US20150174647A1

US20150174647A1 - Method of Manufacturing Ti-Containing Austenitic Stainless Steel Sheet by Twin Roll Strip Caster

Info

Publication number: US20150174647A1
Application number: US14/570,260
Authority: US
Inventors: Sung-Jin Park; Byoung-Jun Song; Dae Sung Lee
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2013-12-20
Filing date: 2014-12-15
Publication date: 2015-06-25
Also published as: KR20150072755A; CN104726788A

Abstract

There is provided a method of manufacturing a titanium (Ti)-containing austenitic stainless steel sheet having a high degree of surface quality by using a twin roll strip caster. The method includes controlling a composition of molten steel such that a TiN precipitation temperature may be higher than a temperature of the molten steel in a tundish (T/D) by at least 50° C. (ΔT≧50° C.), the TiN precipitation temperature being defined by the following formula 2:

Log(N%)=−19,755/(T+273)+7.78+0.07[% Ti]−log [% Ti]+

0.045[% Cr]

T(° C.)=−19,755/log(N%)−7.78−0.07(%Ti)+log(%ti)−0.045(%Cr)−273 [Formula 2]

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of Korean Patent Application No. 10-2013-0160262 filed on Dec. 20, 2013, with the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference.

BACKGROUND

The present disclosure relates to a method of manufacturing a titanium (Ti)-containing austenitic stainless steel sheet using a twin roll strip caster, and more particularly, to a method of manufacturing a Ti-containing austenitic stainless steel sheet having a high degree of surface quality by directly casting molten steel into a strip in a twin roll strip caster instead of using a conventional continuous casting process, so as to prevent a nozzle clogging and the formation of large surface defects (caused by TiN and oxide inclusions).
Titanium (Ti)-containing austenitic stainless steel has a high Ti content of about 0.2% to about 0.5%, and Ti nitrides or oxides are very likely to be formed if Ti-containing austenitic stainless steel is cast in a general continuous casting process. Mixtures of such Ti nitrides and oxides are known to cause clogging of immersion nozzles of ladles or tundishes.
In addition, large clusters formed of Ti nitrides cause many surface defects, and thus most Ti-containing steel sheets are subjected to a post-process such as a hot-rolling process after grinding upper/lower surfaces thereof. If such defects remain on hot-rolled coils, final products may only be produced after performing a coiling grinding process or a cold-rolling process on the hot-rolled coils. Due to these reasons, additional process costs and burdens are increased, and process yields are decreased, to lower profits. Thus, there is a need for a method of solving such problems.
In general, titanium (Ti) is included in highly corrosion-resistant Ti-containing stainless steel in an amount of 0.2% to 0.5% so as to fix carbon in the form of TiC and thus to prevent chromium (Cr), an important element guaranteeing the corrosion resistance of stainless steel, from precipitating in the form of Cr₂₃C₆. Like general stainless steel, Ti-containing stainless steel may be produced through processes using an electric furnace, a refining furnace, a ladle treatment unit, and a continuous caster. That is, in the related art, Ti-containing stainless steel is formed as a slab by a continuous casting method, and the slab is hot-rolled to form a strip having a thickness of 2 mm to 6 mm.
FIG. 1 is a schematic view illustrating a continuous casting process of the related art. As shown in FIG. 1, a continuous casting process may be performed using a ladle 2 containing molten steel 1 produced through a steel making process, a tundish 4 disposed between the ladle 2 and a mold 8 as a buffer, the mold 8 for producing slabs, and a secondary cooling table 9. Molten steel 1 is formed into a slab while passing though the above-mentioned devices, and then the slab is reheated and hot-rolled into a strip in a region denoted by a box in FIG. 1. Then, the strip is water-cooled and coiled.
