Patent Application
20230287549
The present disclosure relates to an austenitic stainless steel with improved deep drawability, and more particularly, to an austenitic stainless steel in which cracks do not occur during a deep drawing process applied for transforming a plate into three-dimensional parts.
With the recent increase in price competitiveness, cost reduction in raw materials applied to parts have been required. Deep drawing is an efficient method for reducing manufacturing costs by omitting additional processes such as welding and stress-removing heat treatment. Meanwhile, in the case of involving formation of cylindrical shapes such as a cup or a battery, materials having excellent deep drawability are required.
Austenitic stainless steel materials may be used to form in complex shapes without causing any problem due to high elongation and have excellent work-hardening ability, as a steel type applied to various fields involving deep drawing.
In general, austenitic stainless steels are deformed by work-hardening occurring during cold working. In this case, it has been known that austenitic stainless steels having excellent work-hardening ability are easily formed.
However, when austenitic stainless steels are applied to deep drawing, strength is continuously increased by work-hardening, and stress is locally concentrated, resulting in fracture.
Meanwhile, although application of intermediate heat treatment may be considered to solve the problems of the increase in strength caused by work hardening, there are limitations in terms of processing time/processing costs.
Therefore, there is a need to develop austenitic stainless steels applicable as a deep drawing material because the intermediate heat treatment may be omitted and an increase in strength caused by work hardening may be minimized during the deep drawing.
Provided is an austenitic stainless steel capable of obtaining forming processability when applied to deep drawing by minimizing an increase in strength caused by work hardening.
In accordance with an aspect of the present disclosure, an austenitic stainless steel with improved deep drawability includes, in percent by weight (wt %), 0.01 to 0.05% of C, 0.01 to 0.25% of N, 1.5% or less of Si (excluding 0), 0.3 to 3.5% of Mn, 17.0 to 22.0% of Cr, 9.0 to 14.0% of Ni, 2.0% or less of Mo (excluding 0), 0.2 to 2.5% of Cu, and the balance of Fe and inevitable impurities, and satisfying Expression (1) below:
Cr+Si+2*Mo+3*(Ni+Cu)+50*(C+N)≥63 Expression (1):
wherein Cr, Si, Mo, Ni, Cu, C, and N represent the content (wt %) of each element.
In addition, according to an embodiment of the present disclosure, the austenitic stainless steel may satisfy Expression (2) below:
0<2.4*Cr+1.7*Mo+3.9*Si−2.1*Ni−Mn−0.4*Cu−58*C−64*N−13<5.5 Expression (2):
wherein Cr, Mo, Si, Ni, Mn, Cu, C, and N represent the content (wt %) of each element.
In addition, according to an embodiment of the present disclosure, the austenitic stainless steel may further include at least one of 0.04% or less of Al (excluding 0), 0.003% or less of Ti (excluding 0), 0.0025% or less of B (excluding 0), 0.035% or less of P, and 0.0035% or less of S.
In addition, according to an embodiment of the present disclosure, a true strain value may be 0.2 or less at a maximum work-hardening exponent in Expression (3) below:
σ=Kεn Expression (3):
wherein σ is a stress, K is a strength coefficient, ε is a strain, and n is a work-hardening exponent.
In addition, according to an embodiment of the present disclosure, a difference between a true strain value at the maximum work-hardening exponent and a true strain value at a work-hardening exponent of 0 may be 0.11 or more.
In addition, according to an embodiment of the present disclosure, an elongation may be 35% or more.
In addition, according to an embodiment of the present disclosure, a tensile strength may be 360 MPa or more.
In addition, according to an embodiment of the present disclosure, cracks do not occur until a fifth stage in the case of multi-stage formation at a drawing ratio of 1.7 to 4.3.
According to an embodiment of the present disclosure, an austenitic stainless steel applicable as a deep drawing material may be provided because intermediate heat treatment may be omitted and an increase in strength caused by work hardening may be minimized during deep drawing.
