KR100286643B1 - Method for manufacturing dual phase stainless steel slab - Google Patents

Abstract

PURPOSE: A method for manufacturing a dual phase stainless steel slab is provided to prevent surface defects on the surface of slab by minimizing the precipitation of intermetallic compounds. CONSTITUTION: The manufacturing method of a dual phase stainless steel slab includes: (i) continuous casting a molten steel into a steel slab, in which the molten steel comprises C 0.03wt.% or less, Si 1.0wt.% or less, Mn 2.0wt.% or less, P 0.04wt.% or less, S 0.004wt.% or less, Cu 2.0wt.% or less, Ni 5.0-8.0wt.%, Cr 22-23wt.%, Mn 1.0-2.0wt.%, W 2.0-5.0wt.%, N 0.13-0.27wt.% and also Creq/Nieq is 2.2-3.0 {Nieq=%Ni+30x%C+0.5x%Mn+0.33x%Cu+30x(%N-0.045), Creq=%Cr+%Mo+1.5 x%Si+0.73x%W} and W/Mo is 2.6-3.4; (ii) cooling the steel slab from the temperature range of 950-800deg.C to the temperature range of 650-700deg.C at a cooling rate higher than 3deg.C/min. Further, the molten steel additionally comprises Ca 0.03wt.% or less, Ce 0.1wt.% or less, B 0.005wt.% or less and Ti 0.5wt.% or less. Also, cooling rate is controlled to be 3-60deg.C/min in the temperature range of 700-950deg.C.

Description

Slab manufacturing method of 2-phase stainless steel

The present invention relates to a method for producing a two-phase stainless steel slab made of a ferrite phase and austenite phase used as a material for seawater facilities.

Generally, two-phase stainless steel (hereinafter referred to as 'two-phase stainless steel') in which ferrite and austenite phases are mixed has excellent corrosion resistance and stress corrosion cracking resistance, so oil drilling pipes, desulfurization facilities for power plants, and paper mills are used. It is widely used in equipments requiring significant corrosion resistance under corrosive environments such as tank equipment, production tanks, seawater pumps and offshore structures.

In general, two-phase stainless steel, which is known to have excellent corrosion resistance, contains a large amount of Cr, an alloying element that improves corrosion resistance, and basically contains Mo and N, but it can be classified into two types. In other words, 21-23% by weight of Cr (hereinafter referred to as '%'), 4.5-6.5% Ni, 2.5-3.5% Mo, 0.08-0.20% N, 2% or less Mn and UNS 31803 steel containing less than 0.03% C, 24-26% Cr, 6-8% Ni, 3-5% Mo, 0.24-0.32% N, 0.5% or less Cu, 1.2 Mn And SAF 2507 steels containing up to% and C up to 0.03%. The steels have almost the same corrosion resistance as that of super austenitic stainless steels, while the hot workability is poor, and thus the steel sheet is very sensitive to edge cracking during hot rolling. Hot rolled tl edge cracks lead to a drastic reduction in plate breakage and error rate, so two-phase stainless steels need to have excellent hot workability.

As a conventional method for improving the hot workability of two-phase stainless steel, there is a method of adding Ce to two-phase stainless steel. et. al., Proc, of Int. Conf. on Stainless steels, ISIJ. Tokyo, 1991, p 807). This method improves hot workability by lowering S content in steel to 30 ppm or less and by adding Ce to prevent segregation of S. In addition, APaul et al. Proposed a method of improving hot workability by increasing the strain rate to promote recrystallization of austenite phase during hot rolling of two-phase stainless steels (Innovation of Stainless Florence, Italy, 1993, p 3297). However, the above methods have a disadvantage in that they are difficult to apply to rolling equipment that cannot compensate for a certain temperature by adjusting the temperature during hot rolling, such as a stekel mill.

In other words, when the two-phase stainless steel slab is hot rolled using a real tandem rolling mill, it must be heated at a high temperature of 1250-1300 ° C. Therefore, in order to reduce surface defects caused by oxidation and increase the error rate, the two-phase stainless steel material is excellent. It should have high temperature oxidation resistance.

