KR101615453B1 - Super duplex stainless steel having superior yield strength and production method thereof - Google Patents

Abstract

The present invention relates to a heat treatment method capable of improving yield strength and corrosion resistance in a super duplex stainless steel S32750 (2507 stainless steel) comprising: 24-26 wt% of chrome (Cr), 6.0-8.0 wt% of nickel (Ni), 3.5-5.0 wt% of molybdenum (Mo), 0.24-0.32 wt% of nitrogen (N), and the remaining consisting of iron (Fe) and inevitable impurities, wherein the super duplex stainless includes austenite and ferrite. When a material is heat-treated, a heating rate from 700°C to an annealing temperature is controlled at 0.33 °C/sec or less. The material is maintained at the annealing temperature of 1,030-1,050°C for 5-60 minutes, or is maintained at the annealing temperature of 1,010-1,030°C for 40-60 minutes in order to induce extraction of a secondary austenite inside the ferrite. As such, the super duplex stainless steel with excellent yield strength and corrosion resistance is able to be provided.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a super duplex stainless steel having excellent yield strength,

The present invention relates to a super duplex stainless steel having excellent yield strength and a method of manufacturing the same.

Generally, 24 to 26% by weight of chromium (Cr). Super duplex stainless steel (S32750 steel) containing 6.0 to 8.0% by weight of nickel (Ni), 3.0 to 4.0% by weight of molybdenum (Mo) and 0.2 to 0.3% by weight of nitrogen (N) is a two-phase structure of austenite and ferrite The present invention relates to a two-phase stainless steel having a pore resistance equivalent number (PREN = Cr + 3.3 (Mo + 1 / 2W) + 16N) of not less than 40 and having excellent mechanical properties and corrosion resistance, Chemical tanks, desalination plants, and seawater pumps. Because it is mainly used for parts that come into contact with seawater in the first place, high yield strength and high corrosion resistance are required. The material properties of S32750 steel specified in ASTM M240 are defined as "0.2% off set yield strength 550 MPa or more, elongation 25% or more". The base structure of super duplex steel has a structure characteristic that ferrite and austenite have the same ratio as that of the austenitic stainless steel, and the strength is higher than that of austenitic stainless steel, and the formula (pitting corrosion) and stress corrosion cracking resistance It is an advantage.

Super Duplex S32750 steel excellent in corrosion resistance has a high content of chromium and molybdenum, and sigma (sigma) phase is easily generated when it is maintained in the range of 750 to 850 ° C to exhibit brittleness and remarkably reduce corrosion resistance. Therefore, in order to suppress the generation of the sigma phase, it is necessary to control the rate of temperature rise upon heating to the target annealing temperature to avoid stagnation in a temperature region where sigma generation is easy. The sigma phase formation of the two-phase stainless steel is shown in [H. Sieurin et al., Co-author: Material Science and Engineering A (2007) 271, J. Dobranszky et al., Fourth Edition: Spectrochemica Acta Part B (2004) 1781] And it is required to be careful in annealing annealing.

A heat treatment method for annealing for solving such problems is known from Korean Patent Registration No. 1,312,783. The heat treatment method of the patent is characterized in that the temperature interval at which the sigma generation is easy is avoided by keeping the heating rate at 10 ° C / s or higher from 600 ° C to the annealing temperature at 1,060 to 1,080 ° C.

The above annealing heat treatment method is mainly applicable to hot-rolled coils having a thickness of 8 mm or less, but the same heat treatment method can be applied to a thick plate having a thickness of 10 mm or more. In fact, the Society or the NORSOK certification requires heat treatment at 1,080 ° C or higher. As mentioned earlier, the S32750 steel is used in chemical plants, desalination plants, and seawater pumps, and plates of various thicknesses ranging from 5 mm to 50 mm in thickness are used. In addition, 0.2% offset yield strength of 550 MPa or more is required over the entire thickness. However, in the case of materials having a thickness of 30 mm or more, there is a phenomenon in which 550 MPa is not satisfied due to insufficient reduction. In the case of this phenomenon, a 0.5% offset yield strength is applied to 550 MPa or more in some cases, but a fundamental solution to the yield strength problem of the material after S32750 steel is needed.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a super duplex stainless steel which is capable of changing the microstructure of a super duplex stainless steel by controlling a heating rate, an annealing temperature and a holding time, And at the same time to provide a super duplex stainless steel similar to the steels produced by the other heat treatment method, and a method of manufacturing the super duplex stainless steel.