For example, scraps and a ferro alloy are melted into molten steel in an electric furnace, and the molten steel is refined in a refining furnace to remove carbon, phosphorous, and sulfur therefrom. Thereafter, the temperature and minor components of the molten metal are adjusted in a ladle treatment unit according to conditions for a casting process. After the temperature and minor component adjustment, the molten steel is continuously cast and formed as a hot-rolled coil product.
However, the addition of titanium (Ti) results in the formation of Ti oxides (e.g. TiO₂) and Ti nitrides (e.g. TiN) because titanium (Ti) has a high affinity for oxygen and nitrogen, and since Ti oxides and Ti nitrides have very high melting points, Ti oxides and Ti nitrides may clog nozzles during a casting process and degrade the surface quality of products. Particularly, if nozzles are clogged during a casting process, molten steel may not be supplied to a tundish or a caster, and thus it may be impossible to perform the casting process. That is, due to such a situation, the production of products may be interrupted.
Materials causing the clogging of nozzles may be included in molten steel as large inclusions. In this case, the surface quality of products may be very poor. As described above, if nozzles are clogged during a casting process, product quality as well as productivity may be largely affected.
In the related art, the following methods have been proposed to prevent the clogging of nozzles.
First, a method of decreasing the amount of titanium (Ti) has been proposed. That is, the amount of titanium (Ti) is decreased to reduce the formation of Ti oxides and Ti nitrides. In this case, however, the amount of carbon (C) also has to be reduced. The basic design concept of Ti-containing stainless steel is to increase the content of titanium (Ti) to a certain value or greater according to the content of carbon (C) so as to obtain a desired degree of corrosion resistance. That is, the ratio of Ti/C, termed a Ti stabilization ratio, is set to be about 5 to about 10, generally at least 5. Therefore, if the addition of titanium (Ti) is reduced, the amount of carbon also has to be reduced to satisfy the Ti stabilization ratio. In this case, a large amount of oxygen may have to be supplied to a refining furnace to decrease the amount of carbon. Due to this, a larger amount of oxygen may be dissolved in molten steel, and thus the formation of Ti oxides may be unexpectedly increased. In addition, since the period of time necessary for a refining process is increased, the temperature of molten steel may increase, and thus a considerable amount of coolant may be necessary for cooling the molten steel after the molten steel is discharged from the refining furnace. In this case, the molten steel may easily make contact with ambient air while being cooled by the coolant and thus may be re-oxidized to cause the formation of large amounts of inclusions.
Secondly, in another method of reducing the amount of titanium (Ti), a large amount of aluminum (Al), having a relatively higher affinity for oxygen than titanium (Ti), is added before titanium (Ti) is added so as to remove dissolved oxygen. This method effectively prevents the formation of Ti oxides. However, a large amount of Al₂O₃that affects the surface quality of products more negatively than Ti oxides is formed as a result of oxygen removal using aluminum (Al), and thus a process of adding calcium (Ca) to molten steel is required to convert Al₂O₃into CaO—Al₂O₃having a less negative effect. However, it is difficult to control the content of calcium (Ca) due to a high degree of volatility of calcium (Ca). In addition, if the content of calcium (Ca) is excessively low or high, inclusions having a desired shape may not be formed, and thus surface defects may increase. Particularly, if the content of calcium (Ca) is excessively high, calcium (Ca) included in molten steel may react with Al₂O₃included in a refractory material of a stopper that is used to adjust the supply of molten steel from a tundish to a caster, and as a result, a compound having a low melting point, CaO—Al₂O₃, is formed to increase erosion of the stopper. In this case, the supply of molten steel may become unstable, and thus the quality of products may be lowered.