On the other hand, all of the two-phase stainless steels do not contain W and are related to Mo-containing steels, but the two-phase stainless steels containing Mo and W are more excellent than the W-free steels in terms of hot workability and corrosion resistance. Recently, research on two-phase stainless steels in which Mo and W are combined is actively conducted. For example, the two-phase stainless steel proposed by BW Oh et al. Contains 20-22% Cr and partially replaces Mo with W. Among them, two-phase stainless steel with 2.7% W and 1.05% Mo is added. It is reported that steel has improved corrosion resistance compared to steel containing 2.78% Mo (Innovation of Stainless, Florence, Italy, 1993, p 359). However, the steel is difficult to improve the high-temperature oxidation resistance because the content of Cr is essentially low, and especially since it does not contain Cu, impact toughness deteriorates due to the formation of an intermetallic compound.

As another example, the European patent EP 0,545,753Al, filed by H. Okamoto, proposes a two-phase stainless steel containing 2-4% Mo and 1.5-5.0% W, which has a high strength. While it is known to have high corrosion resistance and high corrosion resistance, not only is there a high risk of edge cracking during hot rolling, but also phase stability tends to be lowered.

In addition, Korean Patent Application No. 94-38249, filed by the present inventors, includes two-phase stainless steel containing 22.5-23.5% Cr, and Korean Patent Application No. 94-38978, which contains 24-26% Cr. Phase stainless steel has been proposed, and both the two-phase stainless steels are steels in which Mo and W are added in order to improve corrosion resistance. In particular, high temperature oxidation resistance and hot workability are improved to be manufactured in rolling equipment such as tandem rolling mill. It is the rivers. However, when the two-phase stainless steels added with Mo and W are added to structures that require welding, precipitation of intermetallic compounds in the heat-affected zone increases, resulting in deterioration of impact toughness and deterioration of phase stability. There is.

Accordingly, the present inventors have improved the two-phase stainless steels described in the above-mentioned Korean Patent Application Nos. 94-38249 and 94-38978, which are excellent in hot workability, high temperature oxidation resistance, and excellent in corrosion resistance and phase stability of the heat affected zone. Suggested a method of manufacturing a two-phase stainless steel sheet that can be produced by stainless steel and tandem rolling equipment has been filed a patent in Korea Patent Application No. 95-14766.

The present invention is preferably applied to the production of two-phase stainless steel of the Republic of Korea Patent Application No. 95-14766.

Usually, two-phase stainless steel is produced by steelmaking, refining, producing slab, polishing slab surface polishing, heating in a high temperature furnace of 1200-1350 ° C., followed by hot rolling, annealing heat treatment, and pickling.

The playing slab manufacturing process may be divided into a continuous casting process and a slab cooling process, and the continuous casting process may be divided into a first continuous casting cooling section and a second continuous casting cooling section.

When the playing slab is manufactured by a conventional method, intermetallic compounds which are very sensitive to impact characteristics are formed during some sections of the second continuous casting cooling section and the slab cooling process.

When the intermetallic compound is formed as described above, surface cracks may be caused during surface polishing of the slabs for improving the surface quality of the product.

In general, impact toughness is drastically reduced when 3-5% of the intermetallic compound is formed (L karlsson, Application of Stainless Steel 92, 9-11 June 1992, Stockholm, Sweden).

These cracks cause surface defects by forming a nodule-type oxide scale inside the slab when working at a high temperature of 1200-1350 ° C.

Accordingly, the present inventors have proposed that the present invention in consideration of the precipitation of the intermetallic compound related to the surface crack induction during the slab surface polishing is closely related to the cooling rate of the slab, the present invention is a constant temperature during slab production By appropriately controlling the cooling rate in the section to minimize the precipitation of intermetallic compounds, it is to prevent the occurrence of surface defects when polishing the slab surface, the purpose is to.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated.