An austenitic stainless steel with improved deep drawability according to an embodiment of the present disclosure includes, in percent by weight (wt %), 0.01 to 0.05% of C, 0.01 to 0.25% of N, 1.5% or less of Si (excluding 0), 0.3 to 3.5% of Mn, 17.0 to 22.0% of Cr, 9.0 to 14.0% of Ni, 2.0% or less of Mo (excluding 0), 0.2 to 2.5% of Cu, and the balance of Fe and inevitable impurities and satisfies Expression (1) below.
Cr+Si+2*Mo+3*(Ni+Cu)+50*(C+N)≥63 Expression (1):
Here, Cr, Si, Mo, Ni, Cu, C, and N represent the content (wt %) of each element.
Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. These embodiments are provided to fully convey the concept of the present disclosure to those of ordinary skill in the art. The present disclosure may, however, be embodied in many different forms and should not be construed as limited to the exemplary embodiments set forth herein. In the drawings, parts unrelated to the descriptions are omitted for clear description of the disclosure and sizes of elements may be exaggerated for clarity.
Throughout the specification, the term “include” an element does not preclude other elements but may further include another element, unless otherwise stated.
As used herein, the singular forms are intended to include the plural forms as well, unless the context clearly indicates otherwise.
Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings.
An austenitic stainless steel is a steel type used in products having various shapes due to high elongation and excellent formability. Under a stress, the austenitic stainless steel is deformed by transformation, i.e., transformation induced plasticity, from an unstable austenite phase to a martensite phase, at room temperature.
In this regard, since the generated martensite phase has a high strength, strength of the material also increases. In other words, both deformation and an increase in strength simultaneously occur in an austenitic stainless steel by work-hardening. The work-hardening ability is represented using a work-hardening exponent, and the work-hardening exponent varies according to strain.
Austenitic stainless steels having excellent work-hardening ability have been known to easy formed.
However, when a deep drawing process, which is performed while reducing a blank diameter, is applied to an austenitic stainless steel, strength continuously increases in accordance with work-hardening to cause local concentration of stress in the material resulting in fracture. Also, cracks may suddenly occur due to aging cracks.
Therefore, in the deep drawing that causes a large amount of deformation, it is important to uniformly induce deformation over the entire material and minimize variation in strength while deformation occurs. That is, in order to improve deep drawability of the austenitic stainless steel, work-hardening should be inhibited.
Meanwhile, work-hardening of the austenitic stainless steel is related to the degree of stability of an austenite phase. The work-hardening of the austenitic stainless steel may be inhibited by increasing the degree of stability by controlling elements.
However, workability of an austenitic stainless steel represented by elongation is derived from work-hardening due to transformation induced plasticity, and thus a decrease in work-hardening causes a problem of deteriorating workability of the austenitic stainless steel.
The present inventors have made various studies to enhance elongation of an austenitic stainless steel and inhibit an increase in strength caused by work-hardening during a deep drawing process and have found those described below.
In the present disclosure, as a result of examining factors for preventing fracture in an austenitic stainless steel in the case of applying deep drawing, it was found that deep drawability of the austenitic stainless steel may be improved by obtaining a certain amount of deformation without excessively increasing strength while suppressing over work hardening by inhibiting martensite phase transformation induced by stress. To this end, a composition of alloying elements capable of obtaining continuous deformation without excessively increasing strength has been derived.
An austenitic stainless steel with improved deep drawability according to an embodiment of the present disclosure include, in percent by weight (wt %), 0.01 to 0.05% of C, 0.01 to 0.25% of N, 1.5% or less of Si (excluding 0), 0.3 to 3.5% of Mn, 17.0 to 22.0% of Cr, 9.0 to 14.0% of Ni, 2.0% or less of Mo (excluding 0), 0.2 to 2.5% of Cu, and the balance of Fe and inevitable impurities.
