KR0143481B1 - The making method and same product of duplex stainless steel plate - Google Patents

Abstract

It is an object of the present invention to provide austenitic + ferrite two-phase stainless steel having excellent corrosion resistance, high temperature oxidation resistance, hot workability and impact toughness, and a method of manufacturing a steel sheet using the same.

The present invention is a two-phase stainless steel consisting of a ferrite phase and an austenite phase, in weight%, C: 0.03% or less, Si: 1.0% or less, Mn: 2.0% or less, P: 0.04% or less, S: 0.004% Cu: 2.0% or less, Ni: 5.0-8.0%, Cr: 22-27%, Mo: 1.0-2.0%, W: 3.0-5.0%, and N: 0.13-0.27% or include Ca One or two or more selected from the group consisting of: 0.03% or less, Ce: 0.1% or less, B; 0.005% or less and Ti: 0.5% or less;

The ratio of nickel equivalents (Nieq) and chromium equivalents (Creq) (Creq / Nieq) is defined as 2.2-3.0; The gist of the present invention relates to a method for manufacturing a steel sheet using two-phase stainless steel and two steels, which is formed by satisfying a condition in which the weight ratio (W / Mo) of tungsten and molybdenum is 2.6-3.4.

Description

Two-Phase Stainless Steel and Manufacturing Method of Steel Sheet Using the Same

The present invention relates to a two-phase stainless steel used as a material for seawater facilities, and a method for manufacturing a steel sheet using the same. More specifically, a two-phase stainless steel and a steel having a phase ratio of about 50:50 in a ferrite phase and an austenitic phase It relates to a method for producing a two-phase stainless steel sheet using.

Generally, two-phase stainless steel (hereinafter referred to simply 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 papermaking It is widely used in facilities requiring significant corrosion resistance under corrosive environments such as factory tank facilities, powder 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, which can be largely classified into two types. That is, 21-23% by weight of Cr (hereinafter referred to as '%') in the alloy components, 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. When edge cracking occurs during hot rolling, it leads to a drastic reduction of plate breaking and error rate. Therefore, two-phase stainless steel needs 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 (JIK mi, 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, A.Paul suggests a method to improve 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 controlling the temperature during hot rolling, such as a stegelkel 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. Thus, in order to reduce surface defects caused by oxidation and increase the error rate, the two-phase stainless steel material is excellent. It must 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 steels have been proposed, and both of the two-phase stainless steels are steels in which Mo and W are combined to improve corrosion resistance. Steels with improved high temperature oxidation resistance and hot workability to be manufactured in rolling equipment such as tandem rolling mill. However, when the two-phase stainless steels added with Mo and W are added to a structure that requires welding, the 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 repeatedly conducted research and experiments to improve the two-phase stainless steels described in the above-mentioned Korean Patent Application Nos. 94-38249 and 94-38978, and suggest the present invention based on the results. The present invention is to provide a method for producing a two-phase stainless steel sheet that can be produced by tandem rolling equipment using two-phase stainless steel and two steels excellent in hot workability, high temperature oxidation resistance and excellent corrosion resistance and phase stability of the heat affected zone. , Its purpose is.

Hereinafter, the present invention will be described.

The present invention is a two-phase stainless steel consisting of a ferrite phase and an austenite phase, in weight%, C: 0.03% or less, Si: 1.0% or less, Mn: 2.0% or less, P: 0.04% or less, S: 0.004% Cu: 2.0% or less, Ni: 5.0-8.0%, Cr: 22-27%, Mo: 1.0-2.0%, W: 3.0-5.0%, and N: 0.13-0.27%, or herein 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;

The ratio of nickel equivalents (Nieq) and chromium equivalents (Creq) (Creq / Nieq) is defined as 2.2-3.0; It relates to a two-phase stainless steel formed by satisfying the condition that the weight ratio (W / Mo) of tungsten and molybdenum is 2.6-3.4.

