KR101964314B1 - Austenitic stainless steel with excellent workability and resistant of season cracking, and drawing product using the same - Google Patents

Abstract

The present invention discloses an austenitic stainless steel that is excellent in workability and aging crack resistance that does not cause defects such as age cracking or delayed fracture even when applied to a design sink having a radius of curvature of the sink corner of 50 mm or less.
Austenitic stainless steel according to an embodiment of the present invention, in weight percent, C: 0.01 to 0.04%, Si: 0.1 to 1%, Mn: 0.1 to 2%, Ni: 6 to 10%, Cr: 16 To 20%, Cu: 1-2%, Mo: 0.01-0.2%, N: 0.035-0.07%, remaining Fe and inevitable impurities, and represented by the following formula (1): austenite stabilization parameter (ASP) value It is -3 or less, and the work hardening index n value satisfies the range 0.4 to 0.5 in the true strain range of 0.3 to 0.4.
(1) 565-445 * C-495 * N-11.3 * Si-3.81 * Mn-28.6 * Ni-14.9 * Cr-30.0 * Cu

Description

Austenitic stainless steel with excellent processability and age-resistant cracking and drawing processed products using the same {AUSTENITIC STAINLESS STEEL WITH EXCELLENT WORKABILITY AND RESISTANT OF SEASON CRACKING, AND DRAWING PRODUCT USING THE SAME}

The present invention relates to an austenitic stainless steel having excellent workability and age-resistant cracking resistance even when applied to a design sink having a radius of curvature of the sink corner of 50 mm or less.

Sink bowls for kitchen sinks are usually made of stainless steel. Mainly 300 general purpose stainless steels are used, and the shape of a general sink bowl is widely used because there is no problem in formability.

However, in recent years, many attempts have been made to design sink bowls of various and complex shapes to enhance competitiveness in the market. In particular, the design sink, which is a sink using a tight radius corner, usually has a corner radius of curvature R of 50 mm or less. Austenitic stainless steels, such as STS304, are much more workable than ferritic stainless steels, but they often crack at edges when machining with design sinks.

FIG. 1 shows the deformation distribution in the molding of a design sink model having a radius of curvature R of 20 mm. FIG. As shown in Figure 1, during the drawing process, the amount of deformation is concentrated on the edge leading from the flange portion to the floor, thereby causing cracks in the concentrated deformation portion.

In order to solve this problem, there have been attempts to manufacture a product using austenitic stainless steels containing Cu, but the crack problem at the edge part still remains because the specific control method of the material's unique work hardening value cannot be provided. to be.

In addition, in recent years, it has been required to satisfy the yield strength of 230 MPa or more and the tensile strength of 540 MPa or more, which are 304 material standards (EN, KS), and are limited to austenitic stainless steels that are softened by simply adding Cu and Ni. to be.

Embodiments of the present invention provide an austenitic stainless steel having excellent processability and age-resistant cracking resistance that does not cause defects such as age cracking or delayed fracture even when applied to a design sink having a radius of curvature of the sink corner of 50 mm or less. I would like to.

In addition, to provide an austenitic stainless steel excellent in strength and corrosion resistance with workability.

Austenitic stainless steel having excellent workability and anti-aging cracking properties according to an embodiment of the present invention, in weight%, C: 0.01 to 0.04%, Si: 0.1 to 1%, Mn: 0.1 to 2%, Ni: 6 to 10%, Cr: 16 to 20%, Cu: 1-2%, Mo: 0.01 to 0.2%, N: 0.035 to 0.07%, remaining Fe and inevitable impurities, and are represented by the following formula (1) The austenite stabilization parameter (ASP) value is -3 or less, and the work hardening index n value satisfies the range 0.4 to 0.5 in the true strain range of 0.3 to 0.4.

(1) 565-445 * C-495 * N-11.3 * Si-3.81 * Mn-28.6 * Ni-14.9 * Cr-30.0 * Cu

Here, C, N, Si, Mn, Ni, Cr, Cu means the content (wt%) of each element.

In addition, according to an embodiment of the present invention, the stainless steel may satisfy the following formula (2).

