KR101844573B1 - Duplex stainless steel having excellent hot workability and method of manufacturing the same - Google Patents

Abstract

Disclosed are duplex stainless steel with excellent hot workability, reducing generation of an edge crack during hot rolling even if the content of Mn is high, so as to provide the excellent hot workability; and a manufacturing method thereof. According to the present invention, the duplex stainless steel comprises: 0.01 to 0.1 wt% of carbon (C); 0.2 to 1.0 wt% of silicon (Si); 3 to 10 wt% of manganese (Mn); 19 to 23 wt% of chromium (Cr); 0.1 to 0.25 wt% of nitrogen (N); 1.0 wt% or less of nickel (Ni); 1.0 wt% or less of copper (Cu); 1.0 wt% or less of molybdenum (Mo); and the remainder consisting of iron (Fe) and unavoidable impurities. A ferrite stabilization value expressed by formula (1), -8.09-8.05C+1.37Si-0.10Mn+0.56Cr-0.58Ni-0.20Cu-10.43N+0.69Mo, is 1.0 or more, and an austenite stabilization value expressed by formula (2), 3.35+2.11C-0.36Si+0.03Mn-0.15Cr+0.15Ni+0.05Cu+2.74N-0.18Mo, satisfies a range of 0.5 to 1.0.

Description

[0001] DUPLEX STAINLESS STEEL HAVING EXCELLENT HOT WORKABILITY AND METHOD OF MANUFACTURING THE SAME [0002]

The present invention relates to a duplex stainless steel, and more particularly, to a duplex stainless steel having high manganese content and excellent in hot workability by reducing the occurrence of edge cracks during hot rolling, and a method for producing the duplex stainless steel.

Generally, stainless steel is classified as austenitic, ferritic, martensitic or duplex. Ferritic stainless steels are less expensive than austenitic stainless steels and have good surface gloss, drawability and oxidation resistance, and are widely used in kitchen appliances, building exterior materials, home appliances, and electronic parts. On the other hand, an austenitic stainless steel having good workability and corrosion resistance contains iron (Fe) as a base metal and contains chromium (Cr) and nickel (Ni) as main raw materials, and molybdenum (Mo) and copper It has been developed as a variety of steel to meet various applications by adding other elements.

304 and 316 stainless steels having superior corrosion resistance and workability include nickel (Ni) and molybdenum (Mo), which are expensive raw materials, and 200 and 400 stainless steels have been discussed as alternatives to this. However, 200 series and 400 series stainless steels have problems in that the formability and the corrosion resistance are less than those of the 300 series stainless steel. Accordingly, a duplex stainless steel having both advantages of austenite and ferrite has been developed by mixing austenite phase and ferrite phase. In recent years, attempts to develop lean alloys with a low content of expensive alloying elements such as nickel (Ni) and molybdenum (Mo) have increased, and duplex stainless steels are also in this trend.

On the other hand, duplex stainless steel is known to be susceptible to edge cracking during hot rolling in order to improve hot workability. Particularly, in the case of lean duplex steel in which the content of nickel (Ni) is reduced and the content of manganese (Mn) is increased, such hot workability problems arise.

The technical article 'Nickel-free duplex stainless steels' (Scripta Materialia, vol. 40, No.1, pp. 123-129, 1999) replaces nickel (Ni) with manganese (Mn) The present invention relates to an inexpensive duplex stainless steel, and it has been reported that the hot workability is lowered by the influence of manganese (Mn) and nitrogen (N). Korean Patent Laid-Open No. 10-2006-0074400, which is a patent for a duplex stainless steel having a low nickel (Ni) content, shows that when manganese (Mn) content is excessive, manganese (Mn) reacts with sulfur The upper limit of manganese (Mn) is limited to 4.5 wt%.

Korean Patent Laid-Open No. 10-2009-0005252 also relates to an austenitic-ferritic stainless steel characterized by a low nickel (Ni) content and a high nitrogen (N) content, And to form a lean duplex stainless steel so that the elongation rate is maintained at a high level while retaining the characteristics. In this document, if the manganese (Mn) content is excessive, the hot workability tends to be heated, so that the upper limit of the preferable manganese is limited to 7 wt%.

