CN115398022B

CN115398022B - Low cost austenitic stainless steel having high strength and high formability and method of manufacturing the same

Info

Publication number: CN115398022B
Application number: CN202180028609.8A
Authority: CN
Inventors: 宋硕原; 白种洙
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2020-04-22
Filing date: 2021-02-02
Publication date: 2023-11-28
Anticipated expiration: 2041-02-02
Also published as: JP7395769B2; WO2021215630A1; US20230142021A1; KR20210130426A; CN115398022A; KR102385472B1; EP4119693A4; EP4119693A1; JP2023522999A

Abstract

Disclosed herein are a low-cost austenitic stainless steel having high strength and high formability, and a method for manufacturing the same. According to one embodiment of the disclosed low cost austenitic stainless steel with high strength and high formability, the austenitic stainless steel comprises in weight%: greater than 0% and up to 0.08% C, 0.2% to 0.25% N, 0.8% to 1.5% Si, 8.0% to 9.5% Mn, 15.0% to 16.5% Cr, greater than 0% and up to 1.0% Ni, 0.8% to 1.8% Cu, and Fe and other unavoidable impurities in the remainder, and satisfies the expressions (1) to (4) (1) Ni+0.47Mn+15N.gtoreq.7.5 (2) 23 (C+N) +1.3Si+0.24 (Cr+Ni+Cu) +0.1Mn.gtoreq.12 (3) 551-462 (C+N) -9.2Si-8.1Mn-13.7Cr-29 (Ni+09Cu). Ltoreq.70 (4) 11.ltoreq.1+45C-5Si+0.Mn+2.2Ni-0.28-0.6788.6 Cr+8N, expressed in terms of Cu and represented by weight percent of each of (C, N, si, mn, cr, ni% Cu).

Description

Low cost austenitic stainless steel having high strength and high formability and method of manufacturing the same

Technical Field

The present disclosure relates to austenitic stainless steel and a method of manufacturing the same, and more particularly to low-cost austenitic stainless steel having high strength and high formability, and a method of manufacturing the same.

Background

The vehicle market trend is transitioning from the conventional internal combustion engine-based vehicle industry to the battery-based eco-friendly vehicle market. That is, the conventional internal combustion engine vehicle market, which is highly interested in medium-or large-sized vehicles, is shifting to a battery-based vehicle market, which favors small-or light-sized vehicles.

The structural material protecting the battery is required to have high strength to protect the battery from a safety accident such as explosion or from external impact and for the safety of passengers, the structural material is also required to be lightweight to prevent an increase in weight of a small or light vehicle. In addition to structural materials used to protect batteries, general structural materials have become smaller in size and higher in strength to meet environmental regulations. Therefore, there is a need to develop materials suitable for the entire industry with high productivity, excellent stability, high strength and excellent formability.

Stainless steel is a material suitable for the entire industry due to its excellent corrosion resistance. In particular, austenitic stainless steel having excellent elongation is not problematic in forming a complex shape to meet various demands of consumers, and is advantageous in aesthetic appearance.

However, austenitic stainless steel has a lower yield strength than common carbon steel and is economically disadvantageous because expensive alloying elements are used therein. Therefore, there is a need to develop stainless steel for structural materials that has a high level of yield strength and appropriate tensile strength and maintains excellent formability.

Further, there is a problem that alloy elements constituting austenitic stainless steel are expensive as compared with elements constituting most carbon steel. In particular, ni contained in austenitic stainless steel may cause problems in terms of price competitiveness because it is expensive and it is difficult to stably supply Ni due to its unstable supply and demand caused by large price fluctuation. Therefore, there is a need to develop a low-cost austenitic stainless steel in which the content of expensive elements such as Ni is reduced.

Disclosure of Invention

Technical problem

In order to solve the above problems, a low-cost austenitic stainless steel having high strength and high formability is provided.

Technical proposal

According to one aspect of the present disclosure, in order to achieve the above object, a low-cost austenitic stainless steel having high strength and high formability comprises, in weight percent (wt.%): greater than 0% and up to 0.08% of C, 0.2% to 0.25% of N, 0.8% to 1.5% of Si, 8.0% to 9.5% of Mn, 15.0% to 16.5% of Cr, greater than 0% and up to 1.0% of Ni, 0.8% to 1.8% of Cu, and Fe and other unavoidable impurities in the remaining portion, and satisfies the following expressions (1) to (4):

(1)Ni+0.47Mn+15N≥7.5

(2)23(C+N)+1.3Si+0.24(Cr+Ni+Cu)+0.1Mn≥12

(3)551-462(C+N)-9.2Si-8.1Mn-13.7Cr-29(Ni+Cu)≤70

(4)11≤1+45C-5Si+0.09Mn+2.2Ni-0.28Cr-0.67Cu+88.6N≤17

wherein C, N, si, mn, cr, ni and Cu represent the content of the element (wt.%) respectively.