In order to attain the above object, the present invention provides a chromium (Cr) 24 to 26% by weight. (Fe) and inevitable impurities, wherein the austenitic phase and the ferrite (B) contain 6.0 to 8.0% by weight of nickel, 3.5 to 5.0% by weight of molybdenum and 0.24 to 0.32% by weight of nitrogen, A step of rolling a super duplex stainless steel slab having a phase to produce a rolled steel sheet; And a step of subjecting the rolled steel sheet to annealing while controlling the rate of temperature rise from 700 占 폚 to the annealing temperature to 0.33 占 폚 / sec or less. The present invention also provides a method of manufacturing super duplex stainless steel.

In the present invention, the annealing temperature may be maintained for 5 to 60 minutes at an annealing temperature of 1,030 to 1,050 占 폚 or for 40 to 60 minutes at an annealing temperature of 1,010 to 1,030 占 폚.

In the present invention, it is possible to induce the precipitation of the sigma phase and the secondary austenite phase during the temperature increase by controlling the heating rate.

In the present invention, by controlling the annealing temperature and the holding time, the sigma phase precipitated during heating can be dissolved in the ferrite phase while the secondary austenite phase can remain in the ferrite phase.

In addition, the present invention relates to a chromium (Cr) 24 to 26% by weight. (Fe) and inevitable impurities, wherein the austenitic phase and the ferrite (B) contain 6.0 to 8.0% by weight of nickel, 3.5 to 5.0% by weight of molybdenum and 0.24 to 0.32% by weight of nitrogen, Phase, and a secondary austenite phase remains inside the ferrite phase.

The elongation of the super duplex stainless steel according to the present invention may be 25% or more.

The critical temperature of the super-duplex stainless steel according to the present invention may be 80 ° C or higher.

The thickness of the super duplex stainless steel according to the present invention may be 30 mm or more.

The yield strength of the super-duplex stainless steel according to the present invention may be 550 MPa or more.

According to the present invention, unlike the conventional heat treatment for avoiding the precipitation of the sigma phase, the formation of the secondary austenite phase is induced in the ferrite phase by inducing the precipitation of the sigma phase, whereby the yield of the super- The strength can be secured to a level of 550 MPa or more. In addition, the corrosion resistance of super duplex steels similar to steels produced by other heat treatment methods can be provided.

FIG. 1 is a graph showing a sigma phase generation behavior according to a heating rate of S32750 steel. FIG.
Fig. 2 is a graph showing the behavior of precipitates depending on annealing temperature and holding time of S32750 steel.
Fig. 3 is a figure comparing the microstructure of the super-duplex steel produced according to the annealing condition of the present invention and the microstructure at a generally known annealing condition.
4 is a graph showing the yield strength and the elongation change according to the annealing temperature.
5 is a graph showing changes in corrosion resistance characteristics according to the annealing temperature.

Hereinafter, the present invention will be described in detail.

The present invention relates to a super duplex stainless steel excellent in yield strength and a method for manufacturing the super duplex stainless steel.

A method of manufacturing a super duplex stainless steel according to the present invention comprises the steps of: rolling a super duplex stainless steel slab to produce a rolled steel sheet; And annealing the rolled steel sheet for annealing.

In the present invention, the super duplex stainless steel contains 24 to 26% by weight of chromium (Cr). (Ni), 3.5 to 5.0 wt% of molybdenum (Mo), 0.24 to 0.32 wt% of nitrogen (N), and the balance of iron (Fe) and unavoidable impurities. Super-duplex stainless steel (S32750 steel) is a two-phase stainless steel consisting of a two-phase structure of austenite and ferrite.

If the sigma phase remains, the yield strength increases, but the corrosion resistance and mechanical properties may deteriorate. Conversely, when the sigma phase is solidified, the yield strength is reduced, but the corrosion resistance and mechanical properties can be improved. If the secondary austenite phase remains, the yield strength, corrosion resistance characteristics and mechanical properties can be appropriately maintained. On the contrary, when the secondary austenite phase is solidified, the yield strength is lowered and the effect of improving the corrosion resistance and mechanical properties may be insignificant. Therefore, it is preferable that the sigma phase is solidified and the secondary austenite phase remains, thereby ensuring adequate yield strength, corrosion resistance and mechanical properties.

In the annealing heat treatment step according to the present invention, the temperature raising rate can be controlled, and the temperature raising rate can be controlled from 700 deg. C to 0.33 deg. C / sec or less from the annealing temperature. The lower limit of the temperature raising rate is not particularly limited, and may be 0.01 deg. C / s, 0.05 deg. C / s or 0.1 deg. C / s, for example.