In this paper, we investigate edge cracking phenomenon in the sheet metal manufacturing process through the hot rolling process. In this paper, the controllable duplex steel edge crack (Lei Bao et al., Stainless Steel World, January / February, pp.1-3, And it is reported that the occurrence of edge cracks can be reduced by completing the hot rolling at a high temperature of 1,000 ° C or more.

On the other hand, Korean Patent Laid-Open No. 10-2002-0083493 discloses that an increase in the content of manganese (Mn) in a duplex steel containing molybdenum (Mo) and tungsten (W) improves hot workability.

As described above, except for Korean Patent Publication No. 10-2002-0083493, it is considered that the hot workability is deteriorated when the content of manganese (Mn) increases in most literature data.

Korean Patent Publication No. 10-2002-0083493 (published on November 2, 2002) Korean Patent Publication No. 10-2006-0074400 (published on Mar. 3, 2006) Korean Patent Laid-Open No. 10-2009-0005252 (published on Jan. 12, 2009)

Technical Papers 'Nickel-free duplex stainless steels' (Scripta Materialia, vol. 40, No. 1, pp. 123-129, 1999) Technical paper 'Controlling duplex steel edge crack' (Lei Bao et al., Stainless Steel World, January / February, pp.1-3, 2015)

The present invention relates to an economical duplex stainless steel produced by replacing expensive nickel (Ni) with inexpensive manganese (Mn), and a method for producing the duplex stainless steel. The present invention provides a duplex stainless steel which is excellent in hot workability even when manganese (Mn) And to provide a duplex stainless steel in which the occurrence of cracks is suppressed and a manufacturing method thereof.

The duplex stainless steel excellent in hot workability according to an embodiment of the present invention comprises 0.01 to 0.1% of C, 0.2 to 1.0% of Si, 3 to 10% of Mn, 19 to 23% of Cr, N: 0.1 to 0.25% of Ni, not more than 1.0% of Ni, not more than 1.0% of Cu, not more than 1.0% of Mo and the balance of Fe and other unavoidable impurities, wherein the ferrite stability value represented by the following formula (1) The austenite stability value represented by the formula (2) satisfies the range of 0.5 to 1.0.

(1) -8.09-8.05C + 1.37Si-0.10Mn + 0.56Cr-0.58Ni-0.20Cu-10.43N + 0.69Mo

(2) 3.35 + 2.11C-0.36Si + 0.03Mn-0.15Cr + 0.15Ni + 0.05Cu + 2.74N-0.18Mo

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

According to an embodiment of the present invention, the ferrite stability value may be in the range of 1.0 to 2.2.

Also, according to an embodiment of the present invention, the stainless steel may include a ferrite phase in a volume fraction of 55% or more.

Also, according to an embodiment of the present invention, the stainless steel may have an elongation of 30% or more.

A method for producing duplex stainless steel excellent in hot workability according to an embodiment of the present invention includes 0.01 to 0.1% of C, 0.2 to 1.0% of Si, 3 to 10% of Mn, 19 to 23% of Cr, Wherein the ferrite has a ferrite stability value of from 0.1 to 0.25% of N, not more than 1.0% of Ni, not more than 1.0% of Cu, not more than 1.0% of Mo, the balance of Fe and other unavoidable impurities, And austenite stabilization value expressed by the following formula (2) satisfies the range of 0.5 to 1.0, and subjecting the slab to hot rolling and annealing.

(1) -8.09-8.05C + 1.37Si-0.10Mn + 0.56Cr-0.58Ni-0.20Cu-10.43N + 0.69Mo

(2) 3.35 + 2.11C-0.36Si + 0.03Mn-0.15Cr + 0.15Ni + 0.05Cu + 2.74N-0.18Mo

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

Further, according to an embodiment of the present invention, the method further comprises a step of cold rolling after the annealing heat treatment, and the yield strength of the stainless steel at a cold reduction of 60% may be 1,200 MPa or less.

The duplex stainless steel according to the embodiment of the present invention can reduce the production cost by replacing nickel (Ni), which is a high-priced element among the components, with low-priced manganese (Mn). Even if the content of manganese (Mn) It is possible to manufacture a hot-rolled sheet without a high yield.