In the low-cost austenitic stainless steel of each of the present disclosure having high strength and high formability, the yield strength of the cold rolled and annealed steel sheet may be 400MPa or more.

In the low-cost austenitic stainless steel of each of the present disclosure having high strength and high formability, the elongation of the cold rolled and annealed steel sheet may be 55% or more.

In the low-cost austenitic stainless steel of each of the present disclosure having high strength and high formability, the yield strength of the skin-pass cold rolled steel sheet may be 800MPa or more.

In the low-cost austenitic stainless steel of each of the present disclosure having high strength and high formability, the elongation of the skin-pass cold-rolled steel sheet may be 25% or more.

Further, according to an aspect of the present disclosure, in order to achieve the above object, a method of manufacturing a low-cost austenitic stainless steel having high strength and high formability includes: preparing a steel billet comprising, in weight percent (wt%): greater than 0% and up to 0.08% of C, 0.2% to 0.25% of N, 0.8% to 1.5% of Si, 8.0% to 9.5% of Mn, 15.0% to 16.5% of Cr, greater than 0% and up to 1.0% of Ni, 0.8% to 1.8% of Cu, and Fe and other unavoidable impurities in the remaining portion, and satisfies the following expressions (1) to (4); hot rolling the steel slab to produce a hot rolled steel sheet, and then hot annealing the hot rolled steel sheet to produce a hot rolled and annealed steel sheet; cold-rolling the hot-rolled and annealed steel sheet to prepare a cold-rolled steel sheet, and then cold-annealing the cold-rolled steel sheet at 1050 ℃ or more to prepare a cold-rolled and annealed steel sheet; and subjecting the cold rolled and annealed steel sheet to skin-pass rolling to prepare a skin-pass rolled steel sheet:

(1)Ni+0.47Mn+15N≥7.5

(2)23(C+N)+1.3Si+0.24(Cr+Ni+Cu)+0.1Mn≥12

(3)551-462(C+N)-9.2Si-8.1Mn-13.7Cr-29(Ni+Cu)≤70

(4)11≤1+45C-5Si+0.09Mn+2.2Ni-0.28Cr-0.67Cu+88.6N≤17

In a method of manufacturing low-cost austenitic stainless steel each having high strength and high formability, the skin pass rolling may be performed at a reduction ratio of 20% or more.

In the method of manufacturing low-cost austenitic stainless steel each having high strength and high formability, the reduction of area of the steel blank at a high temperature of 800 ℃ or more may be 50% or more.

Advantageous effects

According to one embodiment of the present disclosure, an austenitic stainless steel having excellent yield strength is provided, wherein a cold-rolled and annealed steel sheet prepared by cold annealing at 1050 ℃ or higher after cold rolling has excellent yield strength, and excellent elongation enough to be formed can be obtained after finishing cold rolling to further increase strength. In addition, it is possible to provide a low-cost austenitic stainless steel having high strength and high formability with high productivity even if a reduced amount of expensive alloying elements is used.

Detailed Description

A low-cost austenitic stainless steel having high strength and high formability according to one embodiment of the present disclosure comprises, in weight percent (wt.%): greater than 0% and up to 0.08% of C, 0.2% to 0.25% of N, 0.8% to 1.5% of Si, 8.0% to 9.5% of Mn, 15.0% to 16.5% of Cr, greater than 0% and up to 1.0% of Ni, 0.8% to 1.8% of Cu, and Fe and other unavoidable impurities in the remaining portion, and satisfies the following expressions (1) to (4).

(1)Ni+0.47Mn+15N≥7.5

(2)23(C+N)+1.3Si+0.24(Cr+Ni+Cu)+0.1Mn≥12

(3)551-462(C+N)-9.2Si-8.1Mn-13.7Cr-29(Ni+Cu)≤70

(4)11≤1+45C-5Si+0.09Mn+2.2Ni-0.28Cr-0.67Cu+88.6N≤17

EMBODIMENTS FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present disclosure will be described in detail. However, embodiments of the present disclosure may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the concept of the invention to those skilled in the art.

Furthermore, the terminology used herein is for the purpose of describing particular embodiments only. Unless otherwise indicated, expressions used in the singular are intended to cover plural expressions. Throughout the specification, terms such as "comprise" or "have" are intended to indicate the presence of features, operations, functions, components, or combinations thereof disclosed in the specification, and are not intended to exclude the possibility that one or more other features, operations, functions, components, or combinations thereof may be present or added.

