KR101991000B1 - High corrosion resistant austenitic stainless steel and method of manufacturing the same - Google Patents

Abstract

Disclosed are high anticorrosive austenitic stainless steel capable of lowering a sigma-phase precipitation temperature and realizing excellent corrosion resistance and a manufacturing method thereof. According to one embodiment of the present invention, the high anticorrosive austenitic stainless steel comprises: 0.01 to 0.025 weight percent of C; 0.1 to 0.5 weight percent of Si; 3.5 to 4.5 weight percent of Mn; 24.5 to 25.5 weight percent of Cr; 13.5 to 14.5 weight percent of Ni; 1.0 to 3.0 weight percent of Mo: 0.30 to 0.45 weight percent of N; and the remainder of Fe and inevitable impurities, wherein a critical pitting temperature (TCP) represented by following equation (1), -150 + 7.05*Cr + 1.18*Ni + 11.6*Mo + 57.6*N, is 70°C or higher, and following equation (2), (Mo + Ni) / (Cr +10*N) <= 3, is satisfied.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a high corrosion resistant austenitic stainless steel,

The present invention relates to a high corrosion resistant austenitic stainless steel in which sigma phase formation is suppressed and a method of manufacturing the same. More particularly, the present invention relates to austenitic stainless steel capable of lowering the precipitation temperature of sigma phase and achieving excellent corrosion resistance through control of alloy components, And a manufacturing method thereof.

Generally, stainless steel is classified according to chemical composition or metal structure. According to the metal structure, the stainless steel is classified into an austenitic system (300 system), a ferrite system (400 system), a martensitic system, and an ideal system. Among these stainless steels, austenitic stainless steels are excellent in corrosion resistance and non-magnetic and are widely used in kitchen containers, heavy chemical industry, and building interior and exterior materials. However, stainless steels such as 304 and 316, which represent 300-series stainless steel, can not be used for seawater inlet pipes due to formal corrosion in the seawater atmosphere where chloride ion exists. Therefore, high corrosion resistant austenitic stainless steels having high corrosion resistance even in severe corrosive environments have been developed by adding alloying elements that improve corrosion resistance such as Cr, Mo, and N.

For example, 22.0 to 24.0% by weight of chromium (Cr). (PREN = Cr + 3.3 * Mo) containing 21.0 to 23.0 wt% of nickel (Ni), 6.0 to 7.0 wt% of molybdenum (Mo), and 0.25 to 0.30 wt% of nitrogen + 16 * N) super corrosion resistant super austenitic stainless steel is used as a plate heat exchanger and seawater inlet pipe.

In addition, high corrosion resistant austenitic stainless steels such as S31254 and S31700, which are relatively low in corrosion resistance compared with the above super-austenitic stainless steels, have also been developed for each PREN.

The high corrosion resistant austenitic stainless steels of PREN 30 or higher have high corrosion resistance due to high content of Cr and Mo elements. However, when exposed to the temperature range of 750 ~ 1,150 ° C, sigma phase (σ, Cr / Mo- rich. In addition, when continuous casting is performed by a conventional casting of austenitic stainless steel, Cr and Mo segregation layers are present in the center of the slab, and the temperature of precipitation of the sigma phase is increased to 1,250 ° C. or more, It is impossible to completely disassemble the image. The sigma phase formed in the segregation zone of the slab center remains in the heat treatment process of the subsequent process, and the corrosion resistance and toughness of the final product are drastically lowered.

In order to solve the above problems, the present invention provides a high corrosion resistant austenitic stainless steel which has corrosion resistance similar to that of a conventional high corrosion resistant austenitic stainless steel of PREN 30 or more and does not form a sigma phase even in a segregated core layer, .

The high corrosion resistant austenitic stainless steel according to one embodiment of the present invention comprises 0.01 to 0.025% of C, 0.1 to 0.5% of Si, 3.5 to 4.5% of Mn, 24.5 to 25.5% of Cr, Ni : 13.5 to 14.5%, Mo: 1.0 to 3.0%, N: 0.30 to 0.45%, balance Fe and unavoidable impurities, wherein the critical temperature (CPT) expressed by the following formula (1) , The following expression (2) is satisfied.