(2) Cu / (100 * N) ≤ 0.55

Here, Cu and N mean content (weight%) of each element.

In addition, according to an embodiment of the present invention, the stainless steel may have a yield strength (YS) of 230 MPa or more and a tensile strength (TS) of 540 MPa or more.

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

(3) 100 * N-(Mn + Cu) ≥ 0

Here, N, Mn, Cu means the content (% by weight) of each element.

In addition, according to an embodiment of the present invention, the stainless steel may have a formal potential of 245 mV or more.

The austenitic stainless steel drawing product having excellent workability and aging crack resistance according to an embodiment of the present invention is produced by drawing the stainless steel in a limited drawing ratio of 2.0 to 2.3 using a punch. The amount of strained organic martensite measured at sidewall positions of 10 mm, 20 mm, 30 mm, and 40 mm from the bottom and bottom surfaces of the drawing workpiece satisfies less than 1.0%, 1.0%, 5.0%, 10%, and 15%, respectively.

In addition, according to one embodiment of the present invention, the aging crack may not occur after 24 hours of processing the drawing processed product.

The austenitic stainless steel having excellent workability and age cracking resistance according to an embodiment of the present invention can prevent defects such as age cracking or delayed fracture even when applied to a design sink having a radius of curvature of 50 mm or less. have.

In addition, the yield strength of 230MPa or more and the tensile strength of 540MPa or more can be secured to satisfy the material specifications, and the official potential is 245mV or more, showing excellent corrosion resistance.

Figure 1 shows the deformation distribution in the shaping of the design sink model with a radius of curvature of 20mm corner corners.
2 is a graph showing the true strain-process hardening index correlation between austenitic stainless steel and comparative steel according to an embodiment of the present invention.
3 is a cross-sectional view schematically showing a cup drawing process using a punch.
4 shows the position where the amount of strained organic martensite of the workpiece after cup drawing processing was measured.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. 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.

The present inventors have studied the processing conditions that can design sink-molded austenitic stainless steel sheet without cracking, but the development of a new austenitic stainless steel can not be suppressed by the simple control of the processing conditions.

In recent years, in accordance with the trend of emphasizing the designability of sink products, there is an increasing need for design sinks in which press-molded products have complex formations or radius of curvature R of 50 mm or less. STS304 steels have good deep drawing, but cracks often occur when machining complex shapes or design sinks. From this point of view, in the case of a design sink product in which a complicated shape is formed by press molding or the corner curvature radius R is controlled to 50 mm or less, it is important to control not only the deep drawing property but also the work hardening index.

In the present invention, the work hardening index can be secured by controlling the work hardening index according to the austenitic stabilization parameter (ASP) value together with the component composition.

Austenitic stainless steel having excellent workability and anti-aging cracking properties according to an embodiment of the present invention, in weight%, C: 0.01 to 0.04%, Si: 0.1 to 1%, Mn: 0.1 to 2%, Ni: 6-10%, Cr: 16-20%, Cu: 1-2%, Mo: 0.01-0.2%, N: 0.035-0.07%, remaining Fe and unavoidable impurities.

Hereinafter, the reason for 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 carbon (C) is 0.01 to 0.04%.

As carbon in steel is added to the austenite phase stabilizing element, the austenite phase is stabilized, so it is necessary to add 0.01% or more.However, if it contains 0.04% or more, the part that is severely deformed during sink molding by hardening strain organic martensite Age causes cracking.

The content of silicon (Si) is 0.1 to 1.0%.

Silicon in the steel is a component that is added as a deoxidizer in the steelmaking step, and when a certain amount is added to the bright annealing process, Si-Oxide is formed on the passivation film to improve the corrosion resistance of the steel. However, when containing 1.0% or more, there exists a problem of reducing the ductility of steel.

The content of manganese (Mn) is 0.1 to 2.0%.

The more manganese is contained as austenite-phase stabilizing element, the more austenite phase is stabilized and added at 0.1% or more. However, excessively added manganese inhibits corrosion resistance, so it is limited to 2% or less.