Hereinafter, reasons for numerical limitations on the contents of alloying elements in the embodiment of the present disclosure will be described. Hereinafter, the unit is wt % unless otherwise stated.
The content of C is from 0.01 to 0.05%.
Carbon (C) is an element effective on stabilization of an austenite phase and may be added in an amount of 0.01% or more to inhibit formation of martensite and obtain strength during deformation. However, an excess of C may bind to Cr to induce grain boundary precipitation of a Cr carbide, thereby deteriorating corrosion resistance. Therefore, an upper limit the C content may be controlled to 0.05%.
The content of N is from 0.01 to 0.25%.
Nitrogen (N), like carbon, is an element effective on stabilization of an austenite phase and may be added in an amount of 0.01% or more to obtain deep drawability. However, an excess of Ni may form a nitride, thereby deteriorating the surface quality, and thus an upper limit of the N content may be controlled to 0.25%.
The content of Si is 1.5% or less (excluding 0).
Silicon (Si) is an element serving as a deoxidizer during a steelmaking process and used to obtain strength and corrosion resistance of an austenitic stainless steel. However, an excess of Si, as a ferrite phase-stabilizing element, may promote martensite transformation and precipitate intermetallic compounds such as a 6 phase to deteriorate mechanical properties and corrosion resistance. Thus, an upper limit of the Si content may be controlled to 1.5%.
The content of Mn is from 0.3 to 3.5%.
Manganese (Mn), like carbon (C) and nitrogen (N), is an element stabilizing austenite and has an effect on inhibiting an increase in strength during a forming process, and thus Mn may be added in an amount of 0.3% or more. However, an excess of Mn may form a large amount of S-based inclusions (MnS), thereby deteriorating corrosion resistance and surface gloss of an austenitic stainless steel. Thus, an upper limit of the Mn content may be controlled to 3.5%.
The content of Cr is from 17.0 to 22.0%.
Chromium (Cr) stabilizes ferrite as a basic element contained in stainless steels in the largest amount among the elements used to improve corrosion resistance. In the present disclosure, Cr may be added in an amount of 17.0% or more to obtain corrosion resistance by forming a passivated layer that inhibits oxidation.
However, as excess of Cr, as a ferrite phase-stabilizing element, decreases stability of an austenite phase to promote martensite transformation. Accordingly, an increase in the Ni content increases manufacturing costs, and intermetallic compounds such as a 6 phase are precipitated to deteriorate mechanical properties and corrosion resistance. Therefore, an upper limit of the Cr content may be controlled to 22.0%.
The content of Ni is from 9.0 to 14.0%.
Nickel (Ni) is the strongest austenite phase-stabilizing element. As the Ni content increases, an austenite phase is stabilized to soften a material, and it is essential to include 9% or more of Ni to inhibit work-hardening caused by deformation-induced martensite. However, use of a large amount of Ni, which is a high-priced element, cases an increase in costs of raw materials. Therefore, an upper limit of the Ni content may be controlled to 14.0% in consideration of costs and efficiency of steel materials.
The content of Mo is 2.0% or less (excluding 0).
Molybdenum (Mo) is an element effective on obtaining corrosion resistance. However, an excess of molybdenum, as a ferrite phase-stabilizing element, may decrease stability of an austenite phase making it difficult to obtain deep drawability, and precipitate intermetallic compounds such as a 6 phase to deteriorate mechanical properties and corrosion resistance. Therefore, an upper limit of the Mo content may be controlled to 2.0%.
The content of Cu is from 0.2 to 2.5%%.
Copper (Cu), as an austenite phase-stabilizing element added instead of the high-priced nickel (Ni), may be added in an amount of 0.2% or more to enhance price competitiveness and deep drawability. However, when the Cu content is excessive, c-Cu precipitates with a low-melting point are formed to deteriorate the surface quality. Thus, an upper limit of the Cu content may be controlled to 2.5%.