Meanwhile, unless defined otherwise, all terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the present disclosure pertains. Accordingly, these terms should not be construed in an idealized or overly formal sense unless expressly so defined herein. As used herein, the singular is also intended to include the plural unless the context clearly indicates otherwise.

The terms "about," "substantially," and the like as used throughout the specification mean that when natural manufacturing and substance allowable errors are mentioned, such allowable errors correspond to or are similar to a value, and such value is intended for the purpose of clearly understanding the present invention or preventing an unintended infringer from illegally using the disclosure of the present invention.

A low-cost austenitic stainless steel having high strength and high formability according to one embodiment of the present disclosure comprises, in weight percent (wt.%): greater than 0% and up to 0.08% C, 0.2% to 0.25% N, 0.8% to 1.5% Si, 8.0% to 9.5% Mn, 15.0% to 16.5% Cr, greater than 0% and up to 1.0% Ni, 0.8% to 1.8% Cu, and Fe and other unavoidable impurities in the remainder.

Hereinafter, the reason for numerical limitation concerning the content of the alloy element in the embodiment of the present disclosure will be described.

Carbon (C): greater than 0 wt% and up to 0.08 wt%

Carbon (C) as an element effective for stabilizing the austenitic phase is added to obtain the yield strength of austenitic stainless steel. However, excessive C may not only deteriorate cold workability due to the solid strengthening effect, but also may cause grain boundary precipitation of Cr carbide, thereby adversely affecting ductility, toughness, corrosion resistance, and the like, and deteriorating welding characteristics between elements. Therefore, the upper limit thereof may be set to 0.08 wt%.

Nitrogen (N): 0.2 to 0.25 wt%

Nitrogen (N) is the most important element in the present disclosure. Nitrogen is a strong austenite stabilizing element effective for improving the corrosion resistance and yield strength of austenitic stainless steel. However, an excessive amount of N may cause defects such as nitrogen holes to occur in the preparation of the steel slab, and deteriorate cold workability due to the solid solution strengthening effect. Therefore, the upper limit thereof may be set to 0.25 wt%.

Silicon (Si): 0.8 to 1.5 wt%

Silicon (Si) acting as a deoxidizer during the steelmaking process is an element effective for improving corrosion resistance. Among the substitution elements, si is an effective element for improving the yield strength of the steel. In view of these effects, si may be added in an amount of 0.8 wt% or more in the present disclosure. However, an excessive amount of Si as a ferrite phase stabilizing element can promote the formation of delta ferrite in the cast steel slab, thereby deteriorating not only hot workability but also ductility and impact characteristics of the steel. Therefore, the upper limit of the Si content can be set to 1.5 wt%.

Manganese (Mn): 8.0 to 9.5 wt%

Manganese (Mn) as an austenite phase stabilizing element added as a substitute for Ni may be added in an amount of 8.0 wt% or more to improve cold workability by suppressing the formation of strain-induced martensite. However, excessive Mn may cause an increase in the formation of S-based inclusions (MnS), resulting in deterioration of ductility and toughness of austenitic stainless steel, and may cause formation of Mn smoke during the steelmaking process, resulting in an increase in manufacturing risk. In addition, excessive Mn rapidly deteriorates corrosion resistance of the product. Therefore, the upper limit of the Mn content can be set to 9.5 wt%.

Chromium (Cr): 15.0 to 16.5 wt%

Chromium (Cr) is a ferrite stabilizing element, but is effective in suppressing the formation of a martensite phase. As a basic element for obtaining corrosion resistance required in stainless steel, cr may be added in an amount of 15% or more. However, excessive Cr as a ferrite stabilizing element may promote the formation of delta ferrite in a large amount in a steel billet, resulting in deterioration of hot workability and adverse effects on material properties. Therefore, the upper limit thereof may be set to 16.5 wt%.

Nickel (Ni): greater than 0 wt% and up to 1.0 wt%

Nickel (Ni) as a strong austenite phase stabilizing element is added to improve hot workability and cold workability. However, since Ni is an expensive element, in the case of adding a large amount of Ni, the cost of raw materials may increase. Therefore, the upper limit of the Ni content may be set to 1.0% in consideration of both the cost and efficiency of the steel.

Copper (Cu): 0.8 to 1.8 wt%

In the present disclosure, copper (Cu) as an austenite phase stabilizing element is added in place of nickel (Ni). Further, cu as an element for improving corrosion resistance of the steel material under a reducing environment may be added in an amount of 0.8 wt% or more. However, excessive Cu not only increases the cost of the steel material, but also causes liquefaction and embrittlement at low temperatures. In addition, excessive Cu may segregate on the edge of the steel slab, thereby deteriorating hot workability of the steel. Therefore, the upper limit of the Cu content may be set to 1.8 wt% in consideration of the cost, efficiency and characteristics of the steel material.