The content of nickel (Ni) is 6.0 to 10.0%.

Nickel in steel is added to the austenite phase stabilizing element, the more the austenite phase is stabilized to soften the material, and it is necessary to add 6% or more to suppress the work hardening caused by the generation of strained organic martensite. However, excessive addition of expensive nickel will raise the cost and limit it to 10%.

The content of chromium (Cr) is 16.0 to 20.0%.

In steel, chromium is an essential element for improving corrosion resistance, and it is necessary to add more than 16% to secure corrosion resistance in the air environment and sink applications, but when excessively added, the material is hardened and moldability such as deep drawing property is disadvantageous. The lower limit is 20%.

The content of copper (Cu) is 1.0 to 2.0%.

Copper in steel is an austenite-phase stabilizing element, and the more it is added, the more the austenite phase is stabilized, which has the effect of suppressing work hardening caused by the generation of strained organic martensite, and therefore, 1% or more is added. However, if more than 2% is added, there is a problem of lowering corrosion resistance and a problem of rising cost.

The content of molybdenum (Mo) is 0.01 to 0.2%.

Molybdenum in steel has an effect of improving the corrosion resistance and workability, so it is added at 0.01% or more, and excessive addition is limited to 0.2% or less because it involves a cost increase.

The content of nitrogen (N) is 0.035 to 0.07%.

Nitrogen in steel needs to be added more than 0.035% to stabilize the austenite phase and to increase the strength of the material. During cracking, severe cracking occurs at the site of age cracking.

In addition, the austenite stabilization parameter (ASP) value represented by the following formula (1) together with the component composition satisfies −3 or less.

(1) 565-445 * C-495 * N-11.3 * Si-3.81 * Mn-28.6 * Ni-14.9 * Cr-30.0 * Cu

2 is a graph showing the true strain-process hardening index correlation between austenitic stainless steel and comparative steel according to an embodiment of the present invention.

Referring to FIG. 2, most 300-based austenitic stainless steel materials have a work hardening index (n) in the range of 0.3 to 0.4 at a true strain rate of 10 to 20%, which is an early stage of deformation. At 30-40% phosphorus strain, it has a work hardening index ranging from 0.55 to 0.65. In the case of design sinks with a small corner radius of curvature R, it is important to fabricate materials with similar work hardening indices at the beginning of the deformation and later work in order to avoid the above-mentioned phenomenon.

In the austenitic stainless steel according to the embodiment of the present invention, the austenitic stabilization parameter (ASP) value satisfies −3 or less, so that the work hardening index n value may be in the range of 0.4 to 0.5 in the true strain of 0.3 to 0.4. have.

On the other hand, from the viewpoint of a sink molding company, the excellent formability of the material is important, but there are times when it is essential to guarantee the material specifications of each country. In the case of the high-molded austenitic stainless steel added with Cu, there is a problem in that the strength of the material, which is 304 material, cannot be guaranteed. We wanted to develop steel that can secure corrosion resistance.

Austenitic stainless steel according to an embodiment of the present invention may satisfy the following formula (2).

(2) Cu / (100 * N) ≤ 0.55

By controlling the content of Cu and N to satisfy the above formula (2), the austenitic stainless steel according to the present invention can secure a yield strength (YS) of 230 MPa or more and a tensile strength (TS) of 540 MPa or more.

In addition, the austenitic stainless steel according to an embodiment of the present invention may satisfy the following formula (3).

(3) 100 * N-(Mn + Cu) ≥ 0

When the Mn and Cu content is increased to control the austenite stabilization parameter (ASP) so that the work hardening index n value in the range of true strain from 0.3 to 0.4 is within 0.4 to 0.5, the corrosion resistance of STS304 required in the design sink is increased. It cannot be secured. Therefore, by controlling the content of N, Mn, Cu to satisfy the above formula (3), the austenitic stainless steel according to the present invention can secure a formal potential of 245mV or more.

3 is a cross-sectional view schematically showing a cup drawing process using a punch.

Deep drawing raises the circular blank on the drawing die 2 and then presses the circular blank with the appropriate pressure on the blank holder 3. The punch 1 then draws the circular blank into the die 2, where the material center is drawn slowly while the material outer periphery slides into the die 2 and flows into the die 2.