In addition, according to an embodiment of the present disclosure, the austenitic stainless steel may further include at least one of 0.04% or less of Al (excluding 0), 0.003% or less of Ti (excluding 0), 0.0025% or less of B (excluding 0), 0.035% or less of P, and 0.0035% or less of S.
The content of Al is 0.04% or less (excluding 0).
Aluminum (Al), as a strong deoxidizer, reduces a content of oxygen in molten steels. However, an excess of Al may cause sleeve defects of a cold-rolled strip due to an increase in nonmetallic inclusions, and therefore an upper limit of the Al content may be controlled to 0.04%.
The content of Ti is 0.003% or less (excluding 0).
Titanium (Ti) is an element effective on corrosion resistance of a steel because Ti preferentially binds to interstitial elements such as carbon (C) or nitrogen (N) to form precipitates (carbonitrides), thereby reducing amounts of solute C and solute N in the steel and inhibits formation of a Cr depletion region. However, an excess of Ti may form Ti-based inclusions causing a problem in a manufacturing process and a surface defect such as scabs, and therefore an upper limit of the Ti content may be controlled to 0.003%.
The content of B is 0.0025% or less (excluding 0).
Boron (B) is as an element effective on obtaining satisfactory surface quality by inhibiting occurrence of cracks during a casting process. However, an excess of B may form a nitride (BN) on the surface of a product during an annealing/acid pickling process, thereby deteriorating the surface quality, and thus an upper limit of the B content may be controlled to 0.0025%.
The content of P is 0.035% or less.
Phosphorus (P), as an impurity that is inevitably contained in steels, is a major element causing grain boundary corrosion or deterioration of hot workability, and therefore, it is preferable to control the P content as low as possible. In the present disclosure, an upper limit of the P content is controlled to 0.035%.
The content of S is 0.0035% or less.
Sulfur (S), as an impurity that is inevitably contained in steels, is a major element causing deterioration of hot workability as being segregated in grain boundaries, and therefore, it is preferable to control the S content as low as possible. In the present disclosure, an upper limit of the S content is controlled to 0.0035% or less.
The remaining component of the composition of the present disclosure is iron (Fe). However, the composition may include unintended impurities inevitably incorporated from raw materials or surrounding environments, and thus addition of other alloy components is not excluded. These impurities are known to any person skilled in the art of manufacturing and details thereof are not specifically mentioned in the present disclosure.
As described above, work-hardening of the austenitic stainless steel is caused by transformation of an austenite phase unstable at room temperature into a martensite phase by a stress generated by plastic deformation.
Continuous deformation leads to continuous phase transformation which increases strength of the austenitic stainless steel until the material is fractured. In order to obtain deep drawability, transformation into the martensite phase needs to be inhibited.
In the present disclosure, Expression (1) below was derived in consideration of phase transformation occurring due to deformation of the austenitic stainless steel.
Specifically, in the present disclosure, attempts have been made to increase the degree of stability by increasing the contents of austenite-stabilizing elements such as Mn, N, Cu, and Ni. Accordingly, phase transformation into the martensite phase may be inhibited and work-hardening of the austenitic stainless steel may be inhibited.
Cr+Si+2*Mo+3*(Ni+Cu)+50*(C+N) Expression (1):
Here, Cr, Si, Mo, Ni, Cu, C, and N represent the content (wt %) of each element.
The austenitic stainless steel with improved deep drawability according to an embodiment of the present disclosure satisfies a value, represented by Expression (1), of 63 or more.
The present inventors have found that a variation in strength during deformation caused by an external stress increases as the value of Expression (1) decreases. Specifically, when the value of Expression (1) is less than 63, the austenitic stainless steel including the above-described alloying elements exhibits a rapid deformation-induced martensite transformation behavior by external deformation, or plastic non-uniformity due formation of twin crystals. Accordingly, due to problems of deterioration in elongation of the austenitic stainless steel and reduction in deep drawability in multi-stage formation, a lower limit of Expression (1) is controlled to 63.