(1) -150 + 7.05 * Cr + 1.18 * Ni + 11.6 * Mo + 57.6 * N

(2) (Mo + Ni) / (Cr + 10 * N)? 3.5

Here, Cr, Ni, Mo, and N mean the content (weight%) of each element.

According to an embodiment of the present invention, the stainless steel may satisfy the following formula (3).

(3) Cr + Ni? 40

Here, Cr and Ni mean the content (weight%) of each element.

According to an embodiment of the present invention, the stainless steel may have a sigma phase precipitation temperature expressed by the following formula (4) below 1,000 캜.

(4) 202 - 1505 * C - 24.8 * Si - 0.30 * Mn + 40.1 * Cr - 12.6 * Ni + 76.3 * Mo - 522 * N

Here, C, Si, Mn, Cr, Ni, Mo and N mean the content (weight%) of each element.

A method of manufacturing a high corrosion resistant austenitic stainless steel according to an embodiment of the present invention includes: 0.01 to 0.025% of C, 0.1 to 0.5% of Si, 3.5 to 4.5% of Mn, 24.5 to 25.5% of Cr, (1), Ni: 13.5 to 14.5%, Mo: 1.0 to 3.0%, N: 0.30 to 0.45%, balance Fe and unavoidable impurities, satisfies the following formula (2) ?) hot rolling a slab having a precipitation temperature of 1,000 占 폚 or less; And hot-rolled annealing the hot-rolled steel sheet, wherein the hot-rolled annealing is performed at 1,050 to 1,100 ° C for 10 minutes or less, followed by cooling with water.

(2) (Mo + Ni) / (Cr + 10 * N)? 3.5

(4) 202 - 1505 * C - 24.8 * Si - 0.30 * Mn + 40.1 * Cr - 12.6 * Ni + 76.3 * Mo - 522 * N

Here, C, Si, Mn, Cr, Ni, Mo and N mean the content (weight%) of each element.

According to an embodiment of the present invention, there is provided a method of manufacturing a cold-rolled steel sheet, And cooling and annealing the cold-rolled steel sheet. The cold-rolled annealing may be performed at a temperature of 1,050 to 1,100 ° C for 10 minutes or less, followed by cooling.

Further, according to an embodiment of the present invention, the value of the slab calculated from the following formula (1) may satisfy 70 or more.

(1) -150 + 7.05 * Cr + 1.18 * Ni + 11.6 * Mo + 57.6 * N

Here, Cr, Ni, Mo, and N mean the content (weight%) of each element.

Further, according to an embodiment of the present invention, the slab may satisfy the following formula (3).

(3) Cr + Ni? 40

Here, Cr and Ni mean the content (weight%) of each element.

The austenitic stainless steel according to the embodiment of the present invention not only can realize the corrosion resistance similar to the austenitic stainless steel of PREN 30 or more but also can be applied to the environment where the section is exposed to the outside because there is no precipitation of the sigma phase in the center portion Do.

FIG. 1 is a graph showing the relationship between the CPT and the sigma phase precipitation temperature of the inventive steel and the comparative steel according to the embodiment of the present invention.
2 is a graph showing the relationship between the value of the equation (2) of the invention steel and the comparative steel according to the embodiment of the present invention and the sigma phase precipitation temperature.
FIGS. 3 and 4 are cross-sectional optical photographs of an inventive steel and a comparative steel according to an embodiment of the present invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following embodiments are provided to fully convey the spirit of the present invention to a person having ordinary skill in the art to which the present invention belongs. The present invention is not limited to the embodiments shown herein but may be embodied in other forms. For the sake of clarity, the drawings are not drawn to scale, and the size of the elements may be slightly exaggerated to facilitate understanding.

Also, when an element is referred to as "comprising ", it means that it can include other elements as well, without departing from the other elements unless specifically stated otherwise.

The singular forms &quot; a &quot; include plural referents unless the context clearly dictates otherwise.

The high corrosion resistant austenitic stainless steel according to the present invention is a low-cost, high-strength, high-strength, high-strength, high-strength, high- And austenitic stainless steels.