Also, the duplex stainless steel manufacturing method according to the embodiment of the present invention can easily perform the cold rolling process because the rolling load required for cold rolling is small.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a view showing ferrite stability and edge crack length of inventive steel and comparative steel according to an embodiment of the present invention. Fig.
2 is a photograph of an edge of an inventive steel 7 according to an embodiment of the present invention.
3 is a photograph of the edge of the comparative steel 5 according to the embodiment of the present invention.
4 is a graph showing a change in yield strength according to the cold reduction ratio of the invention steel 7 and the comparative steel 12 according to the embodiment of the present invention.

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

The duplex stainless steel excellent in hot workability according to an embodiment of the present invention is characterized by containing 0.01 to 0.1% of C, 0.2 to 1.0% of Si, 3 to 10% of Mn, 19 to 23% of Cr, N : 0.1 to 0.25%, Ni: 1.0% or less, Cu: 1.0% or less, Mo: 1.0% or less, the balance Fe and other unavoidable impurities.

The role and content of each component included in the duplex stainless steel according to the present invention will be described below.

The content of carbon (C) is 0.01 wt% or more and 0.1 wt% or less.

Carbon (C) can be used as an austenite phase forming element in place of expensive elements such as nickel (Ni), and is an effective element for increasing the strength of a material by solid solution strengthening. However, in the case of excessive addition, segregation and coarse carbides are formed in the center of the material during production, which adversely affects the subsequent hot rolling-hot rolling-hot rolling-cold rolling-cold rolling annealing process, and chromium effective for corrosion resistance at the ferrite-austenite phase boundary (Cr) to lower the chromium (Cr) content around the grain boundaries to reduce the corrosion resistance. Therefore, in order to maximize the corrosion resistance, it is preferable to limit the content to 0.01 to 0.1 wt%.

The content of silicon (Si) is 0.2 wt% or more and 1.0 wt% or less.

Silicon (Si) is added for some deoxidizing effect and is an element that is concentrated into ferrite during annealing with a ferrite phase forming element. Silicon (Si) should be added in an amount of 0.2 wt% or more for proper ferrite phase fraction. However, excessive addition of more than 1.0% by weight rapidly increases the hardness of the ferrite phase, thereby lowering the elongation, lowering the slag fluidity at the time of steelmaking, and forming an inclusion by binding with oxygen (O). Therefore, the content of silicon (Si) is preferably limited to a range of 0.2 to 1.0% by weight.

The content of manganese (Mn) is 3 wt% or more and 10 wt% or less.

Manganese (Mn) is an element that increases the degree of flux control, deoxidizing agent and nitrogen solubility, and is added to replace the expensive nickel (Ni) as an austenite phase forming element. When manganese (Mn) is less than 3% by weight, the economic advantage of substituting expensive elements such as nickel (Ni) is not great. On the other hand, when manganese (Mn) exceeds 10%, it becomes difficult to secure corrosion resistance. Therefore, the content of manganese (Mn) is preferably limited to a range of 3 to 10% by weight.

According to the formula (1) to be described later, manganese (Mn) is a factor for lowering ferrite stability (ferrite stability) which affects hot workability. However, other austenite phase forming elements such as carbon (C), nickel (Cu) and nitrogen (N). Therefore, in the present invention, limiting the upper limit of manganese (Mn) to 10 wt% is not intended to prevent deterioration of hot workability and is intended to prevent deterioration of corrosion resistance.

The content of chromium (Cr) is 19 wt% or more and 23 wt% or less.

Chromium (Cr) is a ferrite phase stabilizing element together with silicon (Si), and is an essential element added not only for securing the ferrite phase but also for ensuring corrosion resistance. Increasing the content of chromium (Cr) increases the corrosion resistance, but it is necessary to increase the content of expensive nickel (Ni) and other austenite phase forming elements to maintain the phase fraction. Therefore, the content of chromium (Cr) is preferably limited to a range of 19 to 23% by weight.

The content of nitrogen (N) is 0.1 wt% or more and 0.25 wt% or less.

Nitrogen (N), together with carbon (C) and nickel (Ni), contributes greatly to the stabilization of the austenite phase and is one of the elements causing enrichment in the austenite phase during annealing. When the content of nitrogen (N) is increased, the corrosion resistance can be increased and the strength can be increased. However, if the nitrogen (N) content is excessively high, nitrogen pores are generated due to excessive use of nitrogen during casting, which makes it difficult to produce steels with surface defects. Therefore, the content of nitrogen (N) is preferably limited to a range of 0.1 to 0.25% by weight.

