KR20160080304A - Duplex stainless steel excellent in deep drawing quality - Google Patents

Abstract

Duplex stainless steels having excellent deep-drawing characteristics according to one embodiment of the present invention are characterized in that the stainless steel has a composition of C: 0.01 to 0.17%, Si: 0.2 to 1.0%, Mn: 3 to 11%, Cr: 18 to 22%, N: %, The remainder Fe and other unavoidable impurities, and has a value such that the value of Modified Md ₃₀ expressed by the following formula has a value smaller than Critical Md ₃₀ .
(Modified Md ₃₀ = 551- [462 * (C + N) / (1-0.01 *?)] - 9.2 * Si- 8.1 * Mn- 13.7 * Cr- 29 * Ni- 29 * Cu-
Critical Md ₃₀ = 2 *? -20,
Cr-144 * N + 11.4 * Mo, each element is wt%), and the molar ratio of Φ (%) = -60.9-148 * C-1.30 * Mn-10.7 * Ni-4.57 * Cu + 5.77 * Si + 7.20 *

Description

{Duplex stainless steel excellent in deep drawing quality}

The present invention relates to a duplex stainless steel excellent in deep-drawing characteristics, and more particularly, to a duplex stainless steel excellent in deep-drawing characteristics, and more particularly, To a duplex stainless steel having a composite structure of austenitic and ferrite two-phase, which is excellent in resistance and can replace the conventional 200-series stainless steel.

Alloying elements Ni is a component traditionally used to make microstructure austenite in stainless steels. However, there is a growing interest in 200-series stainless steel due to the disadvantages of expensive Ni. 200-series stainless steel is a steel grade which is added low-priced Mn and N instead of lowering the content of expensive Ni.

In general, 200-series stainless steel steels are called Cr-Mn stainless steel because they contain 15.5 to 19% Cr, 5.5 to 10% Mn and 1.0 to 6% Ni in weight percent. It is inexpensive, and has an advantage of being excellent in strength and ductility. However, the lower the content of Ni is, the higher the content of Mn is, and the more the content of Cr is lower, the corrosion resistance is disadvantageous. And there is a disadvantage that the delayed fracture occurs severely due to the low content of Ni. The disadvantages of these 200 series stainless steels are well documented in the "New 200-series steels" published in November 2005 by the International Stainless Steel Forum (ISSF). Since the 2000s, non-standard 200 stainless steel with a nickel content of less than 1% and containing copper has emerged on the market. Despite its corrosion resistance and low delayed fracture resistance, The use thereof is increasing.

A number of known techniques for improving the problems of the stainless steel 200 series are disclosed. Patent Document 1 discloses low-Ni, high-N-containing austenite, and ferritic stainless steels. The steel types disclosed in Patent Document 1 are prepared by adjusting the austenite stabilization index expressed by each component system to adjust the ferrite fraction of the two-phase structure to improve the extrudability and the corrosion resistance of the application. On the other hand, Patent Document 2 discloses austenitic and ferritic stainless steels excellent in intergranular corrosion resistance. Patent Document 3 discloses a high strength, high ductility duplex stainless steel in which the amount of fired organic martensite is controlled.

These conventional techniques include nickel, copper, or molybdenum, and there is a problem that there is no countermeasure against delayed breakdown which occurs during drawing processing.

It should be understood that the foregoing description of the background art is merely for the purpose of promoting an understanding of the background of the present invention and is not to be construed as adhering to the prior art already known to those skilled in the art.

Patent Document 1: Korean Patent No. 957664 (May 12, 2010) Patent Document 2: Japanese Patent No. 5021901 (2012.12.12) Patent Document 3: Korean Patent No. 10-1379139 (Apr. 31, 2014)

SUMMARY OF THE INVENTION The present invention has been made to overcome the above problems and it is an object of the present invention to provide a stainless steel material which has a low cost ratio compared to a non-standard 200 stainless steel containing Ni of 1% or less and low corrosion resistance and delayed fracture resistance, It is an object of the present invention to provide a duplex stainless steel having a two-phase composite structure having significantly improved properties.

In order to achieve the above object, the duplex stainless steel excellent in deep-drawing characteristics according to an embodiment of the present invention has 0.01 to 0.17% of C, 0.2 to 1.0% of Si, 3 to 11% of Mn, 18 to 22% of Cr, , N: 0.05 to 0.25%, and the balance of Fe and other unavoidable impurities, and the value of Modified Md ₃₀ expressed by the following formula has a value smaller than Critical Md ₃₀ .

(Modified Md ₃₀ = 551- [462 * (C + N) / (1-0.01 *?)] - 9.2 * Si- 8.1 * Mn- 13.7 * Cr- 29 * Ni- 29 * Cu-

Critical Md ₃₀ = 2 *? -20,

Cr-144 * N + 11.4 * Mo, each element is wt%), and the molar ratio of Φ (%) = -60.9-148 * C-1.30 * Mn-10.7 * Ni-4.57 * Cu + 5.77 * Si + 7.20 *

The value of? May satisfy 25 to 55%.

