Austenitic stainless steel with low Ni content and high tensile strength / ductility properties.

Abstract

Austenitic stainless steel alloy compositions are provided having reduced Ni content, which, in combination with a process including martensitic thermomechanical treatment, preferably with thickness reduction of more than 65%, provide a new generation of advanced high strength steels with a combination of tensile strength and total elongation in the range of 1000 MPa / 35-55% to 1350 MPa / 25-45%, as well as good formability and weldability properties, which are useful in the manufacture of many products, particularly in the automotive industry.

Description

[Technical Field]

[0001] The present invention relates to a low-nickel-content austenitic stainless steel alloy having high tensile strength and elongation properties, a method for manufacturing the same, and articles manufactured therefrom. [Background technology]

[0002] Stainless steel is a steel alloy containing a minimum of 10.5% by mass of chromium and a maximum of 1.2% by mass of carbon, characterized by its corrosion resistance and mechanical properties. Chromium reacts with oxygen in air and water to form a microscopically thin, inert surface film of chromium oxide. This passive layer prevents further corrosion by inhibiting the diffusion of oxygen into the steel surface, thus preventing corrosion from spreading throughout the entire metal.

[0003] Stainless steel can be classified into four main types based on its crystal structure: austenitic, ferritic, martensitic, and duplex (austenite-ferritic structure). Austenitic stainless steel has austenite as its main crystal structure (face-centered cubic (FCC)). This austenitic crystal structure is achieved by adding sufficient amounts of austenite-stabilizing elements, such as nickel, manganese, carbon, and nitrogen. Standard austenitic stainless steel contains 16-25% chromium, at least 8% nickel, and the remainder being iron, and often includes other alloying elements to obtain various properties. Austenitic steel generally has much better formability and weldability than ferritic grades, excellent toughness (impact resistance) even at very low temperatures, and is non-magnetic under annealing conditions, although some magnetism may occur during cold working, for example, on bolts or bent edges. Austenitic steel is known for its good mechanical properties over a wide temperature range (from cryogenic to high temperatures), as well as good workability and corrosion resistance, and is the most widely used grade of stainless steel. They can be easily used in the manufacture of all kinds of products.

[0004] Demand for stainless steel is rapidly increasing worldwide, and the subsequent high demand for alloying metals in steel production is leading to rising metal prices. Nickel, in particular, is experiencing a sharp price increase. Consequently, various attempts are being made to replace nickel in austenitic stainless steel with other alloying elements. However, these steels have proven unsuitable for certain products that require cold working, including significant weight loss.

[0005] Current austenitic stainless steel (ASS) exhibits both high tensile strength and excellent ductility, thus meeting the functional requirements of many vehicles. However, generally, ASS is an expensive option for many components due to its alloy content. Austenitic stainless steel has two main subgroups.

[0006] Conventional stainless steels obtain their austenitic structure primarily through the addition of nickel. Until 2003, nearly 70% of the grades of stainless steel produced were conventional ASS, which typically contained 8-10 wt% Ni to stabilize the austenitic phase at room temperature. The most common of these is grade EN-1.4301, which contains approximately 18% Cr and 8% Ni. This 8% Ni is the minimum amount of Ni that can be added to 18%Cr stainless steel to convert all ferrite into austenite. Another common steel is grade EN-1.4401, which is essentially grade EN-1.4301 with 2% molybdenum (Mo) added to improve corrosion resistance. However, rising prices of alloying elements, particularly Ni, and the extreme fluctuations in these prices have led stainless steel users to have serious concerns about the use of conventional ASS.

[0007] Low-Ni ASS uses manganese (Mn) and nitrogen (N), which are also austenite-forming agents, instead of Ni, making it less susceptible to price fluctuations for manufacturers and ultimately users. Nitrogen is a gas and can only be added in limited amounts before problems, such as chromium nitride and gaseous voids, can form. Since the Mn and N combination is usually not sufficient to convert all ferrite to austenite, some Ni is still added, albeit in smaller amounts compared to those used in conventional ASS grades. Furthermore, low-Ni ASS reduces the amount of Cr, a ferrite-forming agent, thus reducing the amount of austenite-forming agent required. However, the properties of low-Ni ASS are significantly narrower in its range of applications due to its lower corrosion resistance, moldability, and ductility compared to conventional ASS grades.

[0008] On the other hand, because the nitrogen content is higher compared to conventional ASS, the tensile strength and hardness are also improved. For example, grade EN-1.4372 has a yield strength approximately 20% higher than grade EN-1.4301. However, it has the disadvantage of being more difficult to form. Adding copper improves formability, and this has the advantage that copper is an austenite-forming agent, but in this case, the strain hardening rate decreases.

[0009] Some of the most common registered low-Ni ASS grades are shown below, compared to the composition of grade EN-1.4301. [Table 1]

[0010] Some industries require steel with extremely high tensile strength and good ductility and formability. At the same time, it is important that these steels enable lightweighting strategies (by reducing thickness without losing mechanical properties) and are low-cost. Steels with a yield strength exceeding 550 MPa are generally called advanced high-strength steel (AHSS). Steels with a tensile strength exceeding 780 MPa are sometimes called "ultra-high-strength steel."