The remaining component of the composition of the present disclosure is iron (Fe). However, the composition may contain unintended impurities that are inevitably incorporated from the raw materials or the surrounding environment. In the present disclosure, the addition of other unintended alloying elements than the above-described alloying elements is not excluded. Since impurities are known to any person skilled in the art, no specific mention of impurities is made in this disclosure.

Examples of unavoidable impurities include phosphorus (P) and sulfur (S), and according to one embodiment of the present disclosure, may include at least one of P (up to 0.035 wt%) and S (up to 0.01 wt%).

Phosphorus (P): at most 0.035 wt%

Phosphorus (P), which is an impurity inevitably contained in steel, is a main causticizing element of grain boundary corrosion or deterioration of hot workability of steel, and therefore, it is preferable to control the P content as low as possible. In the present disclosure, the upper limit of the P content may be set to 0.035 wt%.

Sulfur (S): at most 0.01 wt%

Sulfur (S), which is an impurity inevitably contained in steel, is a main causative element of deterioration of hot workability due to segregation in grain boundaries, and therefore, it is preferable to control the S content as low as possible. In the present disclosure, the upper limit of S may be set to 0.01 wt%.

Improving the yield strength of steel is important for reducing the weight of the steel and improving the stability. In addition, sufficient elongation should be obtained to manufacture structural materials having various shapes, including battery module cases. In addition, in order to obtain price competitiveness of austenitic stainless steel, it is necessary to reduce the amount of expensive austenite stabilizing elements such as Ni, and the amount of elements replacing the expensive elements such as Mn, N, and Cu should be appropriately adjusted.

However, in the case where the Ni content is reduced and Mn, N, and Cu are added, work hardening is rapidly improved, thereby deteriorating the elongation of the steel material and causing a decrease in resistance to thermal deformation, thereby deteriorating productivity, and thus coordination of each alloy element should be considered. In view of the yield strength, elongation and price competitiveness of the steel material as described above, the alloy element composition may be further limited to satisfy the expressions (1) to (4) in addition to the above-described composition.

In the present disclosure, in order to obtain excellent elongation of a cold rolled and annealed steel sheet prepared by cold rolling and annealing a steel material, expression (1) regarding the austenite phase fraction is derived.

(1)Ni+0.47Mn+15N≥7.5

Here, mn, ni, and N represent the content (wt%) of the element, respectively.

As the value of expression (1) decreases, the fraction of the austenite phase decreases. When the value of expression (1) is less than 7.5, the austenitic stainless steel may contain delta ferrite in an amount of 5% or more, or phase transformation to a martensitic phase occurs during cold rolling. Therefore, the elongation of austenitic stainless steel may deteriorate, and therefore, in the present disclosure, the lower limit of the value of expression (1) may be set to 7.5 to obtain a sufficient elongation.

Furthermore, in order to obtain a high yield strength of austenitic stainless steel, expression (2) is derived in the present disclosure in view of improving the yield strength by the stress field of the steel.

(2)23(C+N)+1.3Si+0.24(Cr+Ni+Cu)+0.1Mn≥12

Here, C, N, si, mn, cr, ni and Cu represent the content (wt%) of the element, respectively.

As the value of expression (2) increases, the stress field between lattices increases due to the dimensional difference between the alloy elements, so that the limit of plastic deformation to external stress increases. When the value of expression (2) is less than 12, it is difficult to obtain the yield strength required in the present disclosure. Therefore, in the present disclosure, the lower limit of the value of expression (2) may be set to 12 to obtain high-strength characteristics.

Further, expression (3) is derived in the present disclosure in consideration of phase transformation caused by deformation of austenitic stainless steel.

(3)551-462(C+N)-9.2Si-8.1Mn-13.7Cr-29(Ni+Cu)≤70

As the value of expression (3) increases, austenite compatibility is easily transformed by external stress. Specifically, when the value of expression (3) exceeds 70, austenitic stainless steel exhibits rapid strain-induced martensitic transformation behavior, resulting in plastic working non-uniformity. Therefore, a problem of deterioration of the elongation of austenitic stainless steel may occur, and thus the lower limit of the value of expression (3) may be set to 70.

Further, expression (4) is derived in consideration of dislocation slip behavior of the steel material due to deformation of austenitic stainless steel.

(4)11≤1+45C-5Si+0.09Mn+2.2Ni-0.28Cr-0.67Cu+88.6N≤17.

As the value of expression (4) decreases, it becomes difficult to exhibit cross sliding of the austenite phase caused by external stress. When the value of expression (4) is less than 11, austenitic stainless steel exhibits only planar sliding behavior with respect to deformation, and dislocations are rapidly accumulated due to external stress. Thus, problems of uneven plastic working and high work hardening may occur. Therefore, the elongation of austenitic stainless steel may deteriorate, it may be difficult to perform skin pass rolling, and hot rolling defects such as edge cracks may occur during deformation at high temperature, resulting in a problem of reduced productivity. In view of this, the lower limit of expression (4) may be set to 11.