The content of nickel (Ni) is 0 to 1.0 wt%.

Nickel (Ni) is an austenite phase stabilizing element together with manganese (Mn), copper (Cu) and nitrogen (N), and plays a major role in securing an austenite phase of duplex stainless steel. However, in order to reduce the cost, instead of decreasing the amount of expensive nickel (Ni) as much as possible, the amount of manganese (Mn) and nitrogen (N), which are other austenite phase forming elements, Balance can be maintained sufficiently. It is preferable not to add or to limit the upper limit to 1.0% by weight or less in order to prevent an increase in production cost of a product due to expensive nickel (Ni).

The content of copper (Cu) is 0 to 1.0 wt%.

Copper (Cu) is an element which inhibits work hardening caused by the formation of the processed organic martensite phase and contributes to softening of the austenitic stainless steel. However, in order to prevent the production cost of the product from rising due to expensive copper (Cu), it is preferable not to add or to limit the upper limit to 1.0 wt% or less.

The content of molybdenum (Mo) is 0 to 1.0 wt%.

Molybdenum (Mo) is a very effective element for improving corrosion resistance while stabilizing ferrite together with chromium (Cr). However, in order to prevent the production cost of the product from rising due to the extremely expensive molybdenum (Mo), it is preferable not to add it, or to limit the upper limit to 1.0 wt% or less

The duplex stainless steel according to one embodiment of the present invention is characterized in that the ferrite stability value represented by the following formula (1) satisfies the range of 1.0 to 2.2, and the austenite stability value represented by the following formula (2) .

(1) -8.09-8.05C + 1.37Si-0.10Mn + 0.56Cr-0.58Ni-0.20Cu-10.43N + 0.69Mo

(2) 3.35 + 2.11C-0.36Si + 0.03Mn-0.15Cr + 0.15Ni + 0.05Cu + 2.74N-0.18Mo

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

Generally, it is known that an increase in the content of manganese (Mn) in duplex stainless steels degrades hot workability. However, even though the content of manganese (Mn) was high, steels excellent in hot workability were identified. As a result of detailed studies on the microstructure properties of hot workability of duplex stainless steel containing high manganese, edge cracking occurred in quenched hot rolled plate It can be seen that the ferrite phase fraction is clearly distinguished.

The ferrite stability represented by the formula (1) is an index indicating the stability of the ferrite phase measured in the quenched hot rolled plate. If the value is high, the ferrite phase fraction of the material is high at the end of hot rolling . The austenite stability expressed by the formula (2) is an index indicating the stability of the austenite phase measured in the quenched hot rolled plate. If the value is high, the austenite fraction of the material at the end of hot rolling Is high.

When the ferrite stability value represented by the above formula (1) is 1.0 or more, edge cracking does not occur or occurs very little and the hot workability is excellent. However, when the ferrite stability value is smaller than 1.0, the deterioration of hot workability rapidly progresses, In order to ensure hot workability, the ferrite stability value should be 1.0 or more.

When the ferrite stability value represented by the formula (1) is 1.0 or more, the duplex stainless steel according to an embodiment of the present invention may include a ferrite phase in a volume fraction of 55% or more.

In the present invention, the upper limit value of the ferrite stability value is set to 2.2, but the upper limit value does not mean a change in the hot working property, but is a value given only by the composition range of the duplex stainless steel according to the present invention.

Further, in the duplex stainless steel according to the present invention, the austenite stability value expressed by the formula (2) must satisfy both the range of 0.5 to 1.0 at the same time, thereby ensuring excellent hot workability.

The ferrite-austenitic structure referred to in the present invention means that the ferrite phase and the austenite phase occupy most of the structure, and the ferrite- It does not mean that it is formed only in the ferrite phase and the austenite phase. For example, when the ferrite phase and the austenite phase occupy most of the structure, it means that the sum of the ferrite phase and the austenite phase accounts for 90% or more in the structure forming the stainless steel, and the ferrite phase and the austenite phase May be occupied by the martensite phase in which the austenite phase is transformed.

Meanwhile, the duplex stainless steel according to one embodiment of the present invention may have an elongation of 30% or more.

