KR102003223B1 - Lean duplex stainless steel with improved bending properties and method of manufacturing the same - Google Patents

Abstract

Lean duplex stainless steel with improved bendability is disclosed. According to the lean duplex stainless steel according to an embodiment of the present invention, in weight%, C: 0.08% or less (excluding 0), Si: 1% or less (excluding 0), Mn: 2 to 4%, Cr: 19 to 22%, N: 0.2-0.3%, Ni: 0.5-1.5, Cu: 1% or less (excluding 0), the balance Fe and other unavoidable impurities, and satisfy the following formula (1).
Equation (1): 551-462 (C + N)-9.2 Si-8.1 Mn-13.7 Cr-29 (Ni + Cu)-18.5 Mo-0.1 x Annealing temperature (° C) ≤ -50
(C, N, Si, Mn, Cr, Ni, Cu, Mo means the weight percent of carbon, nitrogen, silicon, manganese, chromium, nickel, copper, molybdenum, respectively.)

Description

Lean duplex steel with improved bendability and manufacturing method thereof {LEAN DUPLEX STAINLESS STEEL WITH IMPROVED BENDING PROPERTIES AND METHOD OF MANUFACTURING THE SAME}

The present invention relates to a duplex stainless steel having two phases of an austenite phase and a ferrite phase, and more particularly, to control the difference in hardness between the austenite phase and the ferrite phase, and to improve bendability, such as Ni, Mo, Si, Cu, etc. The present invention relates to a lean duplex stainless steel having a lower content of alloying elements.

Among the stainless steels, especially duplex stainless steel is a stainless steel having a microstructure in which an austenitic phase and a ferrite phase are mixed. It shows all the characteristics of an austenitic stainless steel and a ferritic stainless steel. To date, various types of duplex stainless steel have been developed. come.

On the other hand, the most widely used duplex stainless steel in high corrosion resistance environment is ATI A205 (UNS S31803 or S32205) of 22% Cr, 5.5% Ni, 3% Mo, 0.16% N components. This steel provides excellent corrosion resistance in a variety of corrosive environments, showing better corrosion resistance than AISI's austenitic stainless steels such as 304 and 316.

However, since the duplex stainless steel contains expensive elements such as Ni and Mo, not only the manufacturing cost is increased but also the Ni and Mo are consumed, and thus the price competitiveness with other steel grades is inferior.

Recently, in order to solve such a problem, lean duplex has eliminated expensive alloying elements such as Ni and Mo among duplex stainless steels, and added low-cost alloying elements in place of these elements to secure the advantages of alloying cost. ) Interest in stainless steel is increasing.

These lean duplex stainless steels have the same corrosion resistance as 304 and 316 steels, which are roughly classified as conventional austenitic stainless steels, and have a low Ni content so that they can be economically secured and have high strength, and thus require fresh water, pulp, paper, and chemical facilities. Has gained the spotlight for its industrial equipment steels.

Such lean duplex steels include, for example, S32304 (typical component 23Cr-4Ni-0.13N) standardized in ASTMA240, S32101 (typical component 21Cr-1.5Ni-5Mn-0.22N) standardized in ASTMA240.

The steels are designed for cold workability, i.e., strengthening of corrosion resistance rather than formability, so that a technical solution requiring corrosion resistance may be provided, but ductility, which is a factor related to workability, is inferior to austenitic stainless steel, requiring molding and bending There are limitations to their application in various industries.

In particular, crack formation is a frequent problem in bending molding. Specifically, cracking occurs more frequently when the bending direction becomes parallel to the rolling direction. Accordingly, there is a new demand for the development of duplex stainless steel with improved bendability while reducing elongation and corrosion resistance equivalent to austenitic stainless steel while reducing Ni and Mo.

Embodiments of the present invention are to provide a lean duplex stainless steel with improved bendability by controlling the hardness difference between the austenite phase and the ferrite phase, and a manufacturing method thereof.

According to the lean duplex stainless steel with improved bendability according to an embodiment of the present invention, in weight%, C: 0.08% or less (excluding 0), Si: 1% or less (excluding 0), Mn: 2 to 4%, Cr: 19 to 22%, N: 0.2 to 0.3%, Ni: 0.5 to 1.5, Cu: 1% or less (excluding 0), the balance Fe and other unavoidable impurities, and satisfy the following formula (1).