In contrast, when the value of expression (4) is too high, cross sliding frequently occurs, resulting in a problem in which the plastic unevenness in which stress concentrates in the weak portion of the steel material increases. As the strength of steel increases, such embrittlement and plastic non-uniformity tend to increase, and thus the elongation of steel having high strength as in the present disclosure may deteriorate. In view of this, the upper limit of the value of expression (4) may be set to 17.

Since cr—mn steel in which Ni content is reduced has poor hot workability as compared to commercially available 300 series austenitic stainless steel, actual yield may be reduced due to edge cracking occurring during hot working, and correction cost may be increased, or additional equipment may be required to reduce edge cracking. According to the present disclosure, excellent hot workability can be obtained by satisfying the above-described alloy element composition and properly designing the alloy element composition using expressions (1) to (4) without adding a separate process and equipment. According to an embodiment of the present disclosure, the reduction of area of a steel billet having the above alloy element composition at a high temperature of 800 ℃ or more may be 50% or more.

In the low-cost austenitic stainless steel having high strength and high formability according to one embodiment of the present disclosure, the yield strength of the cold rolled and annealed steel sheet may be 400MPa. Further, in low-cost austenitic stainless steel having high strength and high formability, the elongation of the cold-rolled and annealed steel sheet may be 55% or more. In this regard, "cold-rolled and annealed steel sheet" refers to steel material prepared by treating a steel blank through hot rolling, annealing, cold rolling and annealing.

In the low-cost austenitic stainless steel having high strength and high formability according to one embodiment of the present disclosure, the yield strength of the skin-pass cold rolled steel sheet may be 800MPa or more. Further, according to one embodiment, in particular, the yield strength may be 800MPa or more, and the elongation may be 25% or more. In this regard, the "skin-pass cold-rolled steel sheet" refers to a steel material prepared by skin-pass cold-rolling the above cold-rolled and annealed steel sheet.

Hereinafter, a method of manufacturing a low-cost austenitic stainless steel having high strength and high formability according to the present disclosure will be described.

A method of manufacturing a low-cost austenitic stainless steel having high strength and high formability according to one embodiment of the present disclosure includes: preparing a steel billet comprising, in weight percent (wt%): greater than 0% and up to 0.08% of C, 0.2% to 0.25% of N, 0.8% to 1.5% of Si, 8.0% to 9.5% of Mn, 15.0% to 16.5% of Cr, greater than 0% and up to 1.0% of Ni, 0.8% to 1.8% of Cu, and Fe and other unavoidable impurities in the remaining portion, and satisfies the expressions (1) to (4); hot rolling the steel slab to produce a hot rolled steel sheet, and then hot annealing the hot rolled steel sheet to produce a hot rolled and annealed steel sheet; cold-rolling the hot-rolled and annealed steel sheet to prepare a cold-rolled steel sheet, then cold-annealing the cold-rolled steel sheet at 1050 ℃ or more to prepare a cold-rolled and annealed steel sheet, and subjecting the cold-rolled and annealed steel sheet to skin-pass cold-rolling to prepare a skin-pass cold-rolled steel sheet.

The reasons for the content of the alloy element and the numerical limitations of the expressions (1) to (4) are as described above. Hereinafter, each of the manufacturing steps will be described in detail.

The steel slab having the above alloy element composition may be hot rolled at a temperature of 1000 to 1300 ℃ to prepare a hot rolled steel sheet, and then annealed at a temperature of 1000 to 1100 ℃ to prepare a hot rolled and annealed steel sheet. In this regard, the annealing heat treatment may be performed for 10 seconds to 10 minutes.

Subsequently, the hot-rolled and annealed steel sheet is cold-rolled to prepare a cold-rolled steel sheet, and then annealed to prepare a cold-rolled and annealed steel sheet. Conventionally, as a method for improving yield strength of austenitic stainless steel, low temperature annealing heat treatment is performed at a low temperature of 1000 ℃ or less after cold rolling as described above. The low temperature annealing heat treatment is a method of improving strength by using energy accumulated in the steel during cold rolling without completing recrystallization. However, in such austenitic stainless steels that have undergone a low temperature annealing heat treatment, under-pickling may occur during the subsequent pickling process, or the aesthetic appearance and the possibility of quality non-uniformity may not be obtained.

According to one embodiment of the present disclosure, a hot rolled and annealed steel sheet is cold rolled to prepare a cold rolled steel sheet, and then annealed at 1050 ℃ or more to prepare a cold rolled and annealed steel sheet. In this case, the annealing heat treatment may be performed for 10 seconds to 10 minutes.