A method for producing duplex stainless steel excellent in hot workability according to an embodiment of the present invention is characterized by comprising 0.01 to 0.1% of C, 0.2 to 1.0% of Si, 3 to 10% of Mn, 19 to 23% of Cr, , Fe: 0.1 to 0.25%, N: not more than 1.0%, Cu: not more than 1.0%, Mo: not more than 1.0%, and the balance Fe and other unavoidable impurities. 2.2, and austenite stability value expressed by the following formula (2) satisfies the range of 0.5 to 1.0, and subjecting the slab to hot rolling and annealing.

(1) -8.09-8.05C + 1.37Si-0.10Mn + 0.56Cr-0.58Ni-0.20Cu-10.43N + 0.69Mo

(2) 3.35 + 2.11C-0.36Si + 0.03Mn-0.15Cr + 0.15Ni + 0.05Cu + 2.74N-0.18Mo

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

The stainless steel prepared by adjusting the components satisfying the above-mentioned component system, ferrite stability and austenite stability range is subjected to a heat treatment such as reheating, hot rolling, hot rolling, cold rolling, cold annealing and pickling of the slab, Can be prepared.

For example, the slab can be rolled by a conventional method, and the hot rolled steel sheet may have a thickness of 3 to 20 mm. For example, the hot-rolled steel sheet can be heat-treated for annealing for 1 to 60 minutes at 1,000 to 1,300 ° C. Preferably, the hot-rolled steel sheet can be annealed at a temperature of 1,050 to 1,150 캜.

Thereafter, the hot-rolled steel sheet can be cold-rolled by a conventional method, and the cold-rolled steel sheet may have a thickness of 0.1 to 5 mm. For example, the cold-rolled steel sheet may be annealed at a temperature of 1,000 to 1,300 ° C for 10 seconds to 60 minutes. Preferably, the cold-rolled steel sheet can be annealed at a temperature of 1,050 to 1,150 캜.

Hereinafter, the present invention will be described in more detail with reference to preferred embodiments.

Invention river  And Comparative steel

Were cast in the form of a 50 kg ingot having a thickness of 140 mm in a vacuum induction melting furnace so as to include the component system according to the inventive steels and the comparative steels shown in Table 1 below. The cast ingot was subjected to a heat treatment at a temperature of 1,250 ° C. for 3 hours, followed by hot rolling at a width of 200 mm and a thickness of 3 mm. To examine the microstructure immediately after hot rolling, the ingot was quenched immediately after hot rolling using water at room temperature .

The size of edge cracks was measured for the quenched hot rolled plate. The edge cracks were measured at the vertical depth from the edge of the hot rolled plate to the center of the width, and the average depth was measured by measuring the depths of the five cracks having the largest edge cracks.

division C Si Mn Cr Ni Cu N Mo ferrite
Stability Austenite
Stability Inventive Steel 1 0.016 0.41 5.84 20.00 0.1 0.1 0.184 0.1 1.03 0.91 Invention river 2 0.017 0.43 5.91 20.10 0.7 0.5 0.184 1.0 1.29 0.84 Invention steel 3 0.021 0.41 6.09 21.00 0.1 0.1 0.180 0.1 1.56 0.77 Inventive Steel 4 0.099 0.44 5.00 21.00 0.1 0.1 0.107 0.1 1.85 0.69 Invention steel 5 0.064 0.71 5.12 21.10 0.1 0.1 0.199 0.8 2.06 0.63 Invention steel 6 0.099 0.39 5.90 21.20 0.1 0.1 0.104 0.1 1.83 0.70 Invention steel 7 0.049 0.38 5.99 21.26 0.1 0.1 0.197 0.1 1.27 0.84 Inventive Steel 8 0.097 0.37 3.91 21.30 0.1 0.1 0.105 0.1 2.06 0.63 Invention river 9 0.070 0.49 6.05 21.59 0.1 0.1 0.191 0.1 1.50 0.78 Invented Steel 10 0.049 0.25 5.98 22.18 0.5 0.1 0.185 0.1 1.51 0.78 Invention steel 11 0.014 0.40 6.01 22.20 0.1 0.1 0.194 0.1 2.14 0.61 Comparative River 1 0.067 0.48 5.84 19.01 0.1 0.1 0.186 0.1 0.14 1.15 Comparative River 2 0.071 0.51 7.19 19.06 0.1 0.1 0.185 0.1 0.05 1.18 Comparative Steel 3 0.018 0.40 5.99 19.10 0.1 0.1 0.186 0.1 0.46 1.07 Comparative Steel 4 0.048 0.36 6.06 19.10 0.1 0.1 0.190 0.1 0.11 1.16 Comparative Steel 5 0.075 0.53 4.80 19.12 0.1 0.2 0.185 0.1 0.30 1.10 Comparative Steel 6 0.097 0.57 9.29 19.97 0.2 0.1 0.191 0.1 0.10 1.17 Comparative Steel 7 0.049 0.38 6.09 20.00 0.1 0.1 0.191 0.1 0.62 1.02 Comparative Steel 8 0.095 0.55 7.04 20.02 0.1 0.1 0.179 0.1 0.52 1.05 Comparative Steel 9 0.099 0.55 7.99 20.11 0.1 0.1 0.173 0.1 0.51 1.06 Comparative Steel 10 0.071 0.48 5.99 20.11 0.2 0.1 0.177 0.1 0.74 0.99 Comparative Steel 11 0.072 0.50 7.05 20.17 0.1 0.1 0.204 0.1 0.46 1.06 Comparative Steel 12 0.069 0.49 4.79 20.17 0.1 0.1 0.202 0.1 0.72 0.99