Equation (1): 551-462 (C + N)-9.2 Si-8.1 Mn-13.7 Cr-29 (Ni + Cu)-18.5 Mo-0.1 x Annealing temperature (° C) ≤ -50

Here, C, N, Si, Mn, Cr, Ni, Cu, Mo means the weight percent of carbon, nitrogen, silicon, manganese, chromium, nickel, copper, molybdenum, respectively.

In addition, according to an embodiment of the present invention, the lean duplex stainless steel with improved bendability is Mo; It may further comprise 0.5% or less.

In addition, according to an embodiment of the present invention, the lean duplex stainless steel having improved bendability may have a fine hardness variation of less than or equal to 120 Hv in an austenitic phase and a ferrite phase.

In addition, according to an embodiment of the present invention, the ferrite phase fraction of the lean duplex stainless steel with improved bendability may be 45 to 60%.

In addition, according to an embodiment of the present invention, the lean duplex stainless steel having improved bendability may satisfy the following Equation (2) related to Mn, Ni, and Cu contained in the austenite phase.

Equation (2): Mn + 0.5 * Ni + N ≥ 4

Here, Mn, Ni, N means the weight% of manganese, nickel, and nitrogen in an austenite phase, respectively.

Method for producing lean duplex stainless steel with improved bendability according to an embodiment of the present invention in weight%, C: 0.08% or less (excluding 0), Si: 1% or less (excluding 0), Mn: 2 to 4%, Hot rolling a stainless steel comprising Cr: 19-22%, N: 0.2-0.3%, Ni: 0.5-1.5, Cu: 1% or less (excluding 0), balance Fe and other unavoidable impurities; And annealing the hot rolled steel sheet in a range of 1050 to 1200 ° C. to satisfy the following Equation (1).

Equation (1): 551-462 (C + N)-9.2 Si-8.1 Mn-13.7 Cr-29 (Ni + Cu)-18.5 Mo-0.1 x Annealing temperature (° C) ≤ -50

Here, C, N, Si, Mn, Cr, Ni, Cu, Mo means the weight percent of carbon, nitrogen, silicon, manganese, chromium, nickel, copper, molybdenum, respectively.

In addition, according to an embodiment of the present invention, Mo; It may further comprise 0.5% or less.

Further, according to an embodiment of the present invention, cold rolling the hot-rolled annealing steel sheet to obtain a cold-rolled steel sheet; And annealing the cold rolled steel sheet at a temperature of 1050 to 1200 ° C .;

According to the disclosed embodiment, it is possible to provide a lean duplex stainless steel having improved bendability and a method of manufacturing the same by preventing cracks generated at the bent portion during bending.

1 is a photograph of a crack generated in the bent portion of the duplex stainless steel.
2 is a graph calculated through the numerical analysis of the state diagram of the duplex stainless steel.
Figure 3 is a graph showing the correlation between the cracks generated during the evaluation of the hardness and fine hardness deviation of the austenite phase and the ferrite phase according to the embodiment of the present invention.
4 is a graph showing the correlation between the stabilization of the duplex stainless steel according to the embodiment of the present invention and the hardness deviation of the austenitic phase and the ferrite phase.

The following embodiments are presented to sufficiently convey the spirit of the present invention to those skilled in the art. The present invention is not limited to the embodiments presented herein but may be embodied in other forms. The drawings may omit illustrations of parts not related to the description in order to clarify the present invention, and may be exaggerated to some extent in order to facilitate understanding.

Throughout the specification, when a part is said to "include" a certain component, it means that it can further include other components, without excluding other components unless specifically stated otherwise.

Singular expressions include plural expressions unless the context clearly indicates an exception.

Hereinafter, with reference to the accompanying drawings an embodiment according to the present invention will be described in detail. First, the duplex stainless steel will be described, and then the manufacturing method of the duplex stainless steel will be described.

MEANS TO SOLVE THE PROBLEM As a result of carrying out various examination about the cause of the crack which arises when a lean duplex stainless steel is bent and performed, the following knowledge was acquired.

1 is a photograph showing a representative crack shape generated at the bent portion of the duplex stainless steel, it can be seen that the crack occurs at the interface between the two phases. This means that cracks occur because the two phases of the duplex steel have different properties.

In order to compare the characteristics of the austenite phase and the ferrite phase, the microhardness of each phase was evaluated and the microhardness deviation between the two phases was derived. At this time, it is measured by Vickers hardness measurement method and a unit is Hv.