In order to suppress the formation of sigma phase (sigma), there are measures such as annealing annealing temperature upward, Ni content upward, and lowering of Cr, Mo and Si contents. In the case of heating the annealing temperature above 1,200 ℃ for sigma phase full employment, it is difficult to apply because of grain size coarsening and mechanical property degradation problem. In addition to high Ni content, It is also difficult to apply this because it requires more than% Ni content. Therefore, to suppress the sigma phase formation promoting component such as Mo and to secure the corrosion resistance, Cr and N were used to realize the suppression of sigma phase formation and high corrosion resistance simultaneously.

The high corrosion resistant austenitic stainless steel according to one embodiment of the present invention comprises 0.01 to 0.025% of C, 0.1 to 0.5% of Si, 3.5 to 4.5% of Mn, 24.5 to 25.5% of Cr, Ni : 13.5 to 14.5%, Mo: 1.0 to 3.0%, N: 0.30 to 0.45%, balance Fe and unavoidable impurities.

Hereinafter, the reasons for limiting the numerical values of the alloy element content in the examples of the present invention will be described. Unless otherwise stated, the unit is wt%.

The content of C is 0.01 to 0.025%.

C is an element for forming an austenite phase, and is an effective element for increasing the strength of a material by solid solution strengthening. However, in the case of over-addition, segregation and coarse carbides are formed in the center of the material during production, which adversely affects the subsequent hot rolling-annealing-cold rolling-cold rolling annealing process and formation of carbide It is preferable to add in the range of 0.025% or less in order to maximize the corrosion resistance because it is easily combined with the element to reduce the Cr content around the grain boundary to reduce the corrosion resistance.

The content of Si is 0.1 to 0.5%.

Si is partially added for the deoxidizing effect, but when it is added in excess of 0.5%, the formation of the sigma phase is promoted and the corrosion resistance is lowered. Therefore, the Si content is preferably limited to a range of 0.5% or less.

The content of Mn is 3.5 to 4.5%.

Mn is an element that increases the degree of melt flow control, deoxidizing agent and nitrogen solubility, and is known as an austenite phase forming element. The higher the Mn content is, the better the nitrogen solubility is. Therefore, the addition of Mn at the level of 3.5 to 4.5% is required for the addition of 0.3% or more of the high N content.

The content of Cr is 24.5 to 25.5%.

Cr is an indispensable element for securing corrosion resistance. However, when the amount of Cr is excessively increased, the formation of sigma phase is promoted, so that the corrosion resistance of the high corrosion resistant austenitic stainless steel is realized and the content of Cr is limited to 24.5 to 25.5% .

The content of Ni is 13.5 to 14.5%.

Ni together with Mn and N play a major role in inhibiting sigma phase formation as austenite stabilizing elements. As the excessive Ni content increases, the content of Ni is limited to 13.5 to 14.5% in order to balance the cost competitiveness and the sigma-suppressing effect.

The content of Mo is 1.0 to 3.0%.

Mo is a strong corrosion resistance improving element such as Cr. However, there arises a problem that the corrosion resistance is lowered by promoting the formation of the sigma phase, and it is preferable to limit the Mo content to a range of 1.0 to 3.0%.

The content of N is 0.30 to 0.45%.

N, like Cr, is an essential element for ensuring corrosion resistance, and at the same time, it is a main element that inhibits the formation of sigma phase like Ni. However, excessive addition may lead to formation of nitrogen pores, thus limiting the N content to 0.30 to 0.45%.

The high corrosion resistant austenitic stainless steel according to the present invention is characterized in that not more than 30% of Ni is added in order to inhibit the formation of sigma phase, but superabundant austenitic stainless steels having an average resistance index (PREN) of 40 or more containing Ni in the range of 13.5 to 14.5% It can exhibit corrosion resistance similar to that of stainless steel. In order to achieve this, the high corrosion resistant austenitic stainless steel according to the present invention satisfies a critical pitting temperature (CPT) value of 70 ° C or more expressed by the following formula (1).

(1) -150 + 7.05 * Cr + 1.18 * Ni + 11.6 * Mo + 57.6 * N

In addition, with the corrosion resistance equivalent to that of super-austenitic stainless steel, it is possible to suppress the formation of the sigma phase by lowering the precipitation temperature of the sigma phase in the range of the above alloy composition, that is, Ni 13.5 to 14.5%. To achieve this, the high corrosion resistant austenitic stainless steel according to the present invention satisfies the following formulas (2) and (3).