In the present invention, by weight%, C: 0.03% or less, Si: 1.0% or less, Mn: 2.0% or less, P: 0.04% or less, S: 0.004% or less, Cu: 2.0% or less, Ni: 5.0-8.0%, Cr: 22-23%, Mo: 1.0-2.0%, W: 2.0-5.0%, and N: 0.13-0.27%; Nieq equivalent to Nieq =% Ni + 30 x% C + 0.5 x% Mn + 0.33 x% Cu + 30x (% N-0.045), Creq =% Cr +% Mo + 1.5 x% Si + 0.73 x% W The ratio (Creq / Nieq) of (Nieq) and chromium equivalent (Creq) is 2.2-3.0; Molten steel formed so as to satisfy the condition that the weight ratio (W / Mo) of tungsten and molybdenum is 2.6-3.4, or Ca: 0.03% or less, Ce: 0.1% or less, B: 0.005% or less, and Ti: 0.5% or less In the method for producing a slab for producing two-phase stainless steel consisting of a ferrite phase and austenite phase by continuously casting molten steel further selected one or two or more selected from the group consisting of in the slab form and cooling the slab, The continuous casting and slab cooling relates to a method for producing a two-phase stainless steel slab with a cooling rate from a temperature range of 950-800 ° C. to a temperature range of 650-700 ° C. of 3 ° C./min or more.

In addition, the present invention is a weight%, C: 0.03% or less, Si: 1.0% or less, Mn: 2.0% or less, P: 0.04% or less, S: 0.004% or less, Cu: 2.0% or less, Ni: 5.0-8.0 %, Cr: 23-27%, Mo: 1.0-2.0%, W: 2.0-5.0%, and N: 0.13-0.27%; Nieq defined as Nieq =% Ni + 30 x% C + 0.5 x% Mn + 0.33 x% Cu + 30 x (% N-0.045), Creq =% Cr +% Mo + 1.5 x% Si + 0.73 x% W The ratio (Creq / Nieq) of the equivalent (Nieq) and the chromium equivalent (Creq) is 2.2-3.0; Molten steel formed so as to satisfy the condition that the weight ratio (W / Mo) of tungsten and molybdenum is 2.6-3.4, or Ca: 0.03% or less, Ce: 0.1% or less, B: 0.005% or less, and Ti: 0.5% or less In the method for producing a slab for producing two-phase stainless steel consisting of a ferrite phase and austenite phase by cooling the slab after continuous casting in molten steel further containing one or two or more selected from the group consisting of The continuous casting and slab cooling relates to a method for producing a two-phase stainless steel slab with a cooling rate from a temperature section of 1000-800 ° C. to a temperature section of 650-700 ° C. of 5 ° C./min or more.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

Although C is a strong austenite phase stabilizing element, when present in an amount of 0.03% or more, it is preferable to limit chromium carbide to 0.03% or less because it lowers corrosion resistance.

The Si is contained from the deoxidizer, and when contained in a large amount, it is limited to 1.0% or less because it promotes precipitation of the intermetallic compound. More preferably, it is limited to 0.6% or less.

The Mn has the advantage of increasing the solubility of N when dissolving the two-phase stainless steel.

However, since Mn induces a decrease in corrosion resistance by forming MnS, Mn is preferably limited to 2.0% or less.

P is an element contained from scrap or ferroalloy introduced in the steelmaking process, and P is preferably limited to 0.04% or less, more preferably 0.03% or less, because it reduces corrosion resistance and impact toughness when 0.04% or more is present. It is limited to.

S is an element contained from scrap or ferroalloy introduced in the steelmaking process, similar to P, and forms sulfide at grain boundaries to reduce hot rolling. In addition, these sulfides act as sites that cause pitting corrosion, which significantly reduces corrosion resistance. Therefore, the content of S is preferably limited to 0.004% or less, more preferably 0.003% or less, because S reduces corrosion resistance and impact toughness when S is present at 0.004% or more.

Cu is an element that suppresses the formation of an intermetallic compound and exhibits excellent corrosion resistance in an acid in a reducing atmosphere. In particular, in the two-phase stainless steel containing 22.5-23.5% Cr, the impact toughness is increased by adding Cu. However, if the content exceeds 2.0%, the hot workability is reduced, so the Cu content is preferably limited to 2.0%, more preferably 1.0% or less.

Ni is an important element for stabilizing an austenite phase. However, when the Ni content is out of the proper amount, the ratio of the austenite phase and the ferrite phase is not proper, and thus the characteristics of the two-phase stainless steel do not appear. Particularly, when the Ni content is 5% or less, the ferrite phase having low solubility of N increases, and chromium nitride is formed in the ferrite phase to reduce corrosion resistance and impact toughness, so Ni is preferably limited to 5-8%.