An increase in strength due to work-hardening may be explained using a stress-strain curve of
σ=Kεn
Here, σ is a stress, K is a strength coefficient, and ε is a strain.
Meanwhile, the equation is expressed as the following equation by applying the common logarithm to both sides.
log σ=log K+n*log ε
In other words, in the stress-strain log relationship, the work-hardening exponent n corresponds to the slope of the graph, and a higher slope means a more increase in strength of a material during plastic deformation.
In order to improve deep drawability of the austenitic stainless steel of the present disclosure, Expression (2) below was derived in consideration that continuous deformation should be obtained without increasing an excessive increase a strength.
2.4*Cr+1.7*Mo+3.9*Si−2.1*Ni−Mn−0.4*Cu−58*C−64*N−13 Expression (2):
Here, Cr, Mo, Si, Ni, Mn, Cu, C, and Nr represent the content (wt %) of each element.
The austenitic stainless steel with improved deep drawability according to an embodiment of the present disclosure satisfies a value, represented by Expression (2), of 0 or more and 5.5 or less.
The present inventors have found that as the value of Expression (2) increases, transformation into martensite more easily occurs by an external stress resulting in an excessive increase in a strength, thereby deteriorating formability. Specifically, when the value of Expression (2) is 5.5 or more, the strength continuously increases from tensile deformation to immediately before fracture resulting in a problem of occurrence of rapid fracture. Therefore, elongation cannot be obtained, and thus an upper limit of the value of Expression (2) is controlled to 5.5.
On the other hand, it has been confirmed that when the value of Expression (2) is too low, expression of cross slip of an austenite phase by an external stress becomes difficult. Specifically, when the value of Expression (2) is less than 0, the austenitic stainless steel exhibits only a planar slip behavior with respect to deformation and dislocation pile-up by an external stress proceeds to exhibit plastic non-uniformity and high work-hardening. As a result, elongation and yield ratio of the austenitic stainless steel deteriorate, and thus a lower limit of the value of Expression (2) is controlled to 0.
Meanwhile, the austenitic stainless steel with improved deep drawability according to an embodiment of the present disclosure may have a true strain value of 0.2 or less in the case where the work-hardening exponent is a maximum value.
In
Referring to
In consideration of the fact that a certain amount of deformation should be obtained without excessively increasing strength to improve deep drawability of an austenitic stainless steel, it is required to dispose point A having a maximum increase in strength at a relatively small strain and a certain amount of strain needs to obtained from point A to point B.
The austenitic stainless steel with improved deep drawability according to the disclosed embodiment has a true strain value of 0.2 or less when the work-hardening exponent is the maximum value.
In
In the austenitic stainless steel with improved deep drawability according to the disclosed embodiment, a difference between a true stain value at the maximum work-hardening exponent and a true stain value at a work-hardening exponent of 0 is 0.11 or more.
In other words, as long as a maximum work-hardening exponent is obtained at a small strain and continuous deformation is obtained without causing an excessive increase in strength, elongation of the austenitic stainless steel may be obtained while inhibiting occurrence of cracks when applied to multi-stage processing more than 2 stages.
The austenitic stainless steel having improved deep drawability according to the disclosed embodiment satisfying the composition ratio of alloying elements and the above-described relationship may have an elongation of 35% or more and a tensile strength of 360 MPa or more.
In addition, in the case of the austenitic stainless steel having improved deep drawability according to the disclosed embodiment, no cracks occur up to the fifth stage in a multi-stage formation of two or more stages, at a drawing ratio of 1.7 to 4.3.
Hereinafter, the embodiments of the present disclosure will be described in more detail with reference to the following examples.