According to the present disclosure, since low-temperature annealing is not performed after cold rolling, excellent elongation can be obtained, and an appropriate yield strength level can be obtained by designing the alloy element composition.

The yield strength of the cold rolled and annealed steel sheet according to the present disclosure may be 400MPa or more.

The elongation of the cold rolled and annealed steel sheet according to the present disclosure may be 55% or more.

By designing the alloy element composition as described above, the cold rolled and annealed steel sheet can have an appropriate yield strength without a low-temperature annealing treatment by a process that does not put a load on production, thereby ensuring excellent price competitiveness.

Furthermore, according to the present disclosure, high yield strength can be obtained by adjusting the alloy element composition and subsequent skin pass rolling without a low temperature annealing treatment after cold rolling. According to one embodiment of the present disclosure, the yield strength of the skin-pass rolled steel sheet may be 800MPa or more. According to the present disclosure, the skin pass rolling may be performed at a reduction of 20% or more.

The skin pass rolling may improve strength by utilizing a high work hardening phenomenon when austenite phase is transformed into strain-induced martensite during cold deformation or utilizing dislocation stacking of steel. However, the elongation of the steel material may be rapidly deteriorated by the skin pass rolling.

According to the present disclosure, by properly controlling phase transition and dislocation behavior through designing the alloy element composition as described above, it is possible to prevent rapid decrease in elongation of steel caused by skin pass rolling. Thus, according to one embodiment of the present disclosure, a low-cost austenitic stainless steel having high strength and high formability in which a skin-pass cold rolled steel sheet has a yield strength of 800MPa or more and an elongation of 25% or more may be provided.

Hereinafter, the present disclosure will be described in more detail by way of examples. It should be noted, however, that the following examples are only intended to illustrate the present disclosure in more detail and are not intended to limit the scope of the present disclosure. This is because the scope of the present disclosure is determined by what is described in the claims and what can be reasonably inferred therefrom.

{ embodiment }

Billets having the alloy element compositions shown in table 1 below were prepared by ingot melting, heated at 1250 ℃ for 2 hours, and hot-rolled to prepare hot-rolled steel sheets. Then, the hot rolled steel sheet was subjected to an annealing heat treatment at 1100 ℃ for 90 seconds to prepare a hot rolled and annealed steel sheet. Subsequently, the steel material was cold-rolled at a reduction ratio of 70% to prepare a cold-rolled steel sheet, and then subjected to an annealing heat treatment at 1100 ℃ for 10 seconds to prepare a cold-rolled and annealed steel sheet.

The alloy element composition of each of the inventive examples and the comparative examples and the values obtained by substituting the content of the alloy element into the expressions (1) and (4) are shown in the following table 1.

(1)Ni+0.47Mn+15N≥7.5

(2)23(C+N)+1.3Si+0.24(Cr+Ni+Cu)+0.1Mn≥12

(3)551—462(C+N)-9.2Si-8.1Mn-13.7Cr-29(Ni+Cu)≤70

(4)11≤1+45C-5Si+0.09Mn+2.2Ni-0.28Cr-0.67Cu+88.6N≤17

TABLE 1

The yield strength, tensile strength and elongation of each of the cold rolled and annealed steel sheets of the inventive examples and comparative examples were measured. Further, yield strength, tensile strength and elongation of the skin-pass cold-rolled steel sheets prepared by skin-pass cold-rolling the cold-rolled and annealed steel sheets according to the inventive examples and comparative examples, respectively, at 20% were measured.

The measurements of yield strength, tensile strength and elongation were made according to ASTM standards, and the measured yield strength (YS, MPa), tensile strength (TS, MPa) and elongation (EL,%) are shown in table 2 below. Further, the occurrence of cracks in the annealed material was measured after the 180 ° adhesion bending test, and the results are shown in table 2 below.

TABLE 2

Referring to table 2, in the case of satisfying inventive examples 1 to 4 of the alloy element composition proposed by the present disclosure and satisfying expressions (1) to (4), it was determined that cold-rolled and annealed steel sheets had a yield strength of 400MPa or more and an elongation of 55% or more. Further, referring to table 2, the skin-pass cold-rolled steel sheets of invention examples 1 to 4 had a yield strength of 800MPa or more and a sufficient elongation of 25% or more even after skin-pass. Further, it was determined that the steels according to invention examples 1 to 4 were price competitive due to the relatively low Ni content of 1.0 wt% or less.

Referring to tables 1 and 2, steels according to comparative examples will be evaluated.