The present inventors compared the crack lengths with the micro hardness deviations (austenitic-ferrite) of the austenitic phase and the ferrite phase for each alloy, and confirmed that no crack occurred when the micro hardness deviation showed 120 Hv or less.

On the other hand, Transformation Induced Plasticity (TRIP) is a known effect on metastable austenitic steels. Lean duplex stainless steel contains a metastable austenite phase that activates calcined organic transformation and exhibits excellent moldability by calcined organic transformation.

However, the transformation of the metastable austenite phase to the martensite phase leads to an increase in the hardness of the austenite phase region, which makes the hardness deviation from the ferrite phase large. In moldings such as bending, this hardness variation causes cracking at the interface.

More specifically, the hardness of austenite is higher than that of ferrite, and as stress is concentrated in relatively soft ferrite during bending, cracks, bursts, etc. occur at the bent portion.

Accordingly, in order to secure the bendability of the lean duplex stainless steel, the hardness variation was minimized. This can be achieved by controlling the austenite phase stabilization taking into account the alloying component and the annealing temperature. Details thereof will be described later.

Lean duplex stainless steel with improved bendability according to an aspect of the present invention, in weight%, C: 0.08% or less (excluding 0), Si: 1% or less (excluding 0), Mn: 2 to 4%, Cr: 19 to 22%, N: 0.2 to 0.3%, Ni: 0.5 to 1.5, Cu: 1% or less (excluding 0), Mo: 0.5% or less (excluding 0), balance Fe and other unavoidable impurities.

Hereinafter, the reason for the numerical limitation of the content of the alloying component in the embodiment of the present invention will be described. In the following, the unit is% by weight unless otherwise specified.

The content of C is 0.08% or less.

Carbon (C) may be used in place of an expensive element such as Ni as an austenite phase forming element, and is an effective element for increasing the material strength by solid solution strengthening. A small amount of C must be added in order to contribute to the austenite phase stability. However, excessive addition of C forms segregation and coarse carbide in the center of the material during manufacturing, which adversely affects the post-hot rolling-annealing-cold rolling-cold annealing process and the corrosion resistance at the ferrite-austenite phase boundary. In order to maximize corrosion resistance, it is preferable to add within a range of 0.08% or less, because it easily bonds with carbide forming elements such as effective Cr to lower Cr content around grain boundaries to reduce corrosion resistance.

The content of Si is 1.0% or less.

Silicon (Si) is partially added for the deoxidation effect, and is an element that is concentrated in ferrite during annealing heat treatment as a ferrite phase forming element. In duplex stainless steel, Si is preferably added at least 0.2% to secure an appropriate ferrite phase fraction. However, if the content is excessive, the hardness of the ferrite phase is sharply increased to reduce the workability and impact characteristics, the slag fluidity is reduced during steelmaking, and the inclusions form oxygen inclusions to deteriorate the corrosion resistance. The upper limit can be limited to 1.0%.

The content of Mn is 2-4%.

Manganese (Mn) is an element that controls deoxidation and melt flow in the steelmaking process. It is also an element to increase the nitrogen solubility in duplex stainless steel, and as an austenite forming element, it can be used to replace expensive Ni. However, when the content exceeds 4%, excess Mn combines with S in the steel to form MnS, which leads to a decrease in corrosion resistance. On the other hand, when the Mn content is less than 2%, it is difficult to secure an appropriate austenite phase fraction even by adjusting the austenite forming elements Ni, Cu, N, etc. Can not get Therefore, it is preferable to limit the content of Mn to 2 to 4%.

The content of Cr is 19 to 22%.

Chromium (Cr), together with Si, plays a main role in securing the ferrite phase of the duplex stainless steel as a ferrite phase stabilizing element, and is an essential element added for securing corrosion resistance. However, if the content is excessive, there is a problem in that the corrosion resistance is increased but the content of expensive Ni or other austenite forming elements must be increased in order to maintain the phase ratio, and the upper limit thereof may be limited to 22%.

The content of N is 0.20 to 0.30%.

Nitrogen (N) is an element that greatly contributes to stabilization of the austenite phase together with Ni in duplex stainless steel, and is one of the elements in which thickening occurs in the austenite phase due to the fast diffusion rate in the solid phase during annealing heat treatment.

Therefore, by increasing the content of N, it is possible to improve the corrosion resistance and strength incidentally, but since the solid solubility of N can be changed according to the content of Mn added, the content control is necessary.