In the present invention, the sigma phase can be intentionally precipitated by controlling the rate of temperature rise to 0.33 ° C / s or less. By intentionally precipitating the sigma phase, it becomes possible to deposit a large amount of the secondary austenite phase inside the ferrite phase. By controlling the heating rate in this way, it is possible to induce precipitation of the sigma phase and the secondary austenite phase during the temperature rise. If the heating rate exceeds 0.33 ° C / s, the sigma phase may be difficult to precipitate.

In the annealing heat treatment step according to the present invention, the annealing temperature may be maintained at 1,030 to 1,050 占 폚 for 5 to 60 minutes, or the annealing temperature may be maintained at 1,010 to 1,030 占 폚 for 40 to 60 minutes.

Specifically, according to one embodiment of the present invention, the annealing temperature is 1,030 to 1,050 占 폚, and the holding time may be 5 to 60 minutes, preferably 20 to 40 minutes. According to another embodiment of the present invention, the annealing temperature is 1,010 to 1,030 占 폚, and the holding time may be 40 to 60 minutes. That is, when the annealing temperature is 1,010 to 1,030 ° C., if the holding time is increased to 40 to 60 minutes, the same effect as in the case of holding the annealing temperature at 1,030 to 1,050 ° C. for 20 to 40 minutes can be obtained.

If the annealing temperature is only 1,030 占 폚, the sigma phase formed during the heating can not be dissolved and remains, so that the corrosion resistance and the mechanical characteristics may be significantly deteriorated. However, as described above, when the annealing temperature is low, if the holding time is increased, the sigma phase can be dissolved and the austenite phase can remain.

When the annealing temperature is 1,030 to 1,050 占 폚, the entire amount of the sigma phase is solved and the secondary austenite phase remains, so that excellent corrosion resistance and mechanical characteristics can be exhibited at the same time. Also, the holding time can be relatively shortened.

If the annealing temperature exceeds 1,050 占 폚, the entire increase in the yield strength can be lost by solubilizing the entire amount of the austenite phase as well as the sigma phase.

As described above, in the present invention, by controlling the annealing temperature and the holding time, the sigma phase precipitated during heating can be dissolved in the ferrite phase while the secondary austenite phase can remain in the ferrite phase.

The super duplex stainless steel according to the present invention comprises 24 to 26% by weight of chromium (Cr). (Fe) and unavoidable impurities, and having a yield strength of 550 MPa (10 MPa), a tensile strength of 500 MPa Or more. The upper limit of the yield strength is not particularly limited and may be, for example, 1,000 MPa, 800 MPa or 650 MPa. The yield strength can be measured by standard such as ASTM M240 and can generally be measured at 0.2% or 0.5% offset yield strength. In the present invention, as described above, the sigma phase is dissolved in the ferrite phase while the secondary austenite phase is retained in the ferrite phase, thereby securing a yield strength of 550 MPa or more.

The elongation of the super duplex stainless steel according to the present invention may be 25% or more. Elongation is the rate of elongation of the material in the tensile test, expressed as a percentage of the elongation. The upper limit of the elongation is not particularly limited, and may be, for example, 100%, 70% or 40%. The elongation can be measured by a standard such as ASTM M240. In the present invention, as described above, the sigma phase is dissolved in the ferrite phase while the secondary austenite phase is retained in the ferrite phase, so that not only a yield strength of not less than 550 MPa but also an elongation of not less than 25% can be secured.

The critical temperature of the super-duplex stainless steel according to the present invention may be 80 ° C or higher. The critical pitting temperature (CPT) is an indicator of corrosion resistance. The larger the value, the better the corrosion resistance. The upper limit value of the critical temperature is not particularly limited. Critical temperature can be measured by standard such as ASTM G150. For example, to measure the critical temperature, a specimen of 5 mmT with a size of 50 mmL x 50 mmW was annealed at 1,050 deg. C in accordance with ASTM G150, immersed in a 1 M NaCl solution, Voltage is applied. The temperature at which the temperature is raised from 0 ° C and the current density of 100 μA / cm 2 or more is measured for 60 seconds is called the critical temperature. In the present invention, as described above, the sigma phase is dissolved in the ferrite phase while the secondary austenite phase is retained in the ferrite phase, so that not only the yield strength of not less than 550 MPa, the elongation of not less than 25%, and excellent corrosion resistance can be secured.

