. To do.

When casting a ribbon with a twin roll type continuous casting machine, the composition of the molten steel used for casting is C0.20 wt%, Si5.00 wt%, Mn
3.00wt%, Ni50wt%, Cr: 1-50wt% and N0.3w
containing t% and the balance mainly consisting of Fe,
And Creq = (% Cr) + (% Mn) +1.5 (% Si) Nieq = (% Ni) +30 (% C) +30 (% N-0.06) +0.5
The Cr and Ni equivalents calculated by (% Mn) should be set to CrNi equivalent ratio: Creq / Nieq> 2.05. The cooling rate on the roll surface of the ribbon in the region from the surface of the ribbon to 0.2mm Is a cooling rate of 50 to 450 ° C./sec.

In addition, the present invention also relates to an alloy having Nb added as the alloy component, and an alloy having a component composition in which one or more of Mo, Cu and Ti are added together with the Nb, or alone. When the residual ferrite ratio is 20% or more, it is suitable for producing a ribbon without surface defects.

[Action]

As described above, in the casting of strips by the twin roll type strip continuous casting machine, the residual ferrite rate after solidification is the composition of the molten steel and the cooling rate (° C / ° C) of the solidified shell when the molten steel is solidified on the rolls. sec) has been found to be dominated by. Therefore, in the casting of the ribbon by the twin roll type ribbon continuous casting machine, first, regarding the cooling rate, the cooling rate up to the depth of the surface layer of the ribbon of 0 to 0.2 mm is set to 50 to 450 ° C./sec, and the Cr, Ni Equivalence ratio: Cre
It was decided that q / Nieq> 2.05. Under these conditions, the residual ferrite ratio of the cast slab (thin band) after solidification was 2.0% or more, and it was found that cracking on the slab surface could be prevented.

The following describes this in detail.

Regarding the influence of composition; Fig. 2 shows the CrNi equivalent ratio (Creq / Nieq) calculated using the above formula for Cr equivalent and Ni equivalent, and the residual measured by the ferrite meter using the cast piece after solidification. It shows the relationship with the ferrite ratio (%). The X mark in the figure is the case where iron piece cracking and uneven unevenness were observed in the thin band after solidification, while the ○ mark is an example of the case where there is no cast piece cracking and the surface is smooth and has no unevenness. is there.

As is clear from the results shown in this figure, the composition of steel is such that when the CrNi equivalent ratio: Creq / Nieq is 2.05 or more, and the amount of residual ferrite is 2.0% or more in the solidification structure of the slab. In this case, the slab has good surface quality without cracks or uneven unevenness on the surface.

The reasons for limiting the composition of the iron-based alloy of the present invention are as follows.

C: C is added to improve the strength, but when it exceeds 0.20 wt%, its effect saturates, so the content is made 0.20 wt% or less.

Si: Si is added as a deoxidizer or for improving oxidation resistance, but if it exceeds 5.00 wt%, it becomes brittle, so 5.00 wt%
Below.

Mn: Mn is added as a deoxidizing agent, but if it exceeds 3.00 wt%, its effect is saturated, so 3.00 wt% is added as the upper limit.

Ni: Ni is a strong austenite forming element and is added to balance with Cr and to improve corrosion resistance. Excessive addition is expensive, so 50 wt% or less is used.

Cr: Cr is added for corrosion resistance, oxidation resistance and high humidity strength, but if it is less than 1 wt% it does not have the effect, and if it exceeds 50 wt% the effect is saturated, so it is made 1 to 50 wt%.

Nb: Nb can be added to improve corrosion resistance and formability. If it is less than 0.01 wt%, the effect is not obtained, and if it exceeds 3.00 wt%, the effect is saturated, so 0.01 to 3.00 wt% is set.

N: N can be added to improve strength. 0.3wt
%, The content exceeds 0.3% and causes blowholes, etc., so the content should be 0.3 wt% or less.

Mo: Mo is an element usually added for the purpose of improving properties such as corrosion resistance and high humidity strength, and Cu and Ti are corrosion resistance and formability. Since this does not affect CrNi equivalent as much as Cr and Ni, the coefficient is treated as 0 in the calculation formula.

Further, in addition to the above, it is a matter of course that the present invention includes the case where a trace amount component is added for the purpose of improving other characteristics.

Regarding the influence of cooling rate; In a twin roll type continuous strip casting machine, when molten steel is supplied to the roll pool and solidified on the circumferential surface of the roll cylinder, the cooling rate of the solidified shell produced is the rotational speed of the roll, It can be changed by the flow rate of the cooling water supplied into the roll, the flow rate of the cooling water at the time of secondary cooling immediately after casting, and the like. If the cooling rate is less than 50 ° C / sec, there is a risk of swelling of the slab due to unsolidified portions in the slab, and bleeding of molten steel flowing out of the solidified shell. Also, 450 ℃ / sec
If it exceeds, it is difficult to set the residual ferrite amount to 2% or more. Therefore, as an example, the result when the rotation speed of the roll is changed is shown in FIG. Similar to FIG. 2, this FIG. 3 shows the relationship between the CrNi equivalent ratio (Creq / Nieq) and the residual ferrite ratio after solidification. In the figure, when the roll rotation speed is high, it is shown by Δ, when it is medium speed, it is shown by ○, and when it is low, it is shown by □. As can be seen from this figure, when the roll rotation speed is high, the residual ferrite ratio of the slab is higher than when it is low, even if the composition is the same and the CrNi equivalent ratio is the same.

Therefore, the occurrence of cracks on the surface of the cast piece can be prevented by ensuring a residual ferrite content of 2.0% or more in the cast piece after solidification. This is also presumed to be because the ferrite inside the cast piece is more likely to form a solid solution with an impurity component such as PS than austenite.