In addition, the present invention is to heat the steel slab having the above composition in a furnace containing excess oxygen of 2vol% or less to a temperature in the range of 1250-1300 ℃ and the first pass reduction rate during hot rolling to 10-20% After that, increase the reduction rate to 36% and finish hot rolling at 15-25% reduction rate in the temperature range of 1050-1000 ℃, and also hot rolling the deformation rate of the entire hot rolling in the range of 1-8 / sec. It relates to a method for producing a two-phase stainless steel sheet.

Hereinafter, the composition of the inventive steel will be described in detail.

C: C is a strong austenitic stabilizing element, but when present in an amount of 0.03% or more, it is preferable to limit it to 0.03% or less since chromium carbide is precipitated to lower corrosion resistance.

Si: Si is included from the deoxidizer and is limited to 1.0% or less since it promotes precipitation of a large amount of intermetallic compound. More preferably, it is limited to 0.6% or less.

Mn: Mn has the advantage of increasing the solubility of N when dissolving two-phase stainless steel. However, since Mn forms MnS, the corrosion resistance is reduced, so Mn is preferably limited to 2.0% or less.

P: P is an element included in scrap or ferroalloy in the steelmaking process, and when P is present in an amount of 0.04% or more, P: P is preferably limited to 0.04% or less, more preferably 0.03% or less. It is limiting.

S: S, like P, is an element included in scrap or ferroalloy in the steelmaking process. S: S forms sulfide at grain boundaries, thereby degrading hot rolling property. These sulfides also act as sites of pitting corrosion, which significantly reduces corrosion resistance. Therefore, the content of S is preferably limited to 0.004% or less, and more preferably 0.003% or less, because S reduces corrosion resistance and impact toughness when S is present at 0.004% or more.

It is an element that suppresses the formation of Cu: Cu intermetallic compound and exhibits excellent corrosion resistance in an acid in a reducing atmosphere. Especially. In 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: Ni is an important element to stabilize the 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: 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 22-275.

Mo: Mo is an important element that improves 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: 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 3% or less, such an effect is insufficient, whereas when it exceeds 5%, oxidation proceeds rapidly in the atmosphere by high temperature heating, and W is limited to 3-5% because it promotes the formation of intermetallic compounds. It is preferable to.

N: 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. If the half side 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%.

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.

In the two-phase stainless steel having the composition described above, the austenitic phase and the ferrite phase are mixed so that the two phases coexist. And impact toughness. The most preferred phase fraction of two-phase stainless steel is when the fraction of austenite phase and ferrite phase coexists at about 50:50. Since the phase fraction of two-phase stainless steel is largely dependent on the contents of Cr, Ni, Mo, W, N, Cu, Si, and C in the main alloying components, an appropriate chromium equivalent (Creq) is required to make a proper phase ratio between two phases. It is necessary to set the nickel equivalent (Nieq).

Nickel equivalent silver (Nieq)

Can be obtained using In addition, since the formula for calculating the normal chromium equivalent (Creq) does not include the ferrite generation element, W, the weight is given as 0.73 according to the experimental results of F.B.Pickering.

(The Metallurgical Evolution of Stainless Steels, The American Society of Metals, Cleveland, Ohio, 1979, p132).

In order to maintain the phase ratio of the two-phase stainless steel to about 50:50, the ratio of Creq / Nieq must satisfy the range of 2.2-3.0 according to the equation for chromium equivalent (Creq) and nickel equivalent (Nieq). When the ratio of / Nieq is out of the above range, the phase ratio of the two-phase stainless steel is greatly deformed from 50:50, which deteriorates high temperature oxidation resistance, corrosion resistance and hot workability, and is not preferable.

In addition, even if the ratio of chromium equivalent and nickel equivalent is in the above ratio and the weight ratio of W / Mo is not appropriate even in the range where the total content of Mo and W is excellent in hot workability, This may adversely affect the impact characteristics. That is, in the present invention steel having a Cr content in the range of 22-27%, the hot workability is excellent when the weight ratio of W / Mo is 2.6-3.4, and in particular, the generation degree of the intermetallic compound is reduced in the heat affected zone during welding. The phase may be stabilized.