Cr is an important element for improving corrosion resistance. If the Cr content is less than 22%, it does not exhibit the corrosion resistance required for two-phase stainless steels, and if it exceeds 27%, the precipitation rate of the intermetallic compound is increased, thereby significantly reducing corrosion resistance and impact toughness. Therefore, Cr is preferably limited to 22-27%.

Mo is an important element for improving corrosion resistance, such as Cr. In particular, it exhibits excellent pitting resistance in the chloride environment. However, less than 1% does not exhibit sufficient pitting resistance, and more than 2% promotes precipitation of intermetallic compounds, thereby reducing corrosion resistance and impact toughness, so the content of Mo is limited to 1-2%.

W is an important element for improving corrosion resistance such as Mo. In particular, it provides excellent pitting resistance at low pH, and also has an effect of delaying sigma phase precipitation in two-phase stainless steel. However, when the content of W is 2% or less, such an effect is insufficient. On the other hand, when the content is more than 5%, oxidation proceeds rapidly in the atmosphere by high temperature heating, and W is limited to 2-5% because it promotes the formation of intermetallic compounds. It is preferable to.

N is a strong austenite stabilizing element and is an element which improves corrosion resistance. When the content of N is less than 0.13%, it does not have the strength and corrosion resistance to have as a two-phase stainless steel, and it is easy to precipitate intermetallic compounds. On the other hand, when the content exceeds 0.27%, the strength of the austenite phase is too high, which significantly reduces the hot workability during hot rolling. Therefore, the content of N is preferably limited to 0.13-0.27%.

The content of N may be increased to 0.3%. In this case, the content of S should be limited to 0.003% or less, preferably 0.002% or less.

In addition, when one or two or more elements selected from the group consisting of Ca, Ce, B, and Ti are added to the composition, the hot workability of the two-phase stainless steel is further improved, which is more advantageous for hot rolling. However, when it is contained at least 0.03% for Ca, at least 0.1% for Ce, at least 0.005% for B, and at least 0.5% for Ti, these elements act as inclusions in two-phase stainless steels, and thus, are more resistant to corrosion and impact. It is not preferable because it is lowered.

After continuous casting of the molten steel formed as described above in the form of slabs, the slabs are cooled to manufacture the slabs.

The continuous casting process may be divided into a first continuous casting cooling section and a second continuous casting cooling section.

Normally, continuous casting for producing two-phase stainless steel slab starts at 1450-1500 ° C and is completed at about 900-1000 ° C. The first continuous casting cooling section has a temperature corresponding to 1350-1420 ° C from the start of continuous casting. The second continuous casting cooling section is a temperature section from 1350-1420 ℃ to 900-1000 ℃.

In the present invention, the cooling rate is controlled in some sections of the second continuous casting cooling section and some sections of the slab cooling section during the continuous casting process.

That is, in the present invention, when the Cr content is 22-23%, the cooling rate from the temperature range of 950-800 ° C. to the temperature range of 650-700 ° C. during the continuous casting and the slab cooling is 3 ° C./min or more. In addition, when the Cr content is 23-27%, the cooling rate from the temperature range of 1000-800 ° C to the temperature range of 650-700 ° C is set to 5 ° C / min or more.

According to the results of the precipitation behavior of the intermetallic compound of the present inventors, it was found that in the case of 22-23% Cr-containing steel, the maximum precipitation temperature of the intermetallic compound was about 950 ° C.

Therefore, in the present invention, when the Cr content is 22-23%, it is preferable to select a cooling rate of 3 ° C./mim or more from a temperature range of 950-800 ° C. to a temperature range of 650-700 ° C., for that reason. When the Cr content is 22-23%, when the cooling rate in the temperature section is 3 ℃ / min or less, more than 2% of the intermetallic compound is formed and defects due to surface cracks occur, the preferred temperature section is 950 -700 ° C. The preferred cooling rate is 3-60 ° C./min in view of economics.

On the other hand, in the present invention, when the Cr content is 23-27%, it is preferable to select the cooling rate in the temperature range of 1000-800 ° C to 5 ° C / min or more, for the reason of the temperature range of 1000-700 ° C. If the cooling rate is less than 5 ℃ / min is because more than 2% of the intermetallic compound is formed to cause defects due to surface cracks, the preferred cooling rate is 5-180 ℃ / min in consideration of economical efficiency.