SUMMARY OF THE INVENTION

An aspect of the present disclosure may provide a method of manufacturing a titanium (Ti)-containing austenitic stainless steel sheet by using a twin roll strip caster so as to suppress the formation of TiN and effectively prevent nozzle clogging occurring in a general continuous casting process, thereby ensuring casting stability and a high degree of product surface quality.
However, aspects of the present disclosure are not limited thereto. Additional aspects will be set forth in part in the description which follows, and will be apparent from the description to those of ordinary skill in the related art.
According to an aspect of the present disclosure, there may be provided a method of manufacturing a titanium (Ti)-containing austenitic stainless steel sheet having a high degree of surface quality by using a twin roll strip casting process, the method including controlling a composition of molten steel such that a TiN precipitation temperature may be higher than a temperature of the molten steel in a tundish (T/D) by at least 50° C. (ΔT≧50° C.), the TiN precipitation temperature being defined by the following formula 2:
$\begin{matrix} Log (N %) = - 19, 755 / (T + 273) + 7.78 + 0.07 [% Ti] - \log [% Ti] + 0.045 [% Cr] T (° C .) = \frac{- 19, 755}{\begin{matrix} \log (N %) - 7.78 - 0.07 (% Ti) + \\ \log (% Ti) - 0.045 (% Cr) \end{matrix}} - 273 & [Formula 2] \end{matrix}$
The molten steel may include, by weight %, carbon (C): 0.025% to 0.055%, silicon (Si): 0.25% to 0.55%, manganese (Mn): 1.5% to 1.8%, chromium (Cr): 17.1% to 17.7%, nickel (Ni): 9.25% to 9.65%, titanium (Ti): 0.2% to 0.5%, nitrogen (N): 0.025% or less, and the balance of iron (Fe) and inevitable impurities.
The molten steel may have a Ti/C ratio of 8 or greater.

BRIEF DESCRIPTION OF DRAWINGS

The above and other aspects, features and other advantages of the present disclosure will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic view illustrating a continuous casting process of the related art;

FIG. 2 is a schematic view illustrating a strip caster according to an exemplary embodiment of the present disclosure;

FIG. 3 is an electron microscope image of a TiN+TiO₂cluster of a complex oxynitride;

FIG. 4 illustrates an image of a surface of a cast material and an image of an inside of a tundish nozzle in an comparative example; and

FIG. 5 illustrates an image of a surface of a cast material and an image of an inside of a tundish nozzle in an inventive example.