Hereinafter, the manufacturing method of the steel plate using the two-phase stainless steel by this invention is demonstrated in detail.

Steel sheet production using the two-phase stainless steel according to the present invention can be manufactured by a conventional method for manufacturing two-phase stainless steel sheet, but when produced using a general stainless steel production equipment rather than a dedicated production equipment More special conditions are needed, such as a disadvantage that must be adjusted very much.

Conventional stainless steels, such as 304 stainless steel, limit the excess oxygen in the furnace to reheat the slab to about 3 vol% or less.Reheating steel slabs with a Cr content in the range of 22.5-23.5% in such an atmosphere would result in a W content of 4% or more. Oxidation amount increases sharply, and if the steel slab with Cr content in the 24-26% range is reheated, the oxidation amount also increases rapidly when the W content is 6.12% or more. Therefore, the present inventors have reduced heating to reduce the local corrosion which adversely affects the high temperature oxidation amount and the surface shape by controlling the excessive amount of oxygen in the atmosphere by reheating to improve the high temperature oxidation characteristics of the two-phase stainless steel containing a large amount of Mo and W. The method has been proposed and patented in the Republic of Korea Patent Application No. 95-14484, in the present invention, it is preferable to use this heating method when heating two-phase stainless steel slab.

That is, when reheating the two-phase stainless steel slab according to the present invention, the amount of excess oxygen in the atmosphere in the furnace is controlled to 2 vol% or less, and the heating temperature is in the range of 1250-1300 ° C., which is a normal temperature range.

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 as not to cause breakage 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%.

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

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.

Hereinafter, the present invention will be described in detail through examples.

Example 1

After dissolving and casting to have the composition and conditions as shown in Table 1, 50Kg of ingot was prepared, and heat-treated for 3 hours in a heating furnace at 1270 ° C.

The heated slabs were then rolled up to 12 mm using a test mill, and the rolling reduction was initially set at 18% for the first pass rolling, and then gradually increased to a reduction ratio, and then decreased again at around 1050-1000 ° C. It was then quenched with water. The finish rolling temperature was 1000 degreeC or more.

Phase stability was evaluated through hot workability, high temperature oxidation resistance, corrosion resistance, and impact test on the two-phase stainless steel sheet thus prepared, and the results are shown in Table 2 below.

The hot workability evaluation was carried out through a high temperature tensile test, the high temperature tensile test was heated to 1290 ℃ at 20 ℃ / sec using Gleeble 1500 and then maintained for 1 minute and then cooled to 10 ℃ / sec to 1050 ℃ for 10 seconds After holding, it broke at a cross-head speed of 300mm / sec. When the reduction rate at 1050 ℃ exceeded 80%, it was good (●), when it was 70% or more, it was normal (■), and the following was indicated by (▲). .

The high temperature oxidation test was expressed by weight gain after 3 hours of high temperature oxidation in an atmosphere of 3 vol% excess oxygen at 1290 ° C. The heating takes 60 minutes to reach 1290 ° C., and then was maintained at 1290 ° C. for 120 minutes. Evaluation was expressed as good (●) when the oxidative increase exceeded 10 mg / cm <2> * hr, and (▲) when exceeding 10 mg / cm <2> * hr.

The corrosion resistance test used a modified ASTM G-48 test method. That is, the pH of the solution was 0.8-1.0, and the temperature of the pit on the surface was measured after immersion at 2.5 ° C for 24 hours to compare relative formal resistance.

The evaluation of the stability of the phase was performed by heat-treating each test specimen at 900 ° C. for 3 minutes, followed by Charpy impact test at room temperature. Steels with a Cr content in the range of 22-24wt% were evaluated as having good phase stability when the impact energy was 150J or more (●), or when the steel content was 150J or less (▲). Steels having a Cr content of 24-27 wt% were evaluated as having good phase stability (?) When the impact energy was 50J or more, and when they were 50J or less (phase).

As shown in Table 2, the invention steel that satisfies the composition and the condition range according to the present invention can be seen that the hot workability, high temperature oxidation resistance, corrosion resistance and impact toughness is very excellent compared to the comparative steel.