Referring to the relationship between the cooling conditions and Cr content of the slab in the present invention in more detail.

The precipitation rate and precipitation temperature range of the intermetallic compound vary with the Cr content.

The higher the Cr content, the wider the precipitation temperature range, and the faster the intermetallic compound precipitation rate in the same temperature range.

Therefore, in order to control the amount of the intermetallic compound, the cooling rate and the cooling temperature range should be determined according to the Cr content.

When the Cr content is 22-23%, the temperature at which the intermetallic compound begins to precipitate is 950 ° C. or less, and the temperature range indicating the maximum precipitation rate is 800-900 ° C., and the precipitation rate is very slow at 700-650 ° C. or less. .

Therefore, in the present invention, the cooling of the slab is performed at a cooling rate from 950-800 ° C to 650-700 ° C in the case of 22-23% Cr steel at 3 ° C / min or more, preferably 3-60 ° C / min. do.

After cooling to the 650-700 ℃ section as described above it is cooled to room temperature by a conventional method, that is, water cooling or strong air cooling, etc., the amount of precipitates of the intermetallic compound precipitated in the final slab thus produced is less than 2% do.

On the other hand, when the Cr content is 23-27%, the temperature at which the intermetallic compound starts to precipitate is 1050 ° C. or less, and the temperature range indicating the maximum precipitation rate is 800-950 ° C., and the deposition rate at 700-650 ° C. or less is Very slow

Accordingly, in the present invention, the cooling of the slab is performed by setting the cooling rate from 1000-800 ° C to 650-700 ° C in the case of 23-27% Cr steel to 5 ° C / min or more, preferably 5-180 ° C / min. do.

After cooling to the 650-700 ℃ section as described above it is cooled to room temperature by conventional methods, that is, water cooling or strong air cooling, etc., the amount of precipitation of the intermetallic compound precipitated in the final slab thus produced is less than 2% do.

An example of a preferred method for producing two-phase stainless steel using the slab manufactured according to the present invention as described above is as follows.

In order to manufacture two-phase stainless steel, the surface of the two-phase stainless steel slab prepared according to the present invention is surface polished and then reheated in a reheating furnace, followed by hot rolling. When reheating, the amount of excess oxygen in the atmosphere of the heating furnace is 2 vol% or less. It is preferable to limit the temperature, and the heating temperature is preferably limited to 1250-1300 ° C.

In addition, it is preferable to reduce the initial reduction rate during hot rolling of the slab heated as described above, and then gradually increase the reduction rate, and then reduce the rolling reduction rate at around 1050-1000 ° C. again. For example, it is recommended that the first pass rolling be 10-20% so that no breakage occurs in the two-phase stainless steel, and then gradually increase the reduction rate to 36% while gradually increasing the reduction rate, and then the furnace temperature is 1050-. When it reaches the range of 1000 degreeC, it is preferable to carry out finish hot rolling by making the reduction ratio into the range of 15-25%.

More importantly, if the reduction ratio exceeds 25% in the temperature range of 1050-1000 ℃, cracks may occur due to the characteristics of two-phase stainless steels despite excellent hot workability, and it is not preferable in terms of actual productivity when the temperature is 15% or less. .

In addition, it is preferable to perform hot rolling in a range of 1-8 / sec during the hot rolling. When the total strain rate exceeds 8 / sec, the recrystallization behavior (softening behavior) during the hot rolling is insufficient. This is because cracks may be caused, and when it is less than 1 / sec, productivity is significantly reduced, which is not preferable in the actual industry.

By performing the annealing and pickling process of the hot rolled hot rolled steel sheet as described above, the final two-phase stainless steel made of ferrite and austenite is manufactured.

Hereinafter, the present invention will be described in more detail with reference to Examples.

EXAMPLE

After dissolving the steel having a composition as shown in Table 1, and cast to prepare a 50kg ingot.

Next, 950-700 in the case of steel grade (1) using a heat treatment furnace that cuts the test piece to a size of 3 mm (W) × 5 mm (L) × 2 mm (T) from the ingot and arbitrarily adjusts the heating and cooling rate. In the case of the temperature range and the steel grade (2), the precipitation rate of the intermetallic compound was observed according to the cooling rate while changing the cooling rate in the temperature range of 1000-700 ° C as shown in Table 2 below. 2 is shown.