DETAILED DESCRIPTION OF THE INVENTION

Exemplary embodiments of the present disclosure will now be described in detail with reference to the accompanying drawings.
The disclosure may, however, be exemplified in many different forms and should not be construed as being limited to the specific embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art.
In the drawings, the shapes and dimensions of elements may be exaggerated for clarity, and the same reference numerals will be used throughout to designate the same or like elements.
FIG. 2 is a schematic view illustrating a twin roll strip caster 100 according to an exemplary embodiment of the present disclosure.
Referring to FIG. 2, the twin roll strip caster 100 of the exemplary embodiment largely includes casting rolls 110, a ladle 120, a tundish 130, an immersion nozzle 140, a meniscus shield 150, brush rolls 160, and edge dams 170. Reference numeral 180 denotes a strip. In a twin roll strip casting method using such a twin roll strip caster, molten steel is directly cast as a strip having a thickness of 10 mm or less by supplying the molten steel through an injection nozzle into a region between internally-water-cooled twin rolls that are rapidly rotated in opposing directions.
An exemplary embodiment of the present disclosure provides a method of manufacturing a titanium (Ti)-containing austenitic stainless steel sheet using a twin roll strip caster. In the method, the composition of molten steel is controlled to maintain a TiN precipitation temperature defined by the following formula 2 at a level higher than the temperature of the molten steel in a tundish (T/D) by at least 50° C. (ΔT).
In the exemplary embodiment of the present disclosure, a Ti-containing austenitic stainless steel sheet manufactured by the method using a twin roll strip caster may include, by weight %, carbon (C): 0.025% to 0.055%, silicon (Si): 0.25% to 0.55%, manganese (Mn): 1.5% to 1.8%, chromium (Cr): 17.1% to 17.7%, nickel (Ni): 9.25% to 9.65%, titanium (Ti): 0.2% to 0.5%, nitrogen (N): 0.025% or less, and the balance of iron (Fe) and inevitable impurities. This composition of the Ti-containing austenitic stainless steel sheet is a standard composition well known in the related art.
When molten steel having the above-mentioned composition is cast as a strip in a twin roll strip casting process, various casting defects may be formed due to the influence of titanium (Ti).
Generally, materials stuck in a nozzle of a tundish and causing clogging are TiN and TiO₂. TiN functions as nuclei or seeds, and TiO₂gathers around the nuclei or seeds to form a complex inclusion in the form of clusters. For example, FIG. 3 illustrates an electron microscope image of a TiN+TiO₂cluster of a complex oxynitride. Therefore, it is necessary to minimize TiN precipitation (nitride precipitation) and TiO₂formation (oxide formation).
Titanium (Ti) included in molten steel reacts with nitrogen to form a TiN inclusion. Since TiN has a high melting point of about 2000° C., TiN particles may agglomerate together and grow in molten steel. In this case, the surface quality of products is markedly degraded even though TiN has less effect on nozzle clogging than TiO₂. Therefore, it may be important to prevent the formation of TiN in molten steel, especially, before the molten steel is supplied to a tundish.
Therefore, thermodynamic research has been conducted into TiN formation reaction between titanium (Ti) and nitrogen (N) expressed by the following formula 1.
Ti+N=TiN [Formula 1]
Formula 1 is disclosed in academic publications. After analyzing many academic materials, the following formula 2, considered to accurately represent the formation of TiN in actual Ti-containing austenitic stainless steel, is used in the present disclosure.
$\begin{matrix} Log (N %) = - 19, 755 / (T + 273) + 7.78 + 0.07 [% Ti] - \log [% Ti] + 0.045 [% Cr] T (° C .) = \frac{- 19, 755}{\begin{matrix} \log (N %) - 7.78 - 0.07 (% Ti) + \\ \log (% Ti) - 0.045 (% Cr) \end{matrix}} - 273 & [Formula 2] \end{matrix}$