In addition, the invention steel (38-42) in which one, two or more kinds of Ca, Ce, B, and Ti elements are appropriately added, the hot workability is more improved than the invention steel (37) without the addition of these elements. It can be seen.

Example 2

The hot-rolled sheet was manufactured in the same manner as in Example 1 for the inventive steel 16 steel of Example 1, but the rolling conditions were rolled under the conditions shown in Table 3 to obtain a two-phase stainless steel sheet.

The presence of cracks was confirmed for the manufactured steel sheet, and the results are shown in Table 3 below.

As shown in Table 3, the invention is a case in which the reduction of the first pass rolling after increasing the reduction rate to 36% and then lowering again in the finish rolling region (8 pass) ranging from 1000-1050 ℃ temperature range, It can be seen that no crack is generated in the finally obtained steel sheet.

On the other hand, the comparative material (1) continued to increase the reduction rate from the first pass rolling and to drop down even in the 8-9 pass, which reached the temperature range of 1000-1050 ° C, and cracks occurred in the final steel sheet. In the case of 2), after reducing the pressure from the first pass rolling, the reduction rate was gradually increased and the pressure was reduced again at the finishing temperature range.

As described above, the present invention is a two-phase stainless steel excellent in both hot workability, high temperature oxidation resistance and impact resistance as well as corrosion resistance by appropriately controlling the component system in steel while appropriately controlling the relationship between Creq and Nieq and the weight ratio of W / Mo. These two-phase stainless steels are provided and are suitable for installations where corrosion resistance is required under various corrosive environments. In addition, the two-phase stainless steel according to the present invention is particularly excellent in hot workability, it is extremely advantageous to manufacture a steel sheet through the control of the appropriate hot rolling conditions.

Claims (4)

In two-phase stainless steel consisting of a ferritic phase and an austenite phase, in 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: 22-27%, Mo: 1.0-2.0%, W: 3.0-5.0%, and N: 0.13-0.27%; The ratio of nickel equivalents (Nieq) and chromium equivalents (Creq) (Creq / Nieq) is defined as 2.2-3.0; Two-phase stainless steel, characterized in that the composition is satisfied by the condition that the weight ratio (W / Mo) of tungsten and molybdenum is 2.6-3.4 In two-phase stainless steel consisting of a ferritic phase and an austenite phase, in 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: 22-27%, Mo: 1.0-2.0%, W: 3.0-5.0%, and N: 0.13-0.27%; It contains one 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; The ratio of nickel equivalents (Nieq) and chromium equivalents (Creq) (Creq / Nieq) is defined as 2.2-3.0; Two-phase stainless steel, characterized in that the composition is satisfied by the condition that the weight ratio (W / Mo) of tungsten and molybdenum is 2.6-3.4 In the method for producing a two-phase stainless steel sheet consisting of a territic phase and an austenitic phase, a temperature of 1250-1300 ° C in a furnace containing excess oxygen of 2 vol% of a steel slab having the composition of claim 1 After heating to hot rolling, the first pass reduction rate during hot rolling is 10-20%, and then the reduction rate is increased to 36%, and then the final hot rolling is performed at a rolling reduction rate of 15-25% in the temperature range of 1050-1000 ° C. In addition, the manufacturing method of the two-phase stainless steel sheet, characterized in that the hot rolled deformation rate of the entire hot rolling in the range of 1-8 / sec In the method for producing a two-phase stainless steel sheet consisting of a ferrite phase and an austenite phase, the steel slab having the composition of claim 2 is heated at a temperature in the range of 1250-1300 ° C. in a heating furnace containing excess oxygen of 2 vol% or less. After heating, the first pass reduction rate during hot rolling is set to 10-20%, and then the reduction rate is increased to 36%, followed by finishing hot rolling at a reduction rate of 15-25% in the temperature range of 1050-1000 ° C. Method for producing a two-phase stainless steel sheet, characterized in that the hot rolled deformation rate of the entire hot rolling in the range of 1-8 / sec