The ear occurrence index expressed by the following equation may be 2 or less.

(The ear generation predictive index (mm) = 12.41-18.2 * C + 0.05 * Mn + 0.1 * Ni + 0.19 * Cu + 0.71 * Si-0.38 * Cr-17.71 * N + 1.4 * Mo,

The limit drawing ratio may be 2.0 or more.

The stainless steel may contain, by wt%, less than 0.5% of Ni, less than 0.5% of Cu, and less than 0.5% of Mo.

The stainless steel may be characterized by wt%, less than 0.3% of Ni, less than 0.2% of Cu, and less than 0.1% of Mo.

The stainless steel may be characterized by having a critical formaldehyde value of 100 ㎷ or more as measured in a 3.5% NaCl solution at 30 캜.

A specimen having a gage length of 25 mm and a width of 6.25 mm parallel to the rolling direction was subjected to a tensile elongation at a rate of 10 mm per minute at a room temperature from a cold-rolled annealed sheet having a thickness of 1.2 mm made of the stainless steel to obtain a tensile elongation of 35 to 65% .

According to the duplex stainless steel according to the present invention, since expensive Ni, Cu, and Mo are not separately added to the conventional non-standard 200 stainless steels, the cost of raw materials is minimized and stainless steel is manufactured at a low cost. And delayed fracture characteristics of the stainless steel can be remarkably improved.

1 is a photograph showing the generation of delayed fracture of a stainless steel composed of austenite and a ferrite bimodal structure.
2 is a graph showing the relation between the ferrite fraction measured by a ferrite scope and the ear generation height measured under the condition that the drawing ratio DR is 2 for cold-rolled annealed sheets manufactured from No. 1 to No. 28 steels in Table 2. FIG.
3 is a graph showing a predicted fraction of ferrite and Modified Md ₃₀ .
4 is a photograph showing the occurrence of delayed fracture due to a draw ratio.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the invention. The singular forms as used herein include plural forms as long as the phrases do not expressly express the opposite meaning thereto. Means that a particular feature, region, integer, step, operation, element and / or component is specified, and that other specific features, regions, integers, steps, operations, elements, components, and / And the like.

Unless otherwise defined, all terms including technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Commonly used predefined terms are further interpreted as having a meaning consistent with the relevant technical literature and the present disclosure, and are not to be construed as ideal or very formal meanings unless defined otherwise.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a duplex stainless steel excellent in deep processing characteristics according to a preferred embodiment of the present invention will be described with reference to the accompanying drawings.

Typically, the formula resistance of stainless steels is called the pitting corrosion resistance number (PREN) and is generally defined as Cr + 3.3Mo + 30N-Mn. Here, Cr, Mo, Ni, and Mo mean wt% of each component. The higher the PREN value, the higher the formal resistance. Therefore, in order to improve the formal resistance of the 200-series stainless steel, it is essential to increase Cr and decrease Mn. Therefore, the influence of the component, which is an important feature of the present invention, on the ferrite fraction is described by the following expression "ferrite fraction (?)". From the equation [1], it can be clearly seen that the ferrite fraction increases when Mn is reduced and Cr is increased. In addition, it can be seen that Ni, Cu, C and N act as austenite stabilizing element like Mn, and Si and Mo act as ferrite stabilizing elements such as Cr.

[Formula 1]

(%) = -60.9-148 * C-1.30 * Mn-10.7 * Ni-4.57 * Cu + 5.77 * Si + 7.20 * Cr- 144 * N + 11.4 * Mo

200 series stainless steel is widely used for tableware and decorative pipes. Particularly in applications such as aquaculture processing, it is important to determine the yield of materials. It is called earrings that a sheet made of cold-rolled steel forms an ear-shaped or fan-shaped edge around the top edge of the container being drawn due to the orientation of the inner crystal grains. After ear drawing, it is necessary to cut out if ear generation occurs severely, so excessive ear generation is a factor which hinders efficient use of plate material and lowers the error rate. It is known that the ear formation is caused by the directional arrangement of the grains of the metallurgical phase constituting the microstructure. If the grains have a directional arrangement, the plastic anisotropy is increased as a result

To evaluate plastic anisotropy, the Lankford coefficient, commonly referred to as the r-value, is measured. The Lankford coefficient is evaluated by a tensile test with two extensometers to determine the strain in the longitudinal and transverse directions and is defined as follows:

[Formula 2]

r = ε _w / ε _t = ε _w / - (ε _w + ε _L )

Where e _w is the width direction, e _L is the longitudinal direction, and e _t is the thickness direction strain. The planar anisotropy defined by Dr can be calculated by measuring the r value in 0 degree, 45 degree and 90 degree directions with respect to the rolling direction of the plate material.