[0011] To meet the demand for improved safety and fuel efficiency through lighter vehicle structures, the use of AHSS in automobiles is rapidly expanding as research progresses. AHSS is a steel with a microstructure that contains phases other than ferrite, pearlite, and cementite, such as martensite, bainite, austenite, and / or retained austenite, in sufficient quantities to produce unique mechanical properties. The AHSS grades currently in application, or being investigated further by the steel community, are mainly duplex (DP), composite (CP), ferrite-bainite (FB), martensitic (MS), transformation-induced plasticity (TRIP), hot-forming (HF), twinning-induced plasticity (TWIP), and quenching & partitioning (Q&P).

[0012] AHSS grades are designed to meet the functional performance requirements of specific parts. Each type has its own unique microstructural characteristics, alloy additives, processing requirements, and advantages and limitations associated with its use. Recently, funding and research for the development of "third-generation" AHSS have increased. These are steels with improved combinations of tensile strength and ductility, like the steel proposed in this invention. Metastable ASS is one of the important grades of ASS in which austenite can transform into martensite during deformation. Therefore, metastable grades exhibit higher tensile strength and better formability than grades in which the austenite is stable. Metastable ASS is used in a variety of structural applications, such as structural components of railroads and automobiles, due to the need for weight reduction and crash safety in automobiles. However, their structural applications are limited because they have relatively low yield strength.

[0013] International Publication No. 2014 / 135441 describes a stainless steel alloyed with manganese and chromium but without nickel. It is a fully austenitic stainless steel with a special hardening mechanism induced by cold working (cold rolling) followed by heat treatment below the recrystallization temperature. This induces individual dislocations and mechanical twinning in the microstructure, improving properties through twinning-induced plasticity (TWIP). This effect requires a large amount of Mn (more than 20%). This large amount of Mn may reduce the corrosion resistance of the steel.

[0014] U.S. Patent Application Publication 2009 / 0324441 discloses an austenitic cast steel characterized by having content of 2-8% Ni, 5-12% Mn, 12-20% Cr, 0.005-0.500% N, 0.0-2.5% Mo, 0.0-1.2% Nb, 0-2% Cu, 0-4% Si, and 0.01-0.15% C. Furthermore, the alloy contains more than 0% and up to 4% Al as essential components. The presence of Al and Si promotes martensite formation and the TRIP effect at room temperature, improving tensile strength and elongation. The problem is that the aluminum can make the steel brittle, especially during bending, because it forms a B2 crystal structure.

[0015] International Publication No. 2016 / 027009 discloses a low-nickel austenitic stainless steel characterized by containing 0.0-0.4 wt% C, 0-3 wt% Si, 3-20 wt% Mn, 10-30 wt% Cr, 0.0-4.5 wt% Ni, 0-3 wt% Mo, 0-3 wt% Cu, 0.05-0.50 wt% N, 0.0-0.5 wt% Nb, 0.0-0.5 wt% Ti, 0.0-0.5 wt% V, the remainder being Fe, and unavoidable impurities. After cold deformation and annealing at less than 1050°C, the grain size is less than 10 micrometers.

[0016] U.S. Patent No. 4814140A discloses an austenitic stainless steel alloy having improved so-called galling resistance and a tensile strength of approximately 900 MPa, containing 3.57% Ni, 5.94% Mn, 15.96% Cr, 0.16% N, 0.98% Si, and 0.102% C. U.S. Patent No. 4814140A does not mention cold rolling.

[0017] U.S. Patent No. 4,609,577A discloses an alloy containing 2.94% Ni, 6.45% Mn, 16.31% Cr, 0.16% N, 0.21% Mo, 0.63% Cu, 0.90% Si, and 0.05% C for use as a build-up weld to improve the wear resistance between metals in products, such as steelmaking rolls.

[0018] International Publication 2012 / 160594A1 discloses an alloy intended to suppress the increase in magnetic permeability while maintaining a desired hardness. This alloy contains 1.0-2.0% Ni, 7.0-9.0% Mn, 16.0-18.0% Cr, 0.10-0.20% N, 0.00-2.0% Mo, 0.00-0.10% Nb, 0.00-2.3% Cu, 0.00-1.0% Si, and 0.00-0.12% C, with a permeability of -50 ≤ M d30 It is characterized by Mn ≤ -30. With WO2012 / 160594A1 alloy, it is not expected that a good combination of tensile strength and total elongation can be achieved.

[0019] Nevertheless, there is a need for AHSS steel, which offers an improved combination of tensile strength and elongation compared to current grades, good formability, and the potential for lower-cost and more efficient joining, providing a new option to meet the demanding requirements of sectors such as automotive. [Overview of the Initiative]

[0020] The present invention provides a low-Ni austenitic stainless steel alloy composition having high tensile properties, a combination of tensile strength and total elongation in the range of 1000 MPa / 35 - 55% elongation to 1350 MPa / 25 - 45% elongation, good formability and good weldability behavior, and enabling weight reduction. Through a suitable martensitic processing heat treatment including cold rolling that induces martensite transformation from metastable austenite and returning the martensite induced by cold rolling to austenite by heat treatment, these alloys are provided with an austenite microstructure that improves the mechanical properties of the material. Further, good elongation is maintained even after the stamping process, which may help absorb more energy during a collision accident in the automotive industry. This new alloy can be used in automotive applications that require complex shapes and high collision requirements, such as floor tunnels, under-seat beams, side sills, etc.