When the content of N exceeds 0.3% in the Mn content range of the present invention, blow holes and pin holes may occur during casting due to excess nitrogen solubility, resulting in product surface defects. There is a problem that it is difficult to manufacture a stable steel. Therefore, it is preferable to limit the content of nitrogen (N) to 0.20 to 0.30%.

The content of Ni is 0.5 to 1.5%.

Nickel (Ni), together with Mn, Cu, and N, is an austenite stabilizing element, and it is preferable to add 0.5% or more, which plays a major role in securing the austenite phase of duplex stainless steel.

On the other hand, Ni is an expensive element, and instead of reducing the Ni content as much as possible to reduce the cost, there is a tendency to maintain the phase ratio balance by reducing Ni by increasing Mn and N, which are other austenite phase forming elements.

However, 0.5% or more should be added to secure sufficient austenite stability in order to suppress the formation of calcined organic martensite generated during cold working. However, if the content is excessive, it is difficult to secure an appropriate austenite fraction due to the increase of austenite fraction, and in particular, it is difficult to secure competitiveness due to the increase in manufacturing cost of products due to expensive Ni. Therefore, the upper limit should be limited to 1.5%. Can be.

The content of Cu is 1.0% or less.

Copper (Cu) suppresses work hardening due to the formation of work organic martensite phase and increases the manufacturing cost of the product due to expensive Cu in consideration of cost reduction and elements contributing to soft nitriding of austenitic stainless steel. It is preferable to set the upper limit to 1.0% in order to prevent that from happening.

For example, lean duplex stainless steel according to an embodiment of the present invention is Mo; It may further comprise 0.5% or less.

The content of Mo is 0.5% or less.

Molybdenum (Mo) is an element that is very effective for improving corrosion resistance while stabilizing ferrite with Cr, but the upper limit is set to 0.5% in order to prevent an increase in the manufacturing cost of a product due to a very expensive Mo in consideration of cost reduction. It is desirable to.

The remaining component of the present invention is iron (Fe). However, in the conventional manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably mixed, and thus cannot be excluded. Since these impurities are known to those skilled in the art, all of them are not specifically mentioned in the present specification.

Lean duplex stainless steel according to an embodiment of the present invention is composed of a two-phase microstructure of austenite and ferrite.

The austenitic and ferrite structures referred to in the present invention mean that the ferrite phase and the austenite phase occupy most of the structure, and that stainless steel is not formed only with the ferrite phase and the austenite phase. For example, the fact that the ferrite phase and the austenite phase occupy most of the structure means that the sum of the ferrite phase and the austenite phase accounts for 90% or more of the stainless steel forming structures, except for the ferrite phase and the austenite phase. The austenitic martensite phase can be occupied.

According to one embodiment of the present invention, the lean duplex stainless steel satisfying the above-described alloy composition may satisfy the following formula (1).

Equation (1): 551-462 (C + N)-9.2 Si-8.1 Mn-13.7 Cr-29 (Ni + Cu)-18.5 Mo-0.1x Annealing temperature (° C) ≤ -50

Here, C, N, Si, Mn, Cr, Ni, Cu, Mo means the weight percent of carbon, nitrogen, silicon, manganese, chromium, nickel, copper, molybdenum, respectively.

As described above, bendability in bending conditions can be improved only by reducing the hardness difference between the austenitic phase and the ferrite phase of lean duplex stainless steel, and the hardness deviation is due to the transformation of the metastable austenite phase into the martensite phase. Confirmed.

In general, the calcined organic transformation of the austenite phase is predicted by the Nohara formula Md ₃₀ for the austenite composition. Md ₃₀ refers to the temperature at which martensite is 50% formed from austenite at a true strain of 0.3, 551-462 (C + N)-9.2 Si-8.1 Mn-13.7 Cr-29 (Ni + Cu)-18.5 Mo- It is expressed as 68 Nb. The predicted temperature using Md ₃₀ is used to determine the stability of the austenite phase.

In general, the lower the Md ₃₀ temperature, the higher the stability of the austenite phase, and thus the plastic organic transformation is suppressed. Therefore, in order to reduce the micro-hardness variations on the austenite phase and ferrite, it is necessary to inhibit the transformation of martensite phase to the austenite phase, lower Md ₃₀ To this end, It is necessary to adjust the alloying component to have a temperature.

However, since duplex stainless steel contains both austenite phase and ferrite phase, Md ₃₀ There is a limit in indicating the degree of stabilization of the austenoid phase alone.