[Formula 3]

? R = (r ₀ - 2r ₄₅ + r ₉₀ ) / 2

The earring occurring during the deep drawing of the cylindrical shape occurs severely when Δr is large. When Δr is positive according to the sign of Δr, the earing occurs in the rolling direction (0 °) and in the rolling direction (90 °) And if? R is negative, ear generation mainly occurs at ± 45 degrees from the rolling direction. Therefore, in order to reduce the amount of ear, the texture of the material should be controlled so that? R is close to zero. Generally, in ferritic stainless steels, the generation of ear is determined by process parameters such as the size of the cold rolling reduction amount, the number of application of the unit process comprising cold rolling-annealing, annealing temperature and annealing time performed after cold rolling, The plastic anisotropy of the steel is known to be very large compared to the austenitic stainless steels.

In the present invention, a control factor on ear formation was investigated for a stain-in-lease steel having a ferrite fraction of more than 10 and less than 60 predicted by Equation 1. As a result, the results of the conventional ferritic stainless steel It was confirmed that the constituents of the alloys differently controlled ear generation. The influence of the ingredient, which is an important feature of the present invention, on the ear development is described by the expression of "Ear Development Prediction Index (mm)" as shown in the following Equation 4, and if the value of "Ear Development Prediction Index" is large, It shows that it occurs seriously.

[Formula 4]

The predicted ear generation index (mm) is 12.41-18.2 * C + 0.05 * Mn + 0.1 * Ni + 0.19 * Cu + 0.71 * Si-0.38 * Cr-17.71 * N + 1.4 * Mo

In Equation 4, C, Mn, Ni, Cu, Si, Cr, N and Mo mean the content (wt%) of each component. From the equation (4), it can be understood that the content of Mn, Ni, Cu, Si and Mo should be minimized in order to minimize the occurrence of ear. Among the above five elements, Ni, Cu and Mo are expensive elements, and therefore, it is preferable not to add Ni, Cu and Mo for the purpose of replacing inexpensive non-standard 200 stainless steel. Generally, stainless steel uses scrap as a raw material, so it is possible to mix trace amounts of scrap without adding Ni, Cu, or Mo. Therefore, in the present invention, Ni, Cu, and Mo are not intentionally added, but may be 0.5% (wt%) or less in consideration of the content of impurities.

Delayed cracking refers to a cracking phenomenon occurring after a certain time in a molded article made of metastable austenitic stainless steel in which a martensite phase is formed by plastic deformation. 1 is a photograph showing the generation of delayed fracture of a stainless steel composed of austenite and a ferrite bimodal structure.

 It can be seen that macroscopically delayed fracture cracks occur in the rolling direction (0 degrees) and in the rolling direction (90 degrees). However, in detail, when the detailed location of the cracks is observed in detail, it can be seen that crack propagation starts at the boundary between the ridge and the trough of the ear area. Therefore, ear generation is judged to be a factor for promoting crack generation of delayed fracture, so ear generation should be minimized also from the viewpoint of suppressing delayed fracture.

As described above, the 200-series stainless steel is used for a western-style apparatus and has a limited range of applications due to the disadvantage of delayed fracture. It is well known that delayed fracture of 200 series stainless steels occurs severely when the stability of the austenite phase is small.

For example, WO20011 / 138503 A1, which relates to the delayed fracture of a 200-series steel, improves the limit drawing ratio, which is the maximum drawing ratio that can alleviate delayed cracking and form without fracture. , It is important to increase the austenite stability and to inhibit the formation of plasticity-induced martensite in the microstructure during molding.

It is a method for deeper machining of non-standard 200 stainless steel with low nickel content without delayed fracture. The reason why the hot forming method is specifically adopted is to increase the temperature to suppress the generation of fired organic martensite .

Nohara et al. Have proposed a formula for predicting the stability of austenite structure called Md ₃₀ for single-phase austenitic stainless steels 304 and 301 (see "Composition and Grain Size Dependencies of Strain-induced Martensitic Transformation in Metastable Austenitic Stainless Steels ", Tetsu-to-Hagane, page 212-222, No. 5, 1977). Md ₃₀ means a temperature prediction formula where 50% of the martensite phase is generated when 30% is applied as a true strain, and Nohara et al. Proposed the following equation.

[Formula 5]

Md ₃₀ (° C.) = 551-462 (C + N) -9.2Si-8.1Mn-13.7Cr-29 (Ni + Cu) -18.5Mo-68Nb

In the formula 5, C, N, Si, Mn, Cr, Ni, Cu, Mo and Nb mean the content (wt%) of each component. The physical meaning of Equation 5 is that when the value of Md ₃₀ is high, the stability of austenite is lowered, and a large amount of martensite is produced by plastic deformation.