As described above, in the twin roll thin plate type continuous casting machine, if the residual ferrite content of the slab is controlled to 2.0% or more by controlling the cooling rate of the solidified shell together with the composition of molten steel, The occurrence of cracks can be reliably prevented.

〔Example〕

This example illustrates an example in which ribbons of various iron-based alloys are cast by a twin roll ribbon continuous casting machine. First
The table is the composition of the cast ribbon and is calculated from this composition
The CrNi equivalent ratio and the average cooling rate (° C / sec) of the solidified shell calculated from the measured value of the secondary dendrite arm spacing (μm) in the depth range from the surface of the ribbon to 0.2 mm are shown. . The average cooling rate (℃ / sec) of the solidified shell is
For example, it can be controlled by the rotation speed of the roll. Table 1 also shows the relationship between the average cooling rate of the solidified shell and the rotation rate of the roll.

As can be seen from the results of this example, when the composition, particularly the CrNi equivalent ratio, does not satisfy the conditions of the present invention (No. 15, 16), and the cooling rate does not satisfy the conditions of the present invention (No. 14)
In all, the measured ferrite ratio was less than 2.0, and slab cracking occurred, but it was within the conditions of the present invention (No. 1 to
13), such a phenomenon was not observed. In particular, the products of Examples of the present invention were in a state in which the surface could be cold-rolled as it was.

〔The invention's effect〕

According to the present invention as described above, the surface of the ribbon cast by the twin roll type ribbon continuous casting machine, it is possible to reliably prevent the occurrence of cracks and fine cracks, or uneven unevenness,
It is possible to produce a ribbon of Cr- and Ni-containing steel having excellent surface properties.

[Brief description of drawings]

FIG. 1 is a schematic diagram of a twin roll type ribbon continuous casting machine, FIG. 2 is a graph showing the relationship between the CrNi equivalent ratio and the residual ferrite ratio which affect the surface properties of the cast piece, and FIG. 6 is a graph showing the relationship between the CrNi equivalent ratio and the residual ferrite ratio that affect the cooling rate. 1 ... Nozzle, 2 ... Hot water pool, 3 ... Roll, 4 ... Slab

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kagehiro Amano 4-2 Kojima-cho, Kawasaki-ku, Kawasaki-shi, Kanagawa Nihon Metallurgical Industry Co., Ltd. Technical Research Institute (72) Intaker Takeya Kawasaki-ku, Kawasaki-shi, Kanagawa Kojimacho 4-2 Nihon Metallurgical Industry Co., Ltd. Technical Research Center

Claims (3)

[Claims] 1. A molten iron-base alloy is supplied to a pool between a pair of horizontally or inclined cooling rolls to form a solidified shell on the circumferential surface of the roll, and the solidified shell is gradually thickened. And then pulling downward from the gap between the rolls to produce a ribbon of an iron-based alloy, wherein the iron-based alloy is C0.20 wt%, Si5.00 wt%, Mn
3.00wt%, Ni50wt%, Cr: 1-50wt% and N0.3w
containing t% and the balance mainly consisting of Fe,
In addition, the following CrNi equivalent ratio shall satisfy the condition of Cr eq / Ni eq> 2.05, and the cooling rate of the ribbon shall be 50-450 ℃ / sec for the area from the ribbon surface to 0.2mm. The average residual ferrite ratio in the plate thickness direction of the cast ribbon is controlled to 2.0
% Or more, a casting method for a Cr-Ni-containing iron-based alloy by a twin-roll thin strip continuous casting machine. However, in order to obtain the CrNi equivalent ratio: Cr eq / Ni eq, Cr equivalent: Cr eq and Ni equivalent: Ni eq, Cr eq = (% Cr) + (% Mn) + 1.5 (% Si) Ni eq = (% Ni) +30 (% C) +30 (% N-0.06) +0.5
(% Mn) 2. A molten iron-base alloy is supplied to a pool between a pair of cooling rolls arranged horizontally or inclined to form a solidified shell on the circumferential surface of the roll, and the solidified shell is gradually thickened. And then pulling downward from the gap between the rolls to produce a ribbon of an iron-based alloy, wherein the iron-based alloy is C0.20 wt%, Si5.00 wt%, Mn
3.00wt%, Nb: 0.01〜3.00wt%, Ni50wt%, Cr: 1〜5
It has a composition containing 0 wt% and N 0.3 wt% and the balance mainly Fe, and the following CrNi equivalent ratio is Cr eq / Ni e.
Using a material that satisfies the condition of q> 2.05, control the cooling rate of the thin ribbon so that the cooling rate of 50 to 450 ° C / sec is achieved for the area from the surface of the thin ribbon to 0.2 mm. The average residual ferrite ratio in the thickness direction of the strip is 2.0
% Or more, a casting method for a Cr-Ni-containing iron-based alloy by a twin-roll thin strip continuous casting machine. However, in order to obtain the CrNi equivalent ratio: Cr eq / Ni eq, Cr equivalent: Cr eq and Ni equivalent: Ni eq are Cr ed = (% Cr) + (% Mn) + 1.5 (% Si) + 0.5 (% N
b) Ni eq = (% Ni) + 30 (% C) + 30 (% N-0.06) + 0.5
(% Mn) 3. The iron-based alloy according to claim 1 or 2, further containing one or more of Mo: 0.01 to 5 wt%, Cu: 0.01 to 5 wt% and Ti: 0.01 to 3 wt%. A method for casting a Cr-Ni-containing iron-based alloy according to claim 1 or 2 using a twin-roll thin strip continuous casting machine, wherein an iron-based alloy having the composition described below is used.