2 is a graph calculated through the numerical analysis of the state diagram of the duplex stainless steel. Referring to Figure 2, it can be seen that the fraction of the austenite phase and the ferrite phase changes with temperature.

Table 1 below is a value of the austenitic phase fraction according to the temperature and the content of the alloying elements contained in the austenitic phase through numerical analysis.

Temperature (℃) Austenitic phase fraction (%) Alloy element content in austenitic phase (wt%) C Mn Cr Ni Si Cu N 1080 74.5 0.038 3.136 19.439 0.996 0.717 0.864 0.304 1150 62.0 0.043 3.191 19.190 1.035 0.704 0.884 0.342

Referring to Table 1, it can be seen that the fraction of the austenite phase changes with temperature, and the alloying element content in the austenite phase changes. Therefore, it can be predicted that the degree of stabilization of the austenite phase, which changes depending on the content of the element, will also change depending on the annealing temperature.

Applying this, Md ₃₀ and the annealing temperature of the alloying components were used to derive Equation (1) indicating the degree of stabilization of the austenite phase in duplex stainless steel.

Formula (1): 551-462 (C + N)-9.2Si-8.1Mn-13.7Cr-29 (Ni + Cu)-18.5Mo-0.1x Annealing Temperature (℃)

As described above, in order to maintain the fine hardness difference between the austenite phase and the ferrite phase at 120 Hv or less, the fine hardness of the various alloy components was evaluated.

4 is a graph showing the correlation between the stabilization of the duplex stainless steel according to an embodiment of the present invention and the hardness deviation of the austenitic phase and the ferrite phase.

Referring to FIG. 4, as the stabilization temperature of the duplex stainless steel is increased, the fine hardness deviation of the austenitic phase and the ferrite phase increases, and the fine hardness deviation of the austenitic phase and the ferrite phase desired in the present invention is satisfied to 120 Hv or less. In order to confirm that Equation (1) must satisfy the condition of -50 or less.

In addition, according to an embodiment of the present invention, the fine hardness variation of the austenitic phase and the ferrite phase of the lean duplex stainless steel satisfying the aforementioned alloy composition may be Hv 120 or less.

Figure 3 is a graph showing the correlation between the cracks generated when the fine hardness deviation and the bendability evaluation of the austenite phase and the ferrite phase according to the embodiment of the present invention.

Referring to FIG. 3, it can be seen that cracks do not occur when the micro hardness variation of the austenite phase and the ferrite phase is 120 or less.

In addition, according to one embodiment of the present invention, the ferrite phase fraction of the lean duplex stainless steel satisfying the above-described alloy composition may be 45 to 60% by area fraction.

If the ferrite fraction is less than 45%, sufficient corrosion resistance cannot be obtained, and there is a problem of losing the characteristics of the duplex stainless steel, whereas if it exceeds 60%, the work hardening effect does not occur, resulting in the absence of metamorphic organic martensite. It is difficult to secure ductility.

In addition, according to an embodiment of the present invention, the alloying element in the austenitic phase contained in the lean duplex stainless steel satisfying the above-described alloy composition may satisfy the following formula (2).

Equation (2): Mn + 0.5 * Ni + N ≥ 4

Here, Mn, Ni, N means the weight percent of manganese, nickel, and nitrogen, respectively.

The degree of stabilization of the duplex stainless steel defined in the present invention is related to the deformation organic transformation from the austenitic phase to the martensite phase of the two phases during bending molding, and thus to the fraction of the austenitic phase and the component elements in the austenitic phase that change with annealing temperature get affected.

In this regard, the contents of Mn, Ni, and N, which are austenite stabilizing elements, are derived from Equation 2.

For example, when the formula (2) is satisfied in the austenite phase included in lean duplex stainless steel, the degree of stabilization of the duplex stainless steel may be -50 or less.

Accordingly, the lean duplex stainless steel having improved bendability according to an embodiment of the present invention may have a fine hardness variation of less than 120 Hv in the austenitic phase and the ferrite phase. The smaller the hardness variation is, the smaller the crack does not occur, which is advantageous for bendability.

Next, a method of manufacturing lean duplex stainless steel having improved bendability according to another aspect of the present invention will be described.