Therefore, in order to improve the delayed fracture of a material composed of a metastable austenite phase in which a martensite phase is formed by plastic deformation, it is known as a general method to increase the delayed fracture resistance by controlling the component so that the value of Md ₃₀ is lowered.

In the present invention, the influence of the ferrite fraction on the generation and propagation of delayed fracture cracks in a two-phase composite stainless steel composed of austenite and ferrite was investigated. As a result, it was confirmed that the delayed fracture resistance is improved by increasing the ferrite fraction. In studying the cause of this phenomenon, the authors estimated that the residual stress of the formed deep-drawn product decreased when the ferrite fraction increased, and the residual stress, which is the driving force of the occurrence of the delayed fracture, would decrease. The reason for this is that in a steel structure composed of only a single phase of metastable austenite phase, work hardening due to deformation is severe and residual stress is high after deep processing, but when a ferrite phase is partially present, residual stress It is considered that there will be an effect of suppressing the increase of the temperature.

From these conclusions, it was concluded that the occurrence of delayed fracture in the two-phase composite steel consisting of austenite and ferrite will be mainly determined by the austenite stability and ferrite fraction, and experiments on various component steels were conducted. As a result, It was confirmed that the increase of the ferrite phase fraction improves the delayed fracture resistance.

The effect of the ferrite phase fraction, which is an important feature of the present invention, on the delayed fracture resistance can be summarized by the following mechanism. Delayed fracture in the present mechanism for invention The proposal "austenite and ferrite two-phase microstructure consisting of steel is the case where the instability austenite phase, which is represented by Md ₃₀ value is greater than the threshold of Md ₃₀ value (Critical Md ₃₀₎ , Where the Md ₃₀ value of the critical is a function of the ferrite fraction and has a positive correlation with the ferrite fraction ".

The above-mentioned mechanism can be expressed by the following equation.

First, Md ₃₀ prediction formula proposed for austenitic alloy of single phase (Nohara Equation) suggest Md ₃₀ on which is applicable in the two-phase microstructure steel consisting of austenite and ferrite to improve the ferrite fraction of austenite as follows: the do..

[Formula 6]

_{Modified Md 30 = 551- [462 *} (C + N) / (1-0.01 * Φ)] - 9.2 * Si-8.1 * Mn-13.7 * Cr-29 * Ni-29 * Cu-18.5 * Mo

In Equation 6, Φ is the predicted ferrite fraction (%) defined in Equation 1. Equation 6 is a solubility of C and N on the ferrite due to the very low, if ferrite is to be introduced into the microstructure, as the austenite phase is the C and N thickening reflects the effect that a ₃₀ Md lowered.

The effect of increasing the ferrite phase fraction to increase the delayed fracture resistance means that the critical Md ₃₀ (Critical Md ₃₀ ) at which the delayed fracture is triggered has a positive correlation with the ferrite predicted fraction (Φ) (Limit Drawing Ratio) of 1.9 is experimentally determined as follows.

[Equation 7]

Critical Md ₃₀ = 2 *? -20

From Eq. 6 and Eq. 7, the delayed fracture occurring at a constant ferrite fraction can be expressed as occurring when the following condition is satisfied.

[Equation 8]

Modified Md ₃₀ > Critical Md ₃₀

Hereinafter, the reasons for limiting the content of the components used in the present invention and the content thereof will be described. The content of the component is wt% unless otherwise specified below.

C: 0.01 to 0.17%

C is an austenite forming element and can be used in place of expensive elements such as Nirhk. However, in case of over-addition, it is easily bonded to carbide-forming elements such as Cr which is effective for corrosion resistance at the ferrite-austenite phase boundary to lower the Cr content around grain boundaries to reduce the corrosion resistance. %.

Mn: 3 to 11%

Mn is an element for increasing deoxidizing agent and nitrogen solubility. When it is used as an expensive austenite substitute for an austenite forming element, if its content is excessive, it becomes difficult to secure corrosion resistance. In addition, when the content of Mn is less than 1%, it is difficult to secure a proper austenite phase fraction even if Ni, Cu, N and the like as the austenite forming elements are controlled. Therefore, it is preferable to limit the content of Mn to 3 to 11%.

Ni: not added

Ni is an austenite stabilizing element together with Mn, Cu and N, and plays a major role in increasing the stability of the austenite phase. In order to reduce the cost, instead of reducing the Ni content which is high in price, it is possible to increase the Mn and N, which are the other austenite phase forming elements, to sufficiently maintain the phase fraction balance by the reduction of Ni. The manufacturing cost of the product due to the expensive Ni is prevented from rising, and is not separately added in the present invention in order to replace the very low-cost 200-series stainless steel. However, Ni can be contained up to 0.5% as an impurity.

Cu: not added

Cu is an element which inhibits work hardening due to the formation of the processed organic martensite phase and contributes to softening of the austenitic stainless steel. However, there is a disadvantage that the price is high. In order to prevent an increase in the manufacturing cost of a product due to expensive Cu, and to replace a very low-cost 200-series stainless steel, it is not separately added in the present invention. However, Cu may be contained up to 0.5% as an impurity.