The steel material according to comparative example 1, which is a commercially available standard austenitic stainless steel, has a low yield strength because the steel material does not satisfy the alloy element composition and expressions (2), (3) and (4) of the present disclosure. Further, the commercial austenitic stainless steel of comparative example 1 has poor price competitiveness due to a high Ni content far higher than 8.1 wt% of the Ni content according to the present disclosure.

Since comparative example 2 does not satisfy the expression (1), a large amount of initial delta ferrite remains in the steel after cold rolling and annealing. During the forming process, for example, bending the steel, cracks easily occur at the interface between the delta ferrite phase and the austenite phase due to the phase difference, and thus the low value of expression (1) relates to the cracks at the time of bending. Therefore, although comparative example 2 exhibited high yield strength and high elongation due to high Si content, cracks occurred by the bending test due to residual delta ferrite, indicating poor formability including bending characteristics.

All the steels according to comparative examples 3 to 5 are steel types that do not satisfy expressions (1) to (4). Since expression (1) is not satisfied, a large amount of initial delta ferrite remains in the steel after cold rolling and annealing, and thus formability including bending characteristics is poor. Further, since expression (2) is not satisfied, low yield strength is obtained. Further, since the value of expression (3) exceeds 100, plastic unevenness easily occurs due to phase transformation into strain-induced martensite. Further, since the value of expression (4) is too low, serious dislocation accumulation occurs due to planar slip. Therefore, the elongation is deteriorated. In particular, the elongation of comparative examples 3 to 5 deteriorated due to failing to satisfy the expressions (3) and (4) is further deteriorated after the skin-pass rolling, so that the physical properties of the steel material are not suitable as a skin-pass rolled steel sheet.

In comparative example 6, since expression (1) is not satisfied, poor formability including bending characteristics is obtained, and thus a large amount of initial delta ferrite remains in the steel after cold rolling and annealing. Further, although the steel of comparative example 6 has high yield strength due to high Si content and expression (2), elongation is insufficient due to the influence of expressions (3) and (4).

Since expression (1) is not satisfied, the steel of comparative example 7 has poor formability including bending characteristics, and thus a large amount of initial delta ferrite remains in the steel after cold rolling and annealing. Further, since the value of expression (3) is greater than 100 (which does not satisfy expression (3)), plastic unevenness easily occurs during deformation due to phase transformation into strain-induced martensite. Accordingly, the cold rolled and annealed steel sheet and the skin-pass rolled steel sheet have poor elongation.

The steel material of comparative example 8 satisfies the content of alloying elements other than Cu, and satisfies the expressions (1) to (4). Accordingly, the cold rolled and annealed steel sheet has excellent yield strength and elongation. However, comparative example 8 has poor hot workability due to excessive Cu content. The evaluation thereof will be described in more detail below with reference to table 3.

The steels according to comparative examples 9 and 10 have poor hot workability due to excessive Si and Cu. The evaluation thereof will be described in more detail below with reference to table 3.

Since expression (1) is not satisfied, the steels according to comparative examples 11 and 12 have poor formability including bending characteristics due to a large amount of initial delta ferrite remaining in the steels after cold rolling and annealing. Further, since the value of expression (4) is too high, the plastic unevenness in which stress is concentrated on the weak portion of the steel material increases due to frequent cross sliding in comparative examples 11 and 12. Accordingly, the cold rolled and annealed steel sheet and the skin-pass rolled steel sheet have poor elongation. Although in commercial steels the effect of stress concentrated due to cross slip on elongation was negligible, in high strength steels having too high a value of expression (2) as in comparative examples 11 and 12, elongation was significantly deteriorated.

The austenitic stainless steel according to the present disclosure has excellent price competitiveness due to high productivity and high actual yield due to excellent hot workability. For comparative evaluation of hot workability, the reduction of area of the steel billets of the comparative examples and the inventive examples having a high elongation were measured at different temperatures. The reduction of area measurement was performed by the high temperature tensile test according to ASTM standards, and the results are shown in table 3.

TABLE 3

Referring to table 3, it was determined that in the case of inventive examples 1 to 4 satisfying the alloy element composition proposed by the present disclosure and satisfying expressions (1) to (4), a reduction of area of 50% or more was obtained at a high temperature of 800 ℃ or more.

As a commercial standard austenitic stainless steel, the steel according to comparative example 1 has excellent hot workability due to the addition of small amounts of Cu and N to reduce the amounts of Si and Ni required to improve strength. However, a large amount of Ni, which is an expensive element, is contained in commercial 300 series austenitic stainless steel, 300 series austenitic steel having a considerably low price competitiveness. Further, as evaluated in table 2, since the alloy element compositions and expressions (2), (3) and (4) are not satisfied, the steel has poor yield strength.