Formula 2 above expresses a TiN precipitation temperature. According to Formula 2, the TiN precipitation temperature is determined by the contents of titanium (Ti), nitrogen (N), and chromium (Cr), and if the contents of titanium (Ti) and nitrogen (N) reduce or the content of chromium (Cr) increases, the TiN precipitation temperature is reduced. That is, in a caster in which solidification proceeds, the temperature of molten steel in a tundish (T/D) has to be maintained to be higher than the TiN precipitation temperature so as to minimize TiN precipitation before the molten steel solidifies. Therefore, to minimize the formation of oxynitrides (TiN and TiO₂), it may be necessary to minimize the addition of titanium (Ti) on the condition that the content of nitrogen (N) in a refining furnace is minimized by removing nitrogen (N) and preventing the introduction of nitrogen (N).
Therefore, according to the exemplary embodiment of the present disclosure, the composition of molten steel is controlled to maintain the TiN precipitation temperature defined by formula 2 at a level higher than the temperature of the molten steel in a tundish (T/D) by at least 50° C. (ΔT).
In a general continuous casting process, it is difficult to perform high-temperature casting due to a break-out phenomenon (in which non-solidified molten steel breaks out of the inside of a slab due to rupture of the slab). In a strip casting process, however, high-temperature casting (1550° C. or higher) is possible. Thus, a difference (ΔT) between a TiN precipitation temperature and a high-temperature casting temperature may be maintained to be 50° C. or greater, and TiN precipitation may be suppressed. As a result, the formation of a complex oxynitride having TiN+TiO₂clusters formed from TiN seeds may be prevented, and thus nozzle clogging and surface defects may be minimized.
In addition, according to the present disclosure, the corrosion resistance of steel may be guaranteed by maintaining a Ti stabilization ratio (a Ti/C ratio) at a level of 8 or greater. In this case, if the addition of titanium (Ti) is reduced, the amount of carbon (C) also has to be reduced so as to satisfy the Ti stabilization ratio. For this, a large amount of oxygen may be supplied to a refining furnace to decrease the amount of carbon. As a result, a larger amount of oxygen may be dissolved in molten steel, and thus the formation of Ti oxides may be unexpectedly increased. In addition, since the period of time necessary for a refining process is increased, the temperature of molten steel may increase, and thus a considerable amount of coolant may be necessary for cooling the molten steel after the molten steel is discharged from the refining furnace. In this case, the molten steel may make contact with ambient air while being cooled by the coolant and thus may be re-oxidized to cause the formation of large amounts of inclusions.
Although a larger amount of oxygen is dissolved in molten steel and the formation of TiO₂oxide is increased because of a larger amount of oxygen supplied to the refining furnace to remove carbon, since solidification occurs rapidly owing to characteristics of the strip casting process, the size of a TiO₂inclusion may be small or fine, and thus the influence of TiO₂may be small. Large clusters of TiO₂formed from TiN seeds are the representative form of oxide that easily sticks to a nozzle and causes surface defects.
Instead, the content of carbon (C) may be reduced to 0.3% or lower because a large amount of carbon (C) is removed in the refining furnace, and thus the addition of titanium (Ti) may be reduced to satisfy the Ti stabilization ratio. Therefore, the precipitation of TiN and the formation of TiO₂may be fundamentally reduced.
Hereinafter, the exemplary embodiments of the present disclosure will be described more specifically through examples.