Si: 0.2 to 1.0%

Si is partially added for the deoxidation effect and is an element which is enriched in ferrite when annealing with a ferrite forming element. Therefore, 0.1% or more is added in order to ensure a proper ferrite phase fraction. However, the excessive addition of not less than 1.5% rapidly increases the hardness of the ferrite phase to lower the elongation rate, lower the slag fluidity in the steelmaking process, and combine with oxygen to form inclusions and reduce corrosion resistance. Therefore, the Si content is preferably limited to 0.2 to 1.0%.

Cr: 18 to 22%

Chromium (Cr) is a ferrite stabilizing element together with Si, which plays a major role in securing the ferrite phase and is an essential element for ensuring corrosion resistance. Increasing the content increases the corrosion resistance, but it is necessary to increase the content of expensive Ni and other austenite forming elements to maintain the phase fraction. Accordingly, the content of Cr is preferably limited to 18 to 22%.

N: 0.05 to 0.25%

N is an element which contributes greatly to the stabilization of the austenite phase together with C and Ni, and is one of the elements which cause thickening in the austenite phase during annealing. Therefore, the increase of the N content can additionally increase the corrosion resistance and enhance the strength. However, if N content is excessive, it is difficult to produce steels steadily due to surface defects caused by nitrogen pores in the casting process due to exceeding nitrogen solubility. Therefore, the content of N is preferably limited to 0.05 to 0.25%.

Mo: Not added

Mo is a very effective element for improving corrosion resistance while stabilizing ferrite with Cr. However, there is a drawback that the price is very expensive. The production cost of the product due to the expensive Mo is prevented from being increased, and it is not separately added in the present invention in order to replace the very low-cost 200-series stainless steel. However, Mo can be contained up to 0.5% as an impurity.

Hereinafter, the present invention will be described in more detail with reference to Examples. Table 1 and Table 2 below show the major alloy compositions and experimental results for the experimental steel types. The steel types shown in Table 1 (inventive and comparative examples) were each cast in the form of a 50 kg ingot having a thickness of about 140 mm in a vacuum induction melting furnace. The cast ingot was subjected to a heat treatment at a temperature of about 1250 ° C for about 3 hours, followed by hot rolling to a final size of 200 mm in width and 4 mm in thickness, followed by air cooling after hot rolling. The air-cooled hot-rolled sheet was subjected to hot-rolling annealing at a temperature of 1100 ° C for 1 minute, followed by cold rolling to 1.2t after pickling. The cold-rolled sheet was cold-rolled and annealed at a temperature of 1100 ° C for 30 seconds and pickled to prepare test specimens.

Steel grade  C  Mn  Ni Cu  Si  Cr  N Mo Remarks One 0.081 6.00 1.00 2.50 0.50 19.10 0.085 0.00 Comparative Example 2 0.072 7.10 1.00 2.50 0.50 19.00 0.085 0.00 3 0.106 9.90 0.00 1.50 0.50 18.70 0.107 0.00 4 0.097 10.20 0.00 2.00 0.50 18.60 0.099 0.00 5 0.158 9.70 0.00 2.10 0.50 18.60 0.059 0.00 6 0.100 10.00 0.00 2.00 1.00 18.50 0.105 0.00 7 0.069 4.03 0.90 2.02 0.54 19.50 0.118 0.00 8 0.024 1.80 2.05 0.70 0.72 21.41 0.179 0.63 9 0.071 4.07 0.97 2.03 0.47 19.43 0.131 0.00 10 0.068 4.01 0.95 2.01 0.49 20.00 0.129 0.00 11 0.153 10.20 0.00 2.00 1.00 18.70 0.049 0.00 12 0.070 3.98 0.95 1.98 0.49 21.20 0.129 0.00 13 0.068 2.00 2.00 2.00 0.50 18.90 0.110 0.00 14 0.070 4.20 2.00 2.00 0.50 19.10 0.130 0.00 15 0.071 4.20 1.00 2.00 0.50 19.00 0.121 0.00 16 0.074 3.00 1.00 3.10 0.50 19.00 0.116 0.00 17 0.095 10.10 0.00 1.50 0.50 17.90 0.137 0.00 18 0.072 7.20 0.00 0.00 0.52 18.96 0.183 0.00 19 0.068 5.85 0.00 0.00 0.49 18.91 0.184 0.00 20 0.076 4.81 0.00 0.00 0.54 19.02 0.183 0.00 21 0.098 9.30 0.00 0.00 0.58 19.87 0.189 0.00 Honor 22 0.100 8.00 0.00 0.00 0.56 20.01 0.171 0.00 23 0.073 7.06 0.00 0.00 0.51 20.07 0.202 0.00 24 0.069 7.00 0.00 0.00 0.49 21.08 0.197 0.00 25 0.072 6.00 0.00 0.00 0.49 20.01 0.175 0.00 26 0.071 6.06 0.00 0.00 0.50 21.49 0.189 0.00 27 0.070 4.90 0.00 0.00 0.50 20.07 0.200 0.00 28 0.065 5.10 0.00 0.00 0.48 21.00 0.197 0.00 29 0.082 8.50 1.03 1.54 0.54 15.65 0.198 0.00 Comparative Example