Slabs having compositions of alloying elements shown in Table 1 below and having a thickness of 200 mm were prepared by a continuous casting process, heated at 1,250° C. for 2 hours, and hot-rolled to a thickness of 6 mm. After hot rolling, hot annealing was performed at 1,150° C. and wound. Then, the hot-rolled coil was cold-rolled and cold-annealed twice to a thickness of 1 mm. The cold rolling was performed with a reduction ratio of 30 to 70% per pass, and the cold annealing was performed in a furnace at a temperature of 1100 to 1200° C. within 5 minutes.
In Table 1 below, the values of Expressions (1) and (2) are value derived by substituting wt % of each alloying element into Expressions (1) and (2).
Cr+Si+2*Mo+3*(Ni+Cu)+50*(C+N) Expression (1):
2.4*Cr+1.7*Mo+3.9*Si−2.1*Ni−Mn−0.4*Cu−58*C−64*N−13 Expression (2):
The number of multi-stage formation and the work-hardening exponent of each steel sheet were measured. Specifically, deep drawing formation was performed in five stages using a blank having a diameter of 85 mm with a first-stage punch diameter of 50 mm, a second-stage punch diameter of 38 mm, a third-stage punch diameter of 30 mm, a fourth-stage punch diameter of 24 mm, and a fifth-stage punch diameter of 20 mm. Drawing ratios of the respective stages were 1.7 at the first stage, 2.2 at the second stage, 2.8 at the third stage, 3.5 at the fourth stage, and 4.3 at the fifth stage.
In each stage, based on a case where no cracks occur until 48 hours have elapsed after forming a product, maximum numbers of formation are shown in Table 2 below.
Subsequently, a tensile test was conducted on a sample prepared according to the JIS13B standards. Then, true stress-true strain were calculated using stress-strain values obtained during the test, and a maximum work-hardening exponents (a), a true strain value (b) at the maximum work-hardening exponent, a true strain value (c) at the work-hardening exponent of 0, and a difference between the true stain value (b) at the maximum work-hardening exponent and the true stain value (c) at the work-hardening exponent of 0 were obtained and are shown in Table 2 below.
Also, tensile strength (MPa) and elongation (%) measured during the tensile test are shown in Table 2 below.
Referring to Table 2, in the case of Examples 1 to 23 satisfying the composition of alloying elements and values of Expressions (1) and (2) suggested in the present disclosure, not only the tensile strength of 350 MPa or more but also excellent elongation of 35% or more may be obtained. Also, in the case of multi-stage formation of two or more stages at a drawing ratio of 1.7 to 4.3, cracks do not occur until the fifth stage, and thus they are applicable to fields that require deep drawing formation of complex shapes.
In Comparative Examples 1 to 6 and Comparative Examples 17 to 21, strength continuously increases during work hardening because the values of Expression (1) were less than 63, and martensite transformation actively occurs due to deformation resulting in cracks during multi-stage formation because the value of Expression (2) exceeded 5.5.
In Comparative Examples 7 to 16, a rapid increase in strength was caused by formation of twin crystals during processing because the value of Expression (1) was less than 63, and the value of Expression (2) was less than 0. The increase in strength due to formation of twin crystals continuously occurred according to strain, and thus stress became non-uniform during the deep drawing, failing to obtain formation with a sufficient depth.
As such, according to the disclosed embodiment, an austenitic stainless steel having an elongation of 35% or more and a tensile strength of 360 MPa or more without having cracks until the fifth formation in the case of forming at the second or more formation at a drawing ratio of 1.7 to 4.3 may be manufactured by controlling the alloying elements and the relationship therebetween.
While the present disclosure has been particularly described with reference to exemplary embodiments, it should be understood by those of skilled in the art that the scope of the present disclosure is not limited thereby and various changes in form and details may be made without departing from the spirit and scope of the present disclosure.
The present disclosure is applicable to various industrial fields involving deep drawing.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2020-0110646 | Aug 2020 | KR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/KR2021/009569 | 7/23/2021 | WO |