Table 1 shows the ferrite stability value calculated by the formula (1) and the austenite stability value calculated by the formula (2), respectively. In addition, the average value of the edge cracks of the hot rolled plate measured together with the ferrite stability value of Table 1 is shown in Fig.

Referring to FIG. 1, in the range where the ferrite stability value is smaller than 1.0, the edge crack size tends to increase as the ferrite stability value decreases. On the other hand, it was found that, in the range where the ferrite stability value is larger than 1.0, generation of edge cracks was significantly suppressed irrespective of the value.

FIG. 2 and FIG. 3 are photographs of the edges of the invention steel 7 and the comparative steel 5 according to the embodiment of the present invention, showing the difference in the edge crack length of the invention steel and the comparative steel.

Next, the hot-rolled and hot-rolled hot-rolled sheet was subjected to hot-rolling annealing at a temperature of 1,100 ° C for 1 minute, and then cold rolled to a thickness of 1.0 mm after pickling. The cold-rolled sheet was cold-rolled and annealed at a temperature of 1,100 ° C for 30 seconds, and subjected to pickling to prepare duplex stainless steel cold-rolled steel sheet specimens.

Table 2 below shows the tensile test results for inventive steels and comparative steels. In the tensile test, specimens having a gage length of 50 mm and a width of 12.5 mm were taken from a cold-rolled annealed sheet having a thickness of 1.0 mm and subjected to a tensile test at room temperature at a tensile rate of 20 mm per minute. Each specimen was subjected to five tensile tests, and the results are shown in Table 2 below.

Further, a ferrite fraction was measured for a cold-rolled annealed sheet having a thickness of 1.0 mm by using a ferrite scope. The ferrite scope is a device for measuring the ferrite phase fraction by utilizing the magnetic property of the material, and the ferrite fraction was measured using Fisher's "Ferritescope MP30".

division Yield strength
(MPa) The tensile strength
(MPa) Elongation
(%) Cold-annealed plate
Ferrite fraction
(%) 60% cold reduction ratio
Yield strength
(MPa) Inventive Steel 1 439.7 694.3 46.5 57 1,196 Invention river 2 446.8 707.1 34.1 59 1,102 Invention steel 3 428.2 675.0 33.8 61 1,017 Inventive Steel 4 420.2 683.4 32.8 60 1,072 Invention steel 5 455.5 715.3 36.2 58 1,123 Invention steel 6 411.0 677.0 33.4 61 1,103 Invention steel 7 461.0 708.0 34.9 55 1,025 Inventive Steel 8 406.7 673.2 31.8 59 1,130 Invention river 9 509.1 749.4 41.1 58 1,175 Invented Steel 10 428.0 675.2 32.6 62 1,009 Invention steel 11 403.4 648.7 31.5 71 980 Comparative River 1 544.3 1,085.2 47.8 34 - Comparative River 2 549.5 1,002.4 50.1 33 - Comparative Steel 3 478.5 844.6 33.5 43 - Comparative Steel 4 481.4 888.1 33.9 42 1,421 Comparative Steel 5 530.9 1,123.4 47.8 35 - Comparative Steel 6 467.9 738.0 50.1 35 - Comparative Steel 7 481.0 759.1 42.8 52 1,398 Comparative Steel 8 458.8 735.3 52.1 38 - Comparative Steel 9 459.1 727.3 49.1 39 - Comparative Steel 10 528.4 871.5 53.8 45 - Comparative Steel 11 554.9 847.6 52.1 41 - Comparative Steel 12 520.4 871.9 54.1 44 1,334