(2) (Mo + Ni) / (Cr + 10 * N)? 3.5

(3) Cr + Ni? 40

Generally, the conventional super-austenitic stainless steels have a sigma phase precipitation temperature of 1,150 ° C. If this range is avoided and the annealing temperature is heated to 1,200 ° C or more, there arises a problem of mechanical property deterioration due to grain size coarsening.

The present invention can lower the temperature of the sigma phase precipitation to about 1,000 ° C by controlling the value of the formula (2) concerning the composition ratio of Mo, Ni, Cr and N to 3.5 or less even in the low Ni range shown in the formula (3) . Preferably, the sigma phase precipitation temperature may be 1,000 DEG C or less.

The regression equation of the sigma phase precipitation temperature upper limit value calculated according to the composition of the alloy composition can be expressed by the following equation (4). In the high corrosion resistant austenitic stainless steel according to the present invention, the sigma phase precipitation temperature expressed by the following formula (4) can be satisfied at 1,000 占 폚 or lower.

(4) 202 - 1505 * C - 24.8 * Si - 0.30 * Mn + 40.1 * Cr - 12.6 * Ni + 76.3 * Mo - 522 * N

Therefore, a method of producing a high corrosion resistant austenitic stainless steel according to the present invention is characterized in that a slab satisfying the above-mentioned alloy component and the above-mentioned formula (2) and having a sigma phase precipitation temperature calculated according to formula (4) The annealing temperature in the step of hot rolling and hot-rolling the hot-rolled steel sheet can be carried out in the temperature range of 1,050 to 1,100 ° C, which is higher than the downward sigma-phase precipitation temperature range.

In order to prevent crystal grain coarsening, the hot-rolled annealing time can be performed within 10 minutes, and after the annealing, water cooling can be performed to reduce the time for exposure to the sigma-phase precipitation temperature range.

Hereinafter, preferred embodiments of the present invention will be described in detail.

Example

A slab having the alloy composition shown in Table 1 below was hot-rolled, annealed at 1,100 ° C and then water-cooled to prepare a 5.0 mm hot-rolled annealed steel sheet, and cold rolled and cold annealed to obtain a cold rolled annealed steel sheet of 1.5 mm. Cold rolling annealing conditions are the same as hot rolling annealing conditions. The specimens of the cold-rolled annealed steel sheets were evaluated for sigma phase fraction and corrosion resistance characteristics. The critical temperature (CPT) was measured according to ASTM G150.

The inventive steels according to the present invention can be obtained by controlling the Mo, Ni, Cr and N component ratios according to the formula (2) to 3.5 or less and adjusting the sigma phase precipitation temperature according to the formula (4) to the conventional super-austenitic stainless steel Lt; RTI ID = 0.0 &gt; 1,000 C &lt; / RTI &gt;

The comparative steels used samples S31700, S31254 and S32050, which are conventional high corrosion resistant austenitic stainless steels, together with specimens of a component system unsatisfied with Eq. (2) and / or Eq. (4). Comparative steel 1 used S31700, Comparative steel 2 used S31254, and Comparative steel 3 used S32050.

division Composition (% by weight) PREN C Si Mn Cr Ni Mo N Comparative River 1 0.018 0.5 1.3 18.2 11.6 3.2 0.085 30.1 Comparative River 2 0.012 0.4 0.5 20 18 6.1 0.2 43.3 Comparative Steel 3 0.014 0.2 0.3 23 22 6.2 0.28 47.9 Comparative Steel 4 0.019 0.3 4.0 22.1 13 3.95 0.32 37.0 Comparative Steel 5 0.019 0.3 4.02 22.1 12.9 4.99 0.309 43.5 Comparative Steel 6 0.024 0.31 3.97 25.1 14.1 4.89 0.5 49.2 Inventive Steel 1 0.018 0.31 3.95 24.8 13.9 1.02 0.3 32.9 Invention river 2 0.02 0.3 3.95 25.1 14 1.96 0.35 37.2 Invention steel 3 0.024 0.31 3.97 25.1 14.1 2.98 0.408 41.5