Method for producing lean duplex stainless steel with improved bendability according to an embodiment of the present invention, in weight%, C: 0.08% or less (excluding 0), Si: 1% or less (excluding 0), Mn: 2 to 4% Hot rolling a stainless steel comprising Cr: 19-22%, N: 0.2-0.3%, Ni: 0.5-1.5, Cu: 1% or less (excluding 0), balance Fe and other unavoidable impurities; And annealing the hot rolled steel sheet in a range of 1050 to 1200 ° C. to satisfy the following Equation (1).

In addition, cold rolling the hot-rolled annealing steel sheet to obtain a cold-rolled steel sheet; And annealing the cold rolled steel sheet at a temperature of 1050 to 1200 ° C .;

The reason for the numerical limitation of the alloy element content is as described above.

The stainless steel comprising the above composition can be hot rolled and cold rolled to form a final product. In addition, the annealing heat treatment may be performed on the hot rolled steel sheet and the cold rolled steel sheet in order to remove the deformation accumulated in the hot rolling or cold rolling process to improve bendability.

At this time, the annealing heat treatment may be performed for 30 seconds or more at a temperature of 1050 to 1200 ℃.

In order to increase austenite stabilization, the annealing heat treatment temperature must be high. This is because the higher the temperature, the more active the concentration of C and N into the austenite phase in the two phases. Specifically, when the annealing heat treatment temperature is 1050 ° C. or higher, the degree of stabilization of the austenite phase is increased due to the concentration of C and N, and the transformation to the martensite phase can be suppressed.

However, when the annealing heat treatment temperature exceeds 1250 ℃, the diffusion is active at high temperatures, excessive concentration of C, N occurs, stabilization of the austenite phase occurs, the formation of plastic organic martensite is suppressed to stress in the section of deformation It is not possible to obtain a work hardening index that can maintain.

As described above, when temperature is controlled during annealing heat treatment together with the alloying component, it is possible to reduce the fine hardness variation of the austenite phase and the ferrite phase to suppress cracking during bending.

Lean duplex stainless steel produced according to this satisfies the following formula (1).

Equation (1): 551-462 (C + N)-9.2 Si-8.1 Mn-13.7 Cr-29 (Ni + Cu)-18.5 Mo-0.1 x Annealing temperature (° C) ≤ -50

Here, C, N, Si, Mn, Cr, Ni, Cu, Mo means the weight percent of carbon, nitrogen, silicon, manganese, chromium, nickel, copper, molybdenum, respectively.

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

Example

As shown in Table 2, molten steel was produced while changing the content of each component of the steel, and manufactured by applying a twin roll type sheet casting method of manufacturing a thin sheet by passing molten steel between a pair of casting rolls. The thin plate manufactured by passing through a pair of casting rolls was rolled using an inline roller continuously arranged with the pair of casting rolls to produce a plate material having a thickness of 3 mm.

C Si Mn Cr N Ni Cu Mo One 0.042 0.64 3.07 20.1 0.247 1.12 0.79 0.1 2 0.042 0.58 3.11 20.7 0.237 0.84 0.85 0.2 3 0.035 0.75 2.99 21.3 0.225 0.95 0.80 0.0 4 0.041 0.82 2.98 19.8 0.232 1.24 0.82 0.1 5 0.027 0.62 2.89 20.1 0.241 0.98 0.90 0.4 6 0.030 0.75 3.00 20.3 0.240 0.90 0.80 0.2 7 0.035 0.72 2.96 20.4 0.234 0.94 0.74 0.0 8 0.032 0.63 2.84 19.7 0.256 0.80 0.78 0.2 9 0.031 0.70 3.05 20.3 0.217 0.90 0.80 0.4 10 0.035 0.74 2.99 20.0 0.232 0.93 0.79 0.0 11 0.028 0.60 2.82 19.8 0.230 0.90 0.87 0.3

The steel according to Table 2 was used for the experiment.

Hot-rolled annealing was performed continuously on the produced plate. The annealing temperature during the continuous annealing treatment was in the temperature range of 1050 to 1200 ℃, proceeded for 30 seconds or more at each temperature.

For the plate produced in this manner, the micro hardness variation of the austenite phase and the ferrite phase was measured using the Vickers hardness measurement method, and the bendability evaluation was performed. The bendability was evaluated by bending 180 ° using a 20mm wide 120mm thick 3mm specimen and measuring the length of cracks in the bent portion. If no cracks occur, the crack length is zero, and if fully broken, the crack length corresponds to 20 mm, the width of the entire specimen.