The ferrite fraction of each specimen was measured using a ferrite scope for a material having a thickness of 1.2 mm in the cold-rolled annealed state. The ferrite scope is a device for measuring the content of ferrite phase by utilizing the magnetic property of the material and is a result of measurement using Fisher's Ferritescope MP30.

A specimen with a gage length of 25 mm and a width of 6.25 mm was taken from a cold-rolled and annealed sheet of 1.2 mm in thickness in parallel with the rolling direction to measure the recognition strength of each specimen and subjected to a room temperature tensile test at a tensile strength of 10 mm And tensile test characteristics until fracture were measured.

To evaluate the plastic anisotropy of each specimen, the Lankford coefficient, commonly referred to as r-value, was measured. For this, JIS 13B specimen was used and r-value was measured after 20% tensile at room temperature.

A round specimen was discharged from the cold-rolled and annealed sheet at a size of 100 mm in diameter (blank diameter), and the test piece was drawn in a cylindrical cup shape at room temperature under the conditions of a punch diameter of 50 mm and a blanking force of 1 ton. The maximum height corresponding to the ear of the ear and the minimum height corresponding to the ear bone of the specimen drawn by the cup of the cylinder were measured and the value obtained by subtracting the minimum height of the bone from the maximum height of the peak was defined as ear generation .

In order to discriminate the delayed fracture patterns according to the drawing ratio (DR), which is the blank diameter divided by the punch diameter, evaluation of the blankability of the blanks having diameters of 100 mm, 95 mm and 90 mm was carried out at the same time. In order to confirm the delayed fracture pattern due to the draw ratio, it was visually confirmed whether a delayed fracture crack was generated after 30 days from the deep drawing.

Psalter
number ferrite
Fraction
(Actual) Ear
(Actual) ferrite
Forecast fraction Ear
Prediction index Modified
Md ₃₀ (A) Critical
Md ₃₀ (B) A-B LDR Remarks One 27.0 4.3 25.4 3.4 31.9 30.7 1.2 ≥ 1.9 Comparative Example 2 27.0 4.2 24.5 3.7 31.0 29.1 1.9 ≥ 1.9 3 28.0 2.2 25.8 2.6 33.9 31.6 2.3 ≥ 1.9 4 26.0 2.9 24.9 3.1 30.4 29.8 0.6 ≥ 1.9 5 22.0 2.9 21.8 2.7 23.9 23.6 0.2 ≥ 1.9 6 28.0 3.1 26.0 3.3 21.3 32.0 -10.7 ≥ 1.9 7 30.2 3.1 31.3 2.7 35.9 42.6 -6.7 ≥ 1.9 8 47.8 2.3 47.8 2.5 -34.5 75.6 -110.1 ≥ 1.9 9 26.2 2.3 27.4 2.4 31.9 34.8 -2.9 ≥ 1.9 10 29.6 2.3 32.7 2.3 18.9 45.4 -26.5 ≥ 1.9 11 25.0 2.9 27.4 3.3 16.4 34.8 -18.4 ≥ 1.9 12 35.8 0.8 41.2 1.8 -17.6 62.5 -80.1 ≥ 1.9 13 19.0 2.4 19.0 3.1 53.7 18.0 35.7 <1.8 14 16.0 2.5 14.4 2.7 26.7 8.9 17.9 <1.8 15 24.0 2.6 25.6 2.8 45.9 31.1 14.8 <1.8 16 25.0 3.3 22.4 3.0 29.8 24.7 5.1 <1.8 17 15.0 2.1 17.1 2.6 46.6 14.2 32.4 <1.8 18 27.3 1.7 32.2 1.4 54.3 44.5 9.8 <1.8 19 33.1 1.5 33.9 1.4 63.9 47.8 16.0 <1.8 20 37.5 1.4 35.3 1.2 61.5 50.6 10.9 <1.8 21 32.2 1.0 31.7 0.6 4.0 43.4 -39.4 ≥ 2.0 Honor 22 38.8 0.5 36.6 0.8 9.5 53.2 -43.7 ≥ 2.0 23 36.1 1.3 37.5 0.6 11.0 55.0 -44.0 ≥ 2.0 24 46.1 0.4 46.0 0.4 -26.7 72.0 -98.7 ≥ 2.0 25 40.3 0.4 42.3 1.0 25.8 64.7 -38.9 ≥ 2.0 26 51.9 0.8 51.1 0.3 -42.8 82.2 -125.0 ≥ 2.0 27 43.6 0.4 41.0 0.6 20.5 61.9 -41.4 ≥ 2.0 28 51.7 0.2 48.5 0.4 -17.2 76.9 -94.1 ≥ 2.0 29 0.1 0.4 0.0 - 58.9 - - <1.8 Comparative Example

Experimental materials compared with those in Table 2 have a ferrite fraction of 10 to 60% measured by a ferrite scope except for the 29th steel. Steel No. 29 is a 200-story steel containing 1% Ni-1.5% Cu for the purpose of comparison with the present invention steel.