In comparative examples 2, 6, 9 and 10, excessive Si was added to improve yield strength of cold rolled and annealed steel sheets, and excessive Cu was added instead of Ni for price competitiveness. The steels according to comparative examples 2, 6, 9 and 10 have low hot workability due to excessive Si and Cu.

Since Si and Cu that deteriorate hot workability are added in the range set forth in the present disclosure, the steel material according to comparative example 7 has excellent hot workability. However, as evaluated in table 2, since expression (1) is not satisfied, the steel has poor formability, and since expression (3) is not satisfied, the cold-rolled and annealed steel sheet and the skin-pass steel sheet have poor elongation.

The Cu content of comparative example 8 exceeds the range proposed by the present disclosure. Too much Cu segregates on the edges or surfaces of the steel slab, causing embrittlement of the liquid metal, and deteriorating hot workability of comparative example 8. In comparative example 8, since hot workability is poor, actual yield may be reduced due to edge cracks occurring after hot rolling, and thus correction cost may be increased, or additional equipment may be required to reduce the edge cracks.

While the present disclosure has been particularly described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the disclosure.

INDUSTRIAL APPLICABILITY

According to the present disclosure, a low-cost austenitic stainless steel having high strength and high formability, which is suitable in various industrial fields, can be provided.

Claims

1. A low cost austenitic stainless steel having high strength and high formability comprising, in weight percent: greater than 0% and up to 0.08% C, 0.2% to 0.25% N, 0.8% to 1.5% Si, 8.0% to 9.5% Mn, 15.0% to 16.5% Cr, greater than 0% and up to 1.0% Ni, 0.8% to 1.8% Cu, and Fe and other unavoidable impurities in the remainder, and

the following expressions (1) to (4) are satisfied:

(1)Ni+0.47Mn+15N≥7.5

(2)23(C+N)+1.3Si+0.24(Cr+Ni+Cu)+0.1Mn≥12

(3)551-462(C+N)-9.2Si-8.1Mn-13.7Cr-29(Ni+Cu)≤70

(4)11≤1+45C-5Si+0.09Mn+2.2Ni-0.28Cr-0.67Cu+88.6N≤17

wherein C, N, si, mn, cr, ni and Cu represent the content of the element, respectively, in weight%,

wherein the cold rolled and annealed steel sheet has a tensile strength of 850MPa or more, and

wherein the elongation of the cold rolled and annealed steel sheet is 55% or more.

2. The low-cost austenitic stainless steel having high strength and high formability according to claim 1, wherein the yield strength of the cold rolled and annealed steel sheet is 400MPa or more.

3. The low-cost austenitic stainless steel having high strength and high formability according to claim 1, wherein the yield strength of the skin pass cold rolled steel sheet is 800MPa or more.

4. The low-cost austenitic stainless steel having high strength and high formability according to claim 3, wherein the elongation of the skin pass rolled steel sheet is 25% or more.

5. A method of manufacturing a low cost austenitic stainless steel having high strength and high formability, the method comprising:

preparing a steel billet comprising, in weight percent: greater than 0% and up to 0.08% of C, 0.2% to 0.25% of N, 0.8% to 1.5% of Si, 8.0% to 9.5% of Mn, 15.0% to 16.5% of Cr, greater than 0% and up to 1.0% of Ni, 0.8% to 1.8% of Cu, and the remainder of Fe and other unavoidable impurities, and satisfies the following expressions (1) to (4);

hot rolling the steel slab to produce a hot rolled steel sheet, and then hot annealing the hot rolled steel sheet to produce a hot rolled and annealed steel sheet;

cold-rolling the hot-rolled and annealed steel sheet to prepare a cold-rolled steel sheet, and then cold-annealing the cold-rolled steel sheet at 1050 ℃ or more to prepare a cold-rolled and annealed steel sheet; and

subjecting the cold-rolled and annealed steel sheet to skin-pass rolling to prepare a skin-pass rolled steel sheet:

(1)Ni+0.47Mn+15N≥7.5

(2)23(C+N)+1.3Si+0.24(Cr+Ni+Cu)+0.1Mn≥12

(3)551-462(C+N)-9.2Si-8.1Mn-13.7Cr-29(Ni+Cu)≤70

(4)11≤1+45C-5Si+0.09Mn+2.2Ni-0.28Cr-0.67Cu+88.6N≤17

wherein C, N, si, mn, cr, ni and Cu represent the contents of the respective elements in weight%, and

6. The method for manufacturing a low-cost austenitic stainless steel having high strength and high formability according to claim 5, wherein the skin pass rolling is performed at a reduction ratio of 20% or more.

7. The method of manufacturing a low cost austenitic stainless steel having high strength and high formability of claim 5, wherein the reduction of area of the steel blank at a high temperature of 800 ℃ or more is 50% or more.