division CPT (占 폚) Equation (2) Equation (3) Sigma phase precipitation temperature (캜) Actual measurement Equation (1) Actual measurement Equation (4) Comparative River 1 33.2 34.0 29.8 3.8 977 943 Comparative River 2 85.0 94.5 38.0 6.9 1100 1104 Comparative Steel 3 100.0 126.1 45.0 7.1 1136 1139 Comparative Steel 4 78.6 73.7 35.1 3.5 1025 1022 Comparative Steel 5 82.9 96.7 35.0 5.5 1106 1099 Comparative Steel 6 100.0 129.1 39.2 5.4 1080 1083 Inventive Steel 1 73.1 70.1 38.7 1.5 891 900 Invention river 2 90.5 86.3 39.1 2.4 947 950 Invention steel 3 100.0 101.6 3.2 3.5 999 988

Table 1 shows the PREN values of inventive steels and comparative steels. For comparisons of corrosion resistance levels of high corrosion austenitic stainless steels, we limited them to comparative steels with PREN values in the range of 30 to 50. Comparative steel 1 is a comparative example in which the PREN value is 30.1 and comparatively low in corrosion resistance, while Comparative steels 2 to 6 are comparative examples in which the PREN value is 37.0 or more and the corrosion resistance is excellent. On the other hand, the inventive steels according to the present invention have a PREN value of 32 or more, and the critical temperature of 73 ° C or more is also excellent in terms of corrosion resistance by referring to the CPT value shown in Table 2.

Specifically, in the comparative steel 1 (S31700), the content of Cr, Ni, and N was lower than the range of the present invention, and the CPT value was less than 70, so that the corrosion resistance was poor.

The comparative steel 2 (S31254) and the comparative steel 3 (S32050) were satisfactory in terms of Ni content and satisfactory in corrosion resistance, but unsatisfied with equation (2) and / or equation (3), the upper limit of the sigma phase precipitation temperature was 1,100 ° C or more. The sigma phase precipitation temperature calculated according to the formula (4) of the present invention was also found to be 1,100 ° C or higher, and it was confirmed that the error was small in the regression equation and the measured value.

The comparative steel 4 and the comparative steel 5 had a Cr and Ni content slightly out of the range of the present invention. However, the Mo content was so high that the sigma phase precipitation temperature and the measured temperature exceeded 1,020 ° C according to the formula (4).

The comparative steel 6 had a high content of Mo and N, so that the value of equation (2) exceeded 3.5 and 5.4, respectively. The sigma phase precipitation temperature and measured temperature were also high as 1,080 ℃ according to equation (4).

On the other hand, inventive steels according to the present invention satisfied the range of the alloy component, the formula (2), the formula (3) and the formula (4), and the sigma phase precipitation temperature was 1,000 ° C. or less in both the measured value and the regression value, At the same time, the critical temperature (CPT) was 70 ℃ or higher.

Therefore, the high corrosion resistant austenitic stainless steel according to the present invention can be annealed at 1,050 to 1,100 DEG C in the hot-rolled annealing process or the cold-rolled annealing process by lowering the sigma phase precipitation temperature to 1,000 DEG C or less even in the low- It can be seen that it can show similar level to comparative steel types.

FIG. 1 is a graph showing the relationship between the CPT and the sigma phase precipitation temperature of the inventive steel and the comparative steel according to the embodiment of the present invention. Referring to FIG. 1, it can be seen that the comparative steels 2 to 6 except for the comparative steel 1 have a high CPT value but a high sigma phase precipitation temperature. However, inventive steels 1 to 3 according to the embodiments of the present invention have a CPT value of 70 ° C or higher, thereby securing the corrosion resistance characteristics, and the sigma phase precipitation temperature is lowered to 1,000 ° C or lower.

2 is a graph showing the relationship between the value of the equation (2) of the invention steel and the comparative steel according to the embodiment of the present invention and the sigma phase precipitation temperature. Referring to FIG. 2, it can be seen that the sigma phase precipitation temperature is lowered even though the value of the formula (2) shows a low value of 3.5 or less.