The thickness of the super duplex stainless steel according to the present invention may be 30 mm or more. That is, the present invention can be applied to post materials. The upper limit of the thickness is not particularly limited and may be, for example, 100 mm, 70 mm or 50 mm.

In short, the present invention relates to chromium (Cr) 24 to 26% by weight. (Fe) and inevitable impurities, wherein the austenitic phase and the ferrite (B) contain 6.0 to 8.0% by weight of nickel, 3.5 to 5.0% by weight of molybdenum and 0.24 to 0.32% by weight of nitrogen, A method for annealing a black plate produced by rolling a super duplex S32750 steel having a microstructure on a steel sheet by controlling the rate of temperature rise from a temperature of 700 占 폚 to an annealing temperature of 0.33 占 폚 per second or less, Sigma phase and a secondary austenite phase, and further characterized in that the sigma phase and the secondary austenite phase formed during the heating are maintained at the annealing temperature of 1,030 to 1,050 占 폚 for 5 to 60 minutes or the annealing temperature of 1,010 to 1,030 Lt; 0 > C for 40 to 60 minutes to allow the secondary austenite phase to remain in the ferrite phase while finally allowing the sigma phase to solidify inside the ferrite structure And a heat treatment annealing method for a thick plate of a super duplex steel. Accordingly, the present invention can provide a method of manufacturing a super duplex steel improved in yield strength of a post-material having the above-described structure.

As described above, the present invention provides a method of controlling super-duplex stainless steel by controlling the rate of temperature rise from 700 ° C to the annealing temperature to 0.33 ° C / sec or less, maintaining the annealing temperature at 1,030 to 1,050 ° C for 5 to 60 minutes, And is maintained at 1,010 to 1,030 占 폚 for 40 to 60 minutes. Further, the microstructure of the super duplex steel produced by the annealing method induces precipitation of the secondary austenite phase inside the ferrite phase, thereby securing a yield strength of 550 MPa or more, and at the same time, And has similar corrosion resistance characteristics.

Hereinafter, a method of annealing a super duplex steel according to the present invention will be described in detail with reference to the accompanying drawings. Tissue characteristics were observed using an optical microscope (OM), a scanning electron microscope (SEM), and the like.

Fig. 1 is a graph showing the behavior of the sigma phase precipitation according to the heating rate, and the reasons for limiting the heating rate in the present invention will be described. As is well known in Korean Patent Registration No. 1,312,783, it is known that sigma phase is easily formed at a temperature raising rate of 5 ° C / s or less in a temperature range in which a sigma phase is easily generated, thereby significantly lowering impact characteristics and corrosion resistance characteristics. In order to avoid this, it is a generally known heat treatment method to heat a formed sigma phase by raising the temperature raising rate to preferably not lower than 10 캜 / s to inhibit the formation of the sigma phase or keeping it at a temperature of 1,080 캜 or higher for a long time.

In contrast, in the present invention, the temperature raising rate is controlled to be 0.33 ° C / s or less in order to intentionally precipitate the sigma phase. By intentionally precipitating the sigma phase, it becomes possible to deposit a large amount of the secondary austenite phase inside the ferrite phase. On the right side of Fig. 1, a photograph of the structure according to the heating rate was shown. As the heating rate was lowered, the precipitation amount of the sigma phase was increased. That is, when the heating rate is 0.67 ° C / s, the sigma phase is insufficiently precipitated and the secondary austenite phase is hardly precipitated. However, when the heating rate is 0.33 ° C / s or lower, the sigma phase is sufficiently precipitated and secondary austenite It can be confirmed that a large amount of phase was precipitated.

2 is a graph showing changes in precipitates depending on the annealing temperature and the holding time after the temperature rise. In Fig. 2, X indicates that the sigma phase remains, & cir & indicates that the sigma phase is solved and the secondary austenite phase remains, and DELTA indicates that the secondary phase is solidified to the secondary austenite phase. The intentionally formed sigma phase and the secondary austenite phases appear to be different in the manner of being employed depending on the annealing temperature. In the range of 1,000 to 1,020 ° C, the sigma phase formed during the temperature increase is not solubilized and remains, thereby significantly lowering the corrosion resistance and mechanical characteristics. In the range of 1,030 to 1,050 占 폚, the entire amount of the sigma phase is solved and the secondary austenite phase remains, thereby exhibiting excellent corrosion resistance and mechanical characteristics at the same time. In the temperature range of 1,060 占 폚 or more, the entire increase in the yield strength is eliminated by solubilizing the entire amount of the austenite phase as well as the sigma phase.