2 is a graph showing the relation between the ferrite fraction measured by a ferrite scope and the ear generation height measured under the condition that the drawing ratio DR is 2 for cold-rolled annealed sheets manufactured from No. 1 to No. 28 steels in Table 2. FIG. Referring to FIG. 2, it can be seen that a large number of materials exhibiting excellent characteristics with an ear height of 2 mm or less have a ferrite fraction of 25 to 55%. Materials with low ears (measured) of 2 mm are applicable to cases where the value of the ear generation prediction index in Table 2 defined by Equation 4 is 2 or less. Table 2 shows that the occurrence of ears is low in Ni and Cu-free steels.

Indicators for determining the delayed fracture resistance of the experimental steel are shown in Table 2. Critical Md _30, which is a criterion for the occurrence of delayed fracture, is also shown in the ferrite predictive fraction, modified Md _30, and limiting drawing ratio (LDR) of 1.9 in Equation [1]. Also, the range of the experimentally evaluated limit drawing ratio (LDR) is shown in Table 2.

Two phase microstructures of 13 to 20 and single phase austenitic stainless steels of 29 were evaluated to have the lowest retardation resistance with an LDR of less than 1.8. On the other hand, in the case of the present invention, the LDR was evaluated to be 2.0 or more, and the delayed fracture resistance was much better than that of the single-phase austenitic stainless steel of No. 29. This large difference in delayed fracture resistance can be confirmed by the difference between the Modified Md ₃₀ and the Critical Md ₃₀ described in Equation 8 (AB item in Table 2). It can be seen that the 21 to 28 steels with excellent delayed fracture resistance show very large negative values whereas the 16 to 20 steels with extremely low delayed fracture resistance show a large difference showing positive values

The effect of the ferrite phase fraction on the delayed fracture resistance is clearly confirmed in [Table 2]. The No. 14 steel has an LDR of less than 1.8 and is very resistant to delayed fracture. Modified Md ₃₀ value of # 14 is 26.7, which is very similar to the modified Md ₃₀ value of 25 # 25. However, the LDR of the No. 25 steel is 2 or more, which is superior to that of the No. 14 steel. The reason for this is due to the difference in Critical Md ₃₀ , which is a key claim in the present invention. The 25th steel has a higher ferrite content, which means that the critical Md ₃₀ is higher _than that of the 14th steel, which means that even with Modified Md ₃₀ values similar to 14, it does not exhibit delayed failure under DR 1.9 conditions.

In the present invention, 2 *? -20 is proposed as a Critical Md ₃₀ prediction formula in LDR 1.9. The background of the proposal is shown in Fig. FIG. 3 shows the delayed failure control factors (i.e., the predicted ferrite fraction and the modified Md ₃₀ ) of all the steels (1 to 28) except for the single-phase austenitic stainless steel No. 29 shown in Table 2. The critical Md ₃₀ line for LDR 1.9 can be determined from the boundary between the data with an LDR less than 1.8 and the data with an LDR equal to or greater than 1.9.

Table 3 further describes Ni and Cu-free added steels having excellent delayed fracture resistance in the present invention.

ID Drawing Ratio (DR) Delayed Cracking 1.8 1.9 2 ferrite
Forecast fraction Modified
Md ₃₀ (A) Critical
Md ₃₀ (B) A-B LDR 18 X X X 32.2 54.3 44.5 9.8 <1.8 19 X X X 33.9 63.9 47.8 16.0 <1.8 20 X X X 35.3 61.5 50.6 10.9 <1.8 21 O O O 31.7 4.0 43.4 -39.4 ≥ 2.0 22 O O O 36.6 9.5 53.2 -43.7 ≥ 2.0 23 O O O 37.5 11.0 55.0 -44.0 ≥ 2.0 24 O O O 46.0 -26.7 72.0 -98.7 ≥ 2.0 25 O O O 42.3 25.8 64.7 -38.9 ≥ 2.0 26 O O O 51.1 -42.8 82.2 -125.0 ≥ 2.0 27 O O O 41.0 20.5 61.9 -41.4 ≥ 2.0 28 O O O 48.5 -17.2 76.9 -94.1 ≥ 2.0 29 X X X 0.0 58.9 - - <1.8

(O: good, X: fracture)

The materials 18 to 20 in Table 3 are steels not containing Ni and Cu, but the delayed fracture resistance is similar to that of single-phase austenitic stainless steel 200, No. 29. On the other hand, No. 21 to No. 28 show very good delayed fracture resistance compared to No. 29 without containing Ni and Cu (see FIG. 4).