At this time, it cooled by air from 700 degreeC to room temperature.

The amount of precipitation of the intermetallic compounds shown in Table 2 is measured by using an image analyzer after observing using a back scattering eletron of a scanning electron microscope.

As shown in Table 2, when the content of Cr is 22.23% (steel grade 1), the precipitation rate of the intermetallic compound is 2.0% or less at the cooling rate of 3 ° C / min or more, while precipitation is 1 ° C / min. It can be seen that the amount is 3%.

On the other hand, when the Cr content is 24.80% (steel grade 2), the precipitation rate of the intermetallic compound is 2.0% or less at the cooling rate of 5 ° C / min or more, whereas the precipitation amount is 10% at 1 ° C / min. .

As described above, the present invention can prevent the occurrence of surface defects during the production of two-phase stainless steel by maintaining the intermetallic compound precipitation in the slab within 2.0% by appropriately controlling the cooling rate in a constant temperature section during continuous casting and slab cooling. It is effective to provide slabs of two-phase stainless steel.

Claims (6)

By weight%, C: 0.03% or less, Si: 1.0% or less, Mn: 2.0% or less, P: 0.04% or less, S: 0.004% or less, Cu: 2.0% or less, Ni: 5.0-8.0%, Cr: 22 -23%, Mo: 1.0-2.0%, W: 2.0-5.0%, and N: 0.13-0.27%; Nieq = Nickel equivalent defined as% Ni + 30x% C + 0.5x% Mn + 0.33x% Cu + 30x (% N-0.045), Creq =% Cr +% Mo + 1.5 x% Si + 0.73 x% W Nieq) and the ratio of chromium equivalent (Creq) (Creq / Nieq) are 2.2-3.0; Slabs are manufactured by continuously casting molten steel in the form of slab to satisfy the condition that the weight ratio (W / Mo) of tungsten and molybdenum is 2.6-3.4. The slab manufacturing method of the two-phase stainless steel slab, characterized in that the cooling rate from the temperature range of 950-800 ℃ to 650-700 ℃ at least 3 ℃ / min during the continuous casting and slab cooling. The molten steel further comprises one or two or more selected from the group consisting of Ca: 0.03% or less, Ce: 0.1% or less, B: 0.005% or less, and Ti: 0.5% or less by weight%. Slab manufacturing method of two-phase stainless steel, characterized in that The slab manufacturing method of claim 1 or 2, wherein the cooling rate in the continuous casting and slab cooling in the temperature range of 950-700 ℃ is 3-60 ℃ / min. By weight%, C: 0.03% or less, Si: 1.0% or less, Mn: 2.0% or less, P: 0.04% or less, S: 0.004% or less, Cu: 2.0% or less, Ni: 5.0-8.0%, Cr: 23 -27%, Mo: 1.0-2.0%, W: 2.0-5.0%, and N: 0.13-0.27%; Nieq = Nickel equivalent defined as% Ni + 30x% C + 0.5x% Mn + 0.33x% Cu + 30x (% N-0.045), Creq =% Cr +% Mo + 1.5 x% Si + 0.73 x% W Nieq) and the ratio of chromium equivalent (Creq) (Creq / Nieq) are 2.2-3.0; Slabs are manufactured by continuously casting molten steel in the form of slab to satisfy the condition that the weight ratio (W / Mo) of tungsten and molybdenum is 2.6-3.4. The slab manufacturing method of the two-phase stainless steel slab, characterized in that the cooling rate from 1000-800 ℃ temperature section to 650-700 ℃ temperature section at least 5 ℃ / min during the continuous casting and slab cooling. The molten steel further comprises one or two or more selected from the group consisting of Ca: 0.03% or less, Ce: 0.1% or less, B: 0.005% or less, and Ti: 0.5% or less by weight%. Slab manufacturing method of two-phase stainless steel characterized by containing The slab manufacturing method of claim 4 or 5, wherein the cooling rate in the continuous casting and slab cooling in the 1000-700 ℃ temperature section is 5-180 ℃ / min.