In general, in the deep drawing, the bottom portion 10 of the material surrounding the bottom surface of the punch 1 is accompanied by a thickness reduction, and the stress state of this portion is a biaxial tensile state.

In the flange portion 30, since the material is drawn toward the inlet of the die 2 in the radial direction, compression deformation occurs in the circumferential direction and tensile deformation occurs in the radial direction. Due to the peripheral deformation of the compression deformation and the tensile deformation, the thickness increase occurs in the flange portion (30). That is, the flange portion 30 increases the strength of the material as the seal thickness of the material increases and at the same time as the work hardening by compression deformation.

Since the side wall portion 20 is drawn while being stretched up and down along the side of the die 2, the thickness of the material becomes thin and the strength of the material increases due to the work hardening, but in general, the work hardening of the flange part 30. Much lower than that.

As the drawing progresses, the flange portion 30 and the side wall portion 20 of the blank having the same breaking limit have a difference in breaking limit. That is, the flange portion 30 has a breaking limit of [high strength × thick thickness], and the side wall portion 20 has a breaking limit of [somewhat high strength × thin thickness], so that the breaking limit becomes unevenly drawn. Cracks occur due to the concentration of deformation in areas where strength is weak.

Since the compression deformation amount of the corner portion flange portion 30 of the design sink having a small corner radius of curvature R is much larger than that of the general sink, the difference of the breaking limit becomes larger, so that molding is difficult with the general STS304. .

When drawing processing using an austenitic stainless steel according to an embodiment of the present invention, it is possible to prevent generation of age cracking after processing.

The austenitic stainless steel drawing product having excellent workability and aging crack resistance according to an embodiment of the present invention may be manufactured by drawing the stainless steel at a limit drawing ratio (LDR) of 2.0 to 2.3 using a punch. At this time, the amount of strained organic martensite measured at sidewall positions of 10 mm, 20 mm, 30 mm, and 40 mm from the bottom and bottom of the drawing workpiece satisfies less than 1.0%, 1.0%, 5.0%, 10%, and 15%, respectively. .

Limited Drawing Ratio means the ratio (D / d) of the maximum material diameter (D) to the punch diameter (d).

When the austenite stabilization parameter (ASP) value exceeds -3, aging crack may occur after 24 hours of processing due to failure to suppress γ → α 'transformation during drawing processing. Therefore, in the case of drawing processing using the stainless steel having the austenite stabilization parameter (ASP) value of -3 or less according to the present invention, the amount of deformation organic martensite of the drawn workpiece can be controlled to prevent aging cracking. .

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

Machinability and Strength Evaluation

A part of the austenitic stainless steel of the component system shown in Table 1 below is Lab. Ingots were prepared by vacuum melting, and some slabs were prepared through an electric furnace-VOD-casting process. The ingots and slabs thus prepared were reheated at 1,240 ° C. for 1 to 2 hours, and then made of hot rolled materials by rough rolling and continuous finishing rolling mills, followed by cold rolling and cold rolling annealing at temperatures of 1,000 to 1,100 ° C. .

division C Si Mn Ni Cr Cu Mo N Inventive Steel 1 0.02 0.3 1.5 8.3 18.1 1.2 0.1 0.045 Inventive Steel 2 0.02 0.3 1.52 8.1 18 1.43 0.095 0.045 Invention Steel 3 0.021 0.82 1.5 8.1 18.2 1.43 0.1 0.045 Inventive Steel 4 0.019 0.31 1.46 8.14 18.2 One 0 0.061 Inventive Steel 5 0.019 0.31 1.51 8.15 18.2 1.27 0 0.043 Comparative Steel 1 0.11 0.6 0.85 6.6 17.2 0.6 0.1 0.048 Comparative Steel 2 0.055 0.4 1.1 8.1 18.2 0.1 0.1 0.04 Comparative Steel 3 0.11 0.58 0.86 6.7 17.6 0.9 0.1 0.05 Comparative Steel 4 0.01 0.5 1.2 7.6 16.9 2.9 0.1 0.01 Comparative Steel 5 0.02 0.6 2.2 6.6 17.2 2.9 0.1 0.04