That is, although the composition of the materials from 18 to 28 does not seem to be large, there is a great difference in delayed fracture behavior between 18 to 20 (comparative example) and 21 to 28 (inventive) steels. In order to provide a two-phase composite structure steel comprising austenite and ferrite excellent in delayed fracture resistance, the present invention proposes a two-phase composite structure steel excellent in delayed fracture resistance, which is based on the delayed fracture mechanism proposed in the present invention, It means that it is possible to design the component by taking the influence of the ferrite fraction into consideration.

Table 4 summarizes the materials of the steels according to the present invention.

Psalter
number r0 r45 r90 Δr Ear
(Actual) Elongation
(%, L) Yield strength
(MPa) The tensile strength
(MPa) Official potential
(mV) Remarks 21 0.8 0.89 0.76 -0.11 One 45 462 734 227 Honor 22 0.78 0.84 0.8 -0.05 0.5 47 454 721 164 23 0.77 0.86 0.76 -0.1 1.3 53 542 834 204 24 0.77 0.78 0.82 0.02 0.4 43 520 759 177 25 0.74 0.87 0.78 -0.11 0.4 54 518 859 216 26 0.78 0.69 0.87 0.13 0.8 41 503 734 132 27 0.74 0.87 0.79 -0.11 0.4 54 516 863 259 28 0.76 0.69 0.86 0.12 0.2 47 504 750 257 29 1.1 1.09 0.71 -0.19 0.4 60 392 748 107 Comparative Example

As described above, according to the present invention, in order to replace non-standard 200 stainless steel containing Ni of 1% or less, it is necessary to add an expensive Ni and Cu content (impurity level) Phase composite structure in which delayed fracture characteristics are remarkably improved can be provided. As apparent from Table 4, the effect of the present invention from the viewpoint of corrosion resistance is also clear, since the formal dislocations of the inventive steels were measured at a significantly higher value than the 200th stainless steel of No. 29.

While the present invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, You will understand.

It is therefore to be understood that the above-described embodiments are illustrative in all aspects and not restrictive. The scope of the present invention is defined by the appended claims rather than the detailed description, and all changes or modifications derived from the meaning and scope of the claims and their equivalents should be interpreted as being included in the scope of the present invention .

Claims (8)

and the balance contains Fe and other unavoidable impurities, wherein the content of C is 0.01 to 0.17%, the content of Si is 0.2 to 1.0%, the content of Mn is 3 to 11%, the content of Cr is 18 to 22%, the content of N is 0.05 to 0.25%
Wherein a modified Md ₃₀ value expressed by the following formula has a value smaller than Critical Md ₃₀ .
(Modified Md ₃₀ = 551- [462 * (C + N) / (1-0.01 *?)] - 9.2 * Si- 8.1 * Mn- 13.7 * Cr- 29 * Ni- 29 * Cu-
Critical Md ₃₀ = 2 *? -20,
Cr-144 * N + 11.4 * Mo, each element is wt%), and the molar ratio of Φ (%) = -60.9-148 * C-1.30 * Mn-10.7 * Ni-4.57 * Cu + 5.77 * Si + 7.20 *
The method according to claim 1,
Wherein the? Value satisfies 25 to 55%.
The method of claim 2,
Wherein an ear generation prediction index expressed by the following formula is 2 or less.
(The ear generation predictive index (mm) = 12.41-18.2 * C + 0.05 * Mn + 0.1 * Ni + 0.19 * Cu + 0.71 * Si-0.38 * Cr-17.71 * N + 1.4 * Mo,
The method of claim 3,
Wherein the limit drawing ratio is 2.0 or more.
The method according to claim 1,
The stainless steel according to any one of claims 1 to 3, wherein the stainless steel further contains not less than 0.5% of Ni, less than 0.5% of Cu, and less than 0.5% of Mo in wt%.
The method of claim 5,
The stainless steel according to claim 1, wherein the stainless steel has a weight percent of Ni of less than 0.3%, a content of Cu of less than 0.2%, and a content of Mo of less than 0.1%.
The method according to claim 1,
Wherein the stainless steel has a critical formaldehyde value of at least 100 측정 as measured in a 3.5% NaCl solution at 30 캜.
The method according to claim 1,
A specimen having a gage length of 25 mm and a width of 6.25 mm parallel to the rolling direction was subjected to a tensile elongation at a rate of 10 mm per minute at a room temperature from a cold-rolled annealed sheet having a thickness of 1.2 mm made of the stainless steel to obtain a tensile elongation of 35 to 65% Wherein the stainless steel has excellent deep drawing characteristics.

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