Referring to Table 2, it can be seen that the cold rolled and annealed plate ferrite fraction of the inventive steels is 55 to 71%, while the ferrite fraction of the comparative steels is in the range of 33 to 52%. Therefore, it can be deduced from Tables 1 and 2 that the inventive steels having a high ferrite stability value have a high ferrite fraction even in the cold-rolled annealing state as compared with the comparative steels.

Table 2 shows the yield strength values at a reduction ratio of 60%. In order to evaluate the ease of cold rolling of inventive steels, the cold rolled steel sheet was subjected to cold rolling at a different cold rolling reduction ratio, and tensile specimens were taken without annealing to determine yield strength (YS). The tensile test was carried out at a pulling rate of 20 mm per minute and a specimen having a gage length of 50 mm and a width of 12.5 mm perpendicularly to the rolling direction. Each specimen was subjected to five tensile tests.

Referring to Table 2, it can be seen that the inventive steels have a lower yield strength at a cold reduction ratio of 60% than the comparative steels. Therefore, it can be seen that the inventive steels according to the embodiments of the present invention have a great advantage in cold rolling since the rolling load required for cold rolling is smaller than that of comparative steels, and the cold rolling can be easily performed.

4 is a graph showing a change in yield strength according to the cold reduction ratio of the invention steel 7 and the comparative steel 12 according to the embodiment of the present invention.

Referring to FIG. 4, the tendency of the yield strength to change according to the cold rolling reduction rate in the range of the cold reduction rate of 20% is similar to that of the invention steel, but in the range of the cold reduction rate of about 40% It can be seen that there is a clear difference in the curing characteristics.

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

Claims (6)

The steel sheet according to any one of claims 1 to 3, wherein the steel sheet contains 0.01 to 0.1% of C, 0.2 to 1.0% of Si, 3.91 to 10% of Mn, 19 to 23% of Cr, 0.1 to 0.25% of N, , Mo: 1.0% or less, the balance Fe and other unavoidable impurities,
Wherein the ferrite stability value represented by the following formula (1) satisfies the range of 1.0 to 2.2,
A duplex stainless steel excellent in hot workability satisfying an austenite stability value represented by the following formula (2) in a range of 0.5 to 1.0:
(1) -8.09-8.05C + 1.37Si-0.10Mn + 0.56Cr-0.58Ni-0.20Cu-10.43N + 0.69Mo
(2) 3.35 + 2.11C-0.36Si + 0.03Mn-0.15Cr + 0.15Ni + 0.05Cu + 2.74N-0.18Mo
Here, C, Si, Mn, Cr, Ni, Cu, N and Mo mean the content (weight%) of each element. delete The method according to claim 1,
The stainless steel has a volume fraction of 55% or more and contains a ferrite phase, and is excellent in hot workability. The method according to claim 1,
The stainless steel is a duplex stainless steel having an elongation of 30% or more and excellent hot workability. The steel sheet according to any one of claims 1 to 3, wherein the steel sheet contains 0.01 to 0.1% of C, 0.2 to 1.0% of Si, 3.91 to 10% of Mn, 19 to 23% of Cr, 0.1 to 0.25% of N, Austenite stabilization value expressed by the following formula (2), and austenite stabilization value represented by the following formula (2), wherein the austenite stabilization value represented by the following formula 0.5 to 1.0 in a hot rolling process and an annealing heat treatment,
(1) -8.09-8.05C + 1.37Si-0.10Mn + 0.56Cr-0.58Ni-0.20Cu-10.43N + 0.69Mo
(2) 3.35 + 2.11C-0.36Si + 0.03Mn-0.15Cr + 0.15Ni + 0.05Cu + 2.74N-0.18Mo
Here, C, Si, Mn, Cr, Ni, Cu, N and Mo mean the content (weight%) of each element. 6. The method of claim 5,
Further comprising cold-rolling after the annealing heat treatment,
Wherein the stainless steel has a yield strength of 1,200 MPa or less at a cold reduction of 60%.

Images