The ferrite fraction was measured using a ferrite scope on a 3 mm thick hot-annealed material. Ferrite scope is a device for measuring the fraction of ferrite phase by using the magnetism of the material, Fisher's "Ferritescope MP30" was used.

The austenitic stability of the duplex stainless steel according to the annealing temperature in each component, the fine hardness variation of the austenitic phase and the ferrite phase, the ferrite content, the value of the formula (2) and the bendability evaluation results are shown in Table 3 below.

No. Remarks Annealing Temperature
(℃) Stabilization
(℃) Hardness difference
(ΔHv) ferrite
content Mn + 0.5 * Ni + N Crack length
(mm) One Example 1 1050 -50.2 112.9 55.3 4.02 0 Example 2 1100 -55.2 105.5 55.7 4.09 0 Example 3 1150 -60.2 86.9 56.3 4.17 0 Example 4 1200 -65.2 93.9 58.4 4.27 0 2 Example 5 1050 -50.0 113.4 56.4 4.03 0 Example 6 1100 -55.0 108.3 56.8 4.09 0 Example 7 1150 -60.0 107.1 57.4 4.17 0 Example 8 1200 -65.0 105.7 57.2 4.26 0 3 Comparative Example 1 1050 -48.2 124.5 56.5 3.99 3 Example 9 1100 -53.2 112.0 56.8 4.06 0 Example 10 1150 -58.2 109.6 58.1 4.14 0 Example 11 1200 -63.2 113.9 60.2 4.23 0 4 Comparative Example 2 1050 -45.3 128.7 55.4 3.96 16 Example 12 1100 -50.3 112.4 55.6 4.03 0 Example 13 1150 -55.3 115.8 55.6 4.12 0 Example 14 1200 -60.3 113.0 57.1 4.21 0 5 Comparative Example 3 1050 -45.0 132.4 46.3 3.94 12 Example 15 1100 -50.0 120.0 48.2 4.00 0 Example 16 1150 -55.0 117.3 48.7 4.09 0 Example 17 1200 -60.0 114.0 49.5 4.17 0 6 Comparative Example 4 1050 -40.1 126.4 47.8 3.92 10 Comparative Example 5 1100 -45.1 128.6 46.7 3.99 20 Example 18 1150 -50.1 110.5 51.2 4.07 0 Example 19 1200 -55.1 111.2 50.3 4.17 0 7 Comparative Example 6 1050 -36.8 153.5 44.4 3.86 20 Comparative Example 7 1100 -41.8 129.3 42.6 3.93 18 Comparative Example 8 1150 -46.8 149.3 43.9 3.99 20 Example 20 1200 -51.8 109.1 44.9 4.08 0 8 Comparative Example 9 1050 -35.7 144.6 47.7 3.77 20 Comparative Example 10 1100 -40.7 130.3 52.6 3.83 11 Comparative Example 11 1150 -45.7 123.7 50.8 3.91 18 Example 21 1200 -50.7 117.0 52.6 4.00 0 9 Comparative Example 12 1050 -34.3 176.5 52.9 3.74 20 Comparative Example 13 1100 -39.3 129.0 50.4 3.83 20 Comparative Example 14 1150 -44.3 137.2 51.8 3.90 20 Comparative Example 15 1200 -49.3 122.3 52.9 3.99 5 10 Comparative Example 16 1050 -32.8 151.1 43.7 3.78 20 Comparative Example 17 1100 -37.8 129.3 46.3 3.84 20 Comparative Example 18 1150 -42.8 134.7 47.5 3.91 20 Comparative Example 19 1200 -47.8 128.1 52.1 4.00 4 11 Comparative Example 20 1050 -28.8 178.5 52.3 3.70 20 Comparative Example 21 1100 -33.8 149.0 53.1 3.77 20 Comparative Example 22 1150 -38.8 147.2 54.6 3.85 20 Comparative Example 23 1200 -43.8 128.3 53.7 3.94 6

As shown in Table 3 and FIG. 3, it can be seen that cracks do not occur when the hardness variation of the austenite phase and the ferrite phase is 120 Hv or less.

In addition, according to Table 3 and Figure 4, according to Figure 4 it can be seen that the stability of the duplex stainless steel in consideration of the annealing temperature has a hardness deviation of 120 Hv or less when -50 ℃ or less.

In the case of the above embodiments, the formula (1) indicating the stability of the austenite phase in the duplex stainless steel compared to the comparative examples is satisfied, and the hardness deviation of the austenite phase and the ferrite phase is 120 Hv or less, and after bending deformation You can see that no crack occurs.