Austenitic stabilization parameter (ASP) value, Cu / (100 * N) value and mechanical property value of yield strength, tensile strength, work hardening index n value (true strain 0.3 to 0.4) Section) is shown in Table 2 below.

division ASP Cu / (100 * N) Yield strength
(MPa) The tensile strength
(MPa) Work Hardening Index
n Inventive Steel 1 -18.6 0.27 261 574 0.45 Inventive Steel 2 -18.3 0.32 254 569 0.44 Invention Steel 3 -27.6 0.32 275 582 0.43 Inventive Steel 4 -16.9 0.16 281 602 0.43 Inventive Steel 5 -16.6 0.30 264 581 0.46 Comparative Steel 1 18.1 0.13 269 821 0.69 Comparative Steel 2 5.6 0.03 280 620 0.53 Comparative Steel 3 -2.5 0.20 272 742 0.61 Comparative Steel 4 -10.9 2.90 187 498 0.48 Comparative Steel 5 -11.1 0.73 195 501 0.49

Inventive steels 1 to 5 all controlled the Cu / (100 * N) value to 0.55 or less and the ASP value to -3 or less to secure the target yield strength (230 MPa or more) and tensile strength (540 MPa or more). The work hardening index n in the range of true strain 0.3 to 0.4 suitable for design sinks can be obtained below 0.5.

On the other hand, Comparative steels 1 to 3 have Cu / (100 * N) values of 0.13, 0.03, and 0.20, respectively, and have the desired yield strength and tensile strength, but ASP values of the present invention are 18.1, 5.6, and -2.5, respectively. As a result, the work hardening index n indicates a value of 0.5 or more in the range of true strain from 0.3 to 0.4, indicating that it is not suitable for design sink applications.

Comparative steels 4 and 5 are ASP values of -10.9 and -11.1, but belong to the object of the present invention, but Cu / (100 * N) values of 2.90 and 0.73, respectively, are sufficient yield strength of the material outside the object of the present invention. And tensile strength was not secured.

Corrosion Resistance Evaluation

Table 3 below shows the correlation between the formula potential and 100 * N-(Mn + Cu) value of the formula (3). Formal potential was measured using a 3.5% NaCl solution at 30 ° C. after polishing the surface of the steel sheet # 600.

division C Si Mn Ni Cr Cu Mo N 100 * N
-(Mn + Cu) Formal potential Inventive Steel 1 0.02 0.3 1.5 8.3 18.1 1.2 0.1 0.045 1.8 285 Inventive Steel 2 0.02 0.3 1.52 8.1 18 1.43 0.095 0.045 1.6 270 Invention Steel 3 0.021 0.82 1.5 8.1 18.2 1.43 0.1 0.045 1.6 273 Inventive Steel 4 0.019 0.31 1.46 8.14 18.2 One 0 0.061 3.6 325 Inventive Steel 5 0.019 0.31 1.51 8.15 18.2 1.27 0 0.043 1.5 265 Comparative Steel 4 0.01 0.5 1.2 7.6 16.9 2.9 0.1 0.01 -3.1 163 Comparative Steel 5 0.02 0.6 2.2 6.6 17.2 2.9 0.1 0.04 -1.1 194

Inventive steels 1 to 5 are all 100 * N − (Mn + Cu) values of 0 or more, and the formal potential indicating corrosion resistance of the material exhibited 245 mV or more, which is the range for which the present invention is intended. However, the comparative steels 4 and 5, which are designed to have a high content of Mn and Cu in order to increase austenite stabilization, have ASP values of -10.9 and -11.1 (see Table 2), even if they satisfy the object range of the present invention. N-(Mn + Cu) values are -3.1 and -1.1, respectively, which do not satisfy Eq. (3). As a result, the official potential value is 200 mV or less, resulting in STS304 level corrosion resistance (formula potential 245 mV or more) required by the design sink. Could not be secured.