FIGS. 3 and 4 are cross-sectional optical photographs of an inventive steel and a comparative steel according to an embodiment of the present invention.

3 shows the end cold rolled steel t / 2 thick section of inventive steel 1. Referring to FIG. 3, the inventive steel according to the present invention did not have a sigma phase at its center. From this, it can be seen that a high-corrosion austenitic stainless steel having no sigma phase can be produced even when the composition of the present invention and the formula (2) satisfy the low alloy composition. On the other hand, referring to FIG. 4, which shows a cross section of the final cold rolled steel t / 2 of the comparative steel 4, it can be seen that the sigma phase remains in the segregation zone at the center section of the comparative steel.

While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited thereto. Those skilled in the art will recognize that other embodiments may occur to those skilled in the art without departing from the scope and spirit of the following claims. It will be understood that various changes and modifications may be made.

Claims (7)

The steel sheet according to any one of claims 1 to 3, wherein the steel sheet comprises 0.01 to 0.025% of C, 0.1 to 0.5% of Si, 3.5 to 4.5% of Mn, 24.5 to 25.5% of Cr, 13.5 to 14.5% of Ni, 1.0 to 3.0% 0.45%, balance Fe and unavoidable impurities,
The critical temperature (CPT) expressed by the following formula (1) satisfies 70 ° C or higher,
A high corrosion resistant austenitic stainless steel satisfying the following formula (2).
(1) -150 + 7.05 * Cr + 1.18 * Ni + 11.6 * Mo + 57.6 * N
(2) (Mo + Ni) / (Cr + 10 * N)? 3.5
(Where Cr, Ni, Mo, and N mean the content (% by weight) of each element) The method according to claim 1,
Wherein said stainless steel satisfies the following formula (3).
(3) Cr + Ni? 40
(Where Cr and Ni mean the content (weight%) of each element) The method according to claim 1,
Wherein the stainless steel satisfies a sigma phase precipitation temperature expressed by the following formula (4) at 1,000 占 폚 or lower.
(4) 202 - 1505 * C - 24.8 * Si - 0.30 * Mn + 40.1 * Cr - 12.6 * Ni + 76.3 * Mo - 522 * N
(Where C, Si, Mn, Cr, Ni, Mo and N mean the content (% by weight) of each element) The steel sheet according to any one of claims 1 to 3, wherein the steel sheet comprises 0.01 to 0.025% of C, 0.1 to 0.5% of Si, 3.5 to 4.5% of Mn, 24.5 to 25.5% of Cr, 13.5 to 14.5% of Ni, 1.0 to 3.0% 0.45%, the balance Fe and unavoidable impurities, satisfying the following formula (2) and satisfying the sigma phase precipitation temperature represented by the formula (4) is 1,000 占 폚 or lower; And
And hot-rolled and annealed the hot-rolled steel sheet,
Wherein the hot-rolled annealing is performed within 10 minutes at 1,050 to 1,100 ° C, followed by cooling with water.
(2) (Mo + Ni) / (Cr + 10 * N)? 3.5
(4) 202 - 1505 * C - 24.8 * Si - 0.30 * Mn + 40.1 * Cr - 12.6 * Ni + 76.3 * Mo - 522 * N
(Where C, Si, Mn, Cr, Ni, Mo and N mean the content (% by weight) of each element) 5. The method of claim 4,
Cold-rolling the hot-rolled annealed steel sheet; And
And cold rolling and annealing the cold-rolled steel sheet,
Wherein the cold-rolling annealing is carried out at 1,050 to 1,100 ° C for 10 minutes or less, followed by cooling with water, thereby producing a high corrosion resistant austenitic stainless steel. 5. The method of claim 4,
Wherein the slab satisfies a value calculated from the following formula (1) is 70 or more.
(1) -150 + 7.05 * Cr + 1.18 * Ni + 11.6 * Mo + 57.6 * N
(Where Cr, Ni, Mo, and N mean the content (% by weight) of each element) 5. The method of claim 4,
Wherein the slab satisfies the following formula (3): &quot; (1) &quot;
(3) Cr + Ni? 40
(Where Cr and Ni mean the content (% by weight) of each element)

Images