Fig. 3 is a figure comparing the microstructure (left-hand fabricated structure) of a super-duplex steel produced according to the annealing condition of the present invention and the microstructure (right-handed comparative structure) under a generally known annealing condition. As can be seen from Fig. 3, it can be seen that the secondary austenite phases are finely precipitated in the ferrite phase in the inventive structure.

4 is a graph showing the yield strength and elongation according to the structure. When the sigma phase remains in the range of 1,000 to 1,020 ° C, the yield strength is increased but the brittleness of the material is caused and the elongation is very low. The sigma phase was solidified and the yield strength reached 550 MPa, which is the ASTM standard, at the temperature range (1,030 to 1,050 ° C) during which the secondary austenite phase remained, and at the same time, the elongation was much higher than the ASTM standard of 25% . At a temperature interval (1,060 ° C or higher) in which the entire amount of the secondary austenite is solidified, the elongation is high but the yield strength falls to 550 MPa or less.

FIG. 5 is a graph showing the change of the corrosion resistance according to the annealing temperature, and shows the critical temperature (CPT) according to the annealing temperature. The corrosion resistance of super duplex steels is remarkably lowered in the section where the sigma phase remains (less than 1,030 ° C), and the corrosion resistance is improved from the temperature zone (above 1,030 ° C) where all the sigma phase is solidified.

From the results shown in Figs. 1 to 5, it can be seen that, in the case of the super-duplex steel plate, the yield strength is affected by the microstructure. The present invention can provide a plate material after maintaining the corrosion resistance and improving the yield strength, as compared with the conventional heat treatment method.

Therefore, as described above, the present invention is characterized by the heat treatment method of super duplex steel. By controlling the heating rate, annealing temperature and holding time, a structure having a fine secondary austenite phase inside the ferrite phase is realized, The yield strength of the duplex steel can be improved.

That is, the present inventors completed the present invention by repeatedly carrying out research and experiments for securing both the yield strength of 550 MPa or more and the excellent corrosion resistance characteristics while having excellent properties of the super duplex steel. As a result, in the present invention, the sigma phase and the secondary austenite phase are finely precipitated in the ferrite phase during the heat treatment by controlling the heat treatment temperature raising rate to 0.33 캜 / s or less. And the secondary austenite phase remains in the ferrite phase and the sigma phase is entirely dissolved by maintaining the annealing temperature at 1,030 to 1,050 DEG C for 5 to 60 minutes or the annealing temperature at 1,010 to 1,030 DEG C for 40 to 60 minutes, 30 mmT or more, preferably 35 mmT or more.

Claims (10)

24 to 26% by weight of chromium (Cr). (Fe) and inevitable impurities, wherein the austenitic phase and the ferrite (B) contain 6.0 to 8.0% by weight of nickel, 3.5 to 5.0% by weight of molybdenum and 0.24 to 0.32% by weight of nitrogen, A step of rolling a super duplex stainless steel slab having a phase to produce a rolled steel sheet; And
Annealing the rolled steel sheet while controlling the temperature raising rate from 700 DEG C to 0.33 DEG C / sec or less from the annealing temperature,
In the annealing heat treatment step, the annealing temperature is maintained at 1,030 to 1,050 占 폚 for 5 to 60 minutes, or the annealing temperature is maintained at 1,010 to 1,030 占 폚 for 40 to 60 minutes.
delete The method according to claim 1,
Wherein the heating rate is controlled so as to induce precipitation of a sigma phase and a secondary austenite phase during heating.
The method of claim 3,
By controlling the annealing temperature and the holding time, the sigma phase precipitated during the heating is dissolved in the ferrite phase while the secondary austenite phase remains in the ferrite phase.
The method according to claim 1,
Wherein the thickness of the super-duplex stainless steel is 30 mm or more.
24 to 26% by weight of chromium (Cr). (Fe) and inevitable impurities, and further contains, by weight, 6.0 to 8.0% by weight of nickel (Ni), 3.5 to 5.0% by weight of molybdenum (Mo) and 0.24 to 0.32%
An austenite phase and a ferrite phase, wherein a secondary austenite phase remains inside the ferrite phase.
The method according to claim 6,
And an elongation of 25% or more.
The method according to claim 6,
Wherein the critical temperature is at least 80 < RTI ID = 0.0 > C. ≪ / RTI >
The method according to claim 6,
A super duplex stainless steel characterized by a thickness of at least 30 mm.
10. The method according to any one of claims 6 to 9,
Wherein the yield strength is 550 MPa or more.

Images