Aging crack  Property evaluation

Using a punch having a diameter of 50 mm, the austenitic stainless steel according to the embodiment of the present invention was cup drawn at a punch speed of 100 mm / min. The drawing process was carried out within the limit drawing ratio (LDR) in the range of 1.9 to 2.3 and no wrinkles or breakage at the top of the cup. After drawing processing, as shown in FIG. 4, the amount of strained organic martensite (α ′) was measured at sidewall positions of 10 mm, 20 mm, 30 mm, and 40 mm from the bottom surface and the bottom surface of the cup workpiece, and is shown in Table 4 below. It was shown whether aging cracks occurred after 24 hours of processing. The amount of strain organic martensite was measured using a ferrite-scope.

division LDR Modified organic martensite (α ') measurand Aging Crack
Occurrence ① ② ③ ④ ⑤ Inventive Steel 1 2.16 0.36 0.72 2.86 5.88 10.81 Not Occurred Inventive Steel 4 2.20 0.78 0.88 4.12 8.19 14.16 Not Occurred Inventive Steel 5 2.25 0.45 0.77 3.23 6.11 12.54 Not Occurred Comparative Steel 1 1.90 6.78 10.21 30.21 38.25 49.22 Occur Comparative Steel 2 2.04 4.32 8.91 25.22 32.11 40.34 Occur

Inventive steels 1, 4, and 5 have ASP values of -18.6, -16.9, and -16.6 (see Table 2), which suppressed the γ → α 'transformation during molding, so that the amount of martensite produced by the position of the cup workpiece after actual cup drawing was reduced. Although the maximum value was less than 15%, in the case of Comparative Steels 1 and 2, the ASP values were 18.1 and 5.6 (see Table 2), which facilitated the transformation of γ → α 'during molding, and the maximum amount of martensite produced by position after actual cup drawing was 40 to 40%. Up to 49%. For this reason, Comparative Examples 1 and 2 caused a problem in which age cracking occurred 24 hours after the cup drawing process.

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

Claims (7)

By weight, C: 0.01-0.04%, Si: 0.1-1%, Mn: 0.1-2%, Ni: 6-10%, Cr: 16-20%, Cu: 1-2%, Mo: 0.01- 0.2%, N: 0.035 to 0.07%, including the remaining Fe and inevitable impurities,
The austenite stabilization parameter (ASP) value represented by the following formula (1) is -3 or less,
Satisfying the following formula (3),
Austenitic stainless steels with good workability and age-resistant cracking properties in which the work hardening index n value falls within the range of 0.4 to 0.5 in the true strain range of 0.3 to 0.4:
(1) 565-445 * C-495 * N-11.3 * Si-3.81 * Mn-28.6 * Ni-14.9 * Cr-30.0 * Cu
(3) 100 * N-(Mn + Cu) ≥ 0
Here, C, N, Si, Mn, Ni, Cr, Cu means the content (wt%) of each element. The method of claim 1,
The stainless steel is an austenitic stainless steel excellent in workability and aging crack resistance satisfying the following formula (2):
(2) Cu / (100 * N) ≤ 0.55
Here, Cu and N mean content (weight%) of each element. The method of claim 2,
The stainless steel is an austenitic stainless steel having excellent workability and anti-aging cracking resistance of yield strength (YS) of 230 MPa or more and tensile strength (TS) of 540 MPa or more. delete The method of claim 1,
The stainless steel is an austenitic stainless steel having a formal potential of 245 mV or more, which is excellent in workability and aging crack resistance. When drawing the stainless steel according to any one of claims 1 to 3 or 5 using a punch,
Workability and resistance to satisfactory strain organic martensite content measured at sidewall positions of 10 mm, 20 mm, 30 mm, and 40 mm from the bottom and bottom of the drawing workpiece of less than 1.0%, 1.0%, 5.0%, 10%, and 15%, respectively. Austenitic stainless steel drawing products with excellent age cracking properties. The method of claim 6,
The drawing processed article is an austenitic stainless steel drawing processed article excellent in workability and aging crack resistance, characterized in that no age crack after 24 hours of processing.

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