Examples

TABLE 1

	Comparative Samples
	(general continuous	Inventive Samples
	casting)	(strip casting)

		1	2	3	1	2	3	4	5

Molten steel	C	0.025	0.03	0.027	0.035	0.031	0.03	0.029	0.028
composition	N	0.0147	0.015	0.013	0.011	0.015	0.012	0.01	0.0091
(wt %)	Ti	0.157	0.24	0.22	0.20	0.24	0.26	0.25	0.26
	Ti/C	6.28	8.0	8.14	5.71	7.74	8.66	8.62	8.21

TiN precipitation	1487	1516	1501	1485	1519	1509	1493	1490
temperature (° C.)
T/D molten steel	1502	1510	1511	1545	1571	1559	1558	1562
temperature (° C.)

Casting	Nozzle	20	10	5	1	2	2	1	1
results	clogging
	(mm)
	*Grinding	10	8	8	—	—	—	—	—

*Number of times of surface grinding (sum of counts for upper and lower surfaces)

Ti-containing austenitic stainless steel strips were manufactured using molten steel having compositions as shown in Table 1 above. In Table 1, Comparative Samples 1 to 3 are strips manufactured through a general continuous casting process, and Inventive Samples 1 to 5 are strips manufactured using a twin roll strip caster. The TiN precipitation temperature values shown in Table 1 are values calculated using the above-described formula 2.
As shown in Table 1, in a general continuous casting process, if the temperature of molten steel is increased due to decarbonization reaction heat as the period of time of refining increases, since high-temperature casting is impossible due to a break-out phenomenon (strip rupture), a considerable amount of coolant is supplied to decrease the temperature of the molten steel after the molten steel is discharged from a furnace. In this case, the molten steel may easily make contact with ambient air while being cooled by the coolant, and thus may be re-oxidized to cause the formation of large amounts of inclusions. In addition, since casting is performed at a temperature around the TiN precipitation temperature (1490° C. to 1515° C.) due to the decreased temperature of the molten steel, a large amount of TiN may precipitate. Therefore, the degree of nozzle clogging in Comparative Samples 1 to 3 was high at 5 mm or greater.
In addition, since the strips produced by the general continuous casting process had surface defects due to complex inclusions formed of TiN+TiO₂, grinding was performed four or more times on each of the upper and lower surfaces of the strips. FIG. 4 illustrates an image of a surface of a cast material and an image of an inside of a tundish nozzle in a comparative example.
On the contrary, high-temperature casting (1550° C. or higher) was possible by a twin roll strip casting method, and thus the difference between the TiN precipitation temperature and the temperature of casting could be maintained to be 50° C. or greater (ΔT≧50° C.) as in Inventive Samples 1 to 5. Therefore, in Inventive Samples 1 to 5, TiN precipitation could be suppressed to prevent the formation of a complex oxynitride having TiN+TiO₂clusters formed from TiN seeds, and thus nozzle clogging and surface defects could be minimized.
That is, it can be understood that the method of the present disclosure suppresses TiN precipitation to maintain the degree of tundish nozzle clogging at a level of 1 mm or lower and thus allows a casting process to be normally completed. In addition, the degree of surface quality of Ti-containing austenitic steel strips manufactured by the strip casting method of the present disclosure was high such that surface grinding was not performed.
Referring to Table 1, Inventive Samples 3 to 5 having a Ti/C ratio of 8 or greater had a higher degree of corrosion resistance than Inventive Samples 1 and 2 having a relatively low Ti/C ratio.
It is necessary to confirm the purpose of Ti/C control based on the background of development of Ti-containing stainless steel. Ti-containing stainless steel was developed by adding titanium (Ti) to STS304 steel as a carbon stabilizing element so as to decrease grain boundary sensitivity for use in a grain boundary sensitivity range (450° C. to 850° C.) That is, in general stainless steel, chromium (Cr) and carbon (C) combine into Cr₂₃C₆carbide, and a Cr-depleted zone is formed around the Cr₂₃C₆carbide. Therefore, the Cr-depleted zone is easily corroded due to a relatively low chromium (Cr) content therein. Furthermore, in high-temperature applications (500° C. to 800° C.) such as boiler heat exchangers or high-temperature pipes, corrosion occurs more rapidly due to high reactivity. Therefore, titanium (Ti) is generally added to stainless steel for high-temperature applications so as to cause reaction between carbon (C) and titanium (Ti) rather than reaction between carbon (C) and chromium (Cr) and thus to fix chromium (Cr) which is an element for improving corrosion resistance. In general, a Ti/C ratio of 5 to 6 guarantees stainless steel having stable corrosion resistance, and a higher Ti/C ratio guarantees stainless steel having more stable corrosion resistance.
However, in the general continuous casting method of the related art, if the Ti/C ratio is increased, Ti oxynitrides are excessively formed because a large amount of titanium (Ti) is added, and thus surface defects may be excessively formed. Thus it is difficult to increase the Ti/C ratio.
However, in the strip casting process, although the addition of titanium (Ti) is increased to obtain a Ti/C ratio of 8 or greater, the formation of Ti oxides and TiN precipitation may be controlled and minimized to prevent surface defects. That is, the Ti/C ratio may be effectively maintained to be 8 or greater.
As set forth above, according to the exemplary embodiments of the present disclosure, nozzle clogging occurring in a general continuous casting process may be effectively prevented, and the formation of TiN may be suppressed. Therefore, casting stability may be guaranteed, and a Ti-containing austenitic stainless steel sheet having a high degree of surface quality may be effectively manufactured using a twin roll strip caster.
While exemplary embodiments have been shown and described above, it will be apparent to those skilled in the art that modifications and variations could be made without departing from the scope of the present invention as defined by the appended claims.

Claims

What is claimed is:

1. A method of manufacturing a titanium (Ti)-containing austenitic stainless steel sheet having a high degree of surface quality by using a twin roll strip casting process, the method comprising controlling a composition of molten steel such that a TiN precipitation temperature is higher than a temperature of the molten steel in a tundish (T/D) by at least 50° C. (ΔT≧50° C.), the TiN precipitation temperature being defined by the following formula 2:

\begin{matrix} Log (N %) = - 19, 755 / (T + 273) + 7.78 + 0.07 [% Ti] - \log [% Ti] + 0.045 [% Cr] T (° C .) = \frac{- 19, 755}{\begin{matrix} \log (N %) - 7.78 - 0.07 (% Ti) + \\ \log (% Ti) - 0.045 (% Cr) \end{matrix}} - 273 & [Formula 2] \end{matrix}

2. The method of claim 1, wherein the molten steel comprises, by weight %, carbon (C): 0.025% to 0.055%, silicon (Si): 0.25% to 0.55%, manganese (Mn): 1.5% to 1.8%, chromium (Cr): 17.1% to 17.7%, nickel (Ni): 9.25% to 9.65%, titanium (Ti): 0.2% to 0.5%, nitrogen (N): 0.025% or less, and the balance of iron (Fe) and inevitable impurities.

3. The method of claim 1, wherein the molten steel has a Ti/C ratio of 8 or greater.