Referring to Table 3, even if the annealing heat treatment temperature range is satisfied, it can be confirmed that the crack occurs when the formula (1) indicating the degree of stability of the duplex stainless steel in the present invention is not satisfied.

More specifically, in the case of the comparative examples, the value of the formula (1) is greater than -50, which is explained by the low stability of the austenite, resulting in the formation of calcined organic martensite, thereby increasing the hardness deviation of the austenitic austenite phase and the ferrite phase. . In the case of the comparative examples it can be seen that cracks occur in the bent portion is poor quality of the bent portion.

In addition, in the case of Examples, the value of Formula (2) related to the content of Mn, Ni, and N, which are austenite stabilizing elements, is 4 or more as compared with Comparative Examples. In comparison, in the case of the comparative examples, the value of Equation (2) is less than four. Through this, it can be seen that the degree of stabilization of the duplex stainless steel is directly affected by the contents of the austenite stabilizing elements Mn, Ni, and N contained in the austenite phase which varies with annealing temperature.

The lean duplex stainless steel manufactured according to the embodiment of the present invention derives a component system in consideration of the austenite stabilization degree, and formally measures the stability of the austenite in relation to the annealing temperature, thereby connecting with the bendability result. In addition, it was found that the bend quality was improved in an appropriate alloy range.

That is, by controlling the annealing temperature it was possible to reduce the hardness variation between the phase (phas) while ensuring the austenite stabilization, thereby improving the bend quality.

As described above, the exemplary embodiments of the present invention have been described, but the present invention is not limited thereto, and a person skilled in the art does not depart from the spirit and scope of the following claims. It will be understood that various changes and modifications are possible in the following.

Claims (8)

By weight, C: 0.08% or less (excluding 0), Si: 1% or less (excluding 0), Mn: 2-4%, Cr: 19-22%, N: 0.2-0.3%, Ni: 0.5-1.5 , Cu: 1% or less (excluding 0), including residual Fe and other unavoidable impurities,
Lean duplex stainless steel with improved bendability that satisfies the following formula (1).
Equation (1): 551-462 (C + N)-9.2 Si-8.1 Mn-13.7 Cr-29 (Ni + Cu)-18.5 Mo-0.1 x Annealing temperature (° C) ≤ -50
(C, N, Si, Mn, Cr, Ni, Cu, Mo means the weight percent of carbon, nitrogen, silicon, manganese, chromium, nickel, copper, molybdenum, respectively.) The method of claim 1,
Mo; Lean duplex stainless steel with improved bendability further comprising 0.5% or less. The method of claim 1,
Lean duplex stainless steel with improved bendability of less than 120 Hv in fine hardness between austenitic and ferritic phases. The method of claim 1,
Lean duplex stainless steel with improved bendability with ferritic phase percentages of 45 to 60%. The method of claim 1,
Lean duplex stainless steel with improved bendability that satisfies the following formula (2) related to Mn, Ni and Cu contained in the austenitic phase.
Equation (2): Mn + 0.5 * Ni + N ≥ 4
(Wherein Mn, Ni, N means the weight percent of manganese, nickel, and nitrogen in the austenite phase, respectively.) By weight, C: 0.08% or less (excluding 0), Si: 1% or less (excluding 0), Mn: 2-4%, Cr: 19-22%, N: 0.2-0.3%, Ni: 0.5-1.5 , Cu: hot rolling a stainless steel comprising 1% or less (excluding 0), balance Fe and other unavoidable impurities;
Annealing heat treatment the hot-rolled steel sheet in the range of 1050 to 1200 ℃ to satisfy the following formula (1); manufacturing method of lean duplex stainless steel including improved bendability.
Equation (1): 551-462 (C + N)-9.2 Si-8.1 Mn-13.7 Cr-29 (Ni + Cu)-18.5 Mo-0.1 x Annealing temperature (° C) ≤ -50
(C, N, Si, Mn, Cr, Ni, Cu, Mo means the weight percent of carbon, nitrogen, silicon, manganese, chromium, nickel, copper, molybdenum, respectively.) The method of claim 6,
Mo; Method for producing lean duplex stainless steel with improved bendability further comprising 0.5% or less. The method of claim 6,
Cold rolling the hot-rolled annealed steel sheet to obtain a cold-rolled steel sheet; And annealing the cold rolled steel sheet at a temperature of 1050 to 1200 ° C .;

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