[0021] Thus, in a first aspect, the present invention relates to Ni: 2.00 - 3.60 wt%, Mn: 6.0 - 7.0 wt%, Cr: 15.0 - 16.5 wt%, N: 0.085 - 0.180 wt%, Mo: 0.00 - 0.50 wt%, Nb: 0.00 - 0.10 wt%, Cu: 0.00 - 1.00 wt%, Si: 0.50 - 1.00 wt%, C: 0.065 - 0.095 wt%, Fe: the balance, and unavoidable impurities, and relates to an alloy composition containing the same.

[0022] In another aspect, the present invention relates to a method for manufacturing austenitic stainless steel from the alloy of the present invention, the following steps, a) hot rolling the alloy defined above at a temperature of 1200°C - 1300°C, for example 1260°C - 1285°C, b) A step of solution annealing the alloy from step (a) at a temperature of 1000°C to approximately 1200°C, for example, 1080°C to 1120°C for 70 to 170 seconds, c) A process in which the alloy obtained from process (b) is cold-rolled to obtain a thickness reduction rate of more than 50%, d) The alloy obtained from step (c) is annealed at a temperature of 900°C to 1200°C, for example 950°C to 1100°C, for 30 seconds to 300 seconds, for example 30 seconds to 200 seconds. Includes.

[0023] The inventors have discovered that this martensitic heat treatment process yields an austenitic microstructure that improves the tensile and ductile properties of the material.

[0024] The present invention also relates to austenitic stainless steel obtained by previously defined methods. Preferably, the austenitic stainless steel of the present invention has a combination of tensile strength and total elongation ranging from 1000 MPa / 35-55% elongation to 1350 MPa / 25-45% elongation.

[0025] In another aspect, the present invention relates to the use of specified austenitic stainless steel in the automotive industry. [Modes for carrying out the invention]

[0026] The present invention provides novel alloys exhibiting austenitic microstructure stainless steel with good productivity and mechanical properties, pitting corrosion resistance, and weldability after suitable martensitic heat treatment. These alloys offer a good combination of tensile strength and total elongation, exceeding 1000 MPa and elongation of 25% or more, preferably in the range of 1000 MPa / 35-55% elongation to 1350 MPa / 25-45% elongation. This allows for a reduction in the thickness of the components, and therefore the steel of the present invention meets the requirement for weight reduction and is useful for industrial applications.

[0027] In this invention, the total elongation is measured according to the standard UNE-EN ISO 6892-1:2017.

[0028] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as they would be generally understood by those skilled in the art to whom this disclosure belongs.

[0029] alloy composition The alloy composition of the present invention is carefully designed to ensure the industrial production of austenitic stainless steel with a low Ni content and an excellent combination of tensile and elongation properties without impairing corrosion resistance, weldability, and formability, making it suitable for use in automotive parts where weight reduction and mechanical and formability requirements are high.

[0030] In composition design, equations and experimental data that allow for the analysis of many combinations of alloying elements and their content are used to analyze various parameters (SFE, M) related to the successful fabrication and performance of new alloys. d30 The ferrite index and the PRE-Mn coefficient were considered.

[0031] Parameters SFE (Stack Fault Energy) and M d30 This relates to the stability of austenite that transforms into martensite during the molding process. In particular, SFE refers to dislocation movement, and the lower the SFE, the more likely austenite is to transform into martensite when it is formed.

[0032] M d30 In this case, this parameter specifies the temperature at which 50% of the austenite transforms into martensite after 30% true tensile strain. d30 A higher value indicates lower austenite stability, and therefore susceptibility to martensite formation. Nohara et al. found that M d30The following empirical formula for determining [it] was proposed (Nohara K., Ono Y., and Obashi N.: "Composition and grain size dependencies of strain-induced martensitic transformation in metastable austenitic stainless steels", Tetsu-to-Hagane, Vol. 63 (1977) 772-782), Equation (I): M d30 (°C) = 551 - 462(%C + %N) - 9.2%Si - 8.1%Mn - 13.7%Cr - 29(%Ni + %Cu) - 18.5%Mo - 68%Nb.

[0033] In Equation (I), the % of each element should be understood as weight percentage, wt%, and thus, the % of each element in Equation (I) means the amounts disclosed herein for each element in each embodiment. The examples show the M d30 values of three exemplary alloy compositions of the present invention.

[0034] In one embodiment, in any of the embodiments disclosed herein, the alloy of the present invention has an M d30 value of at least 55, preferably at least 60, more preferably at least 65.

[0035] Thus, those skilled in the art will readily understand how much the amount of each element in the alloy of the present invention that results in an M d30 value of at least 55 is.

[0036] The alloy of the present invention can be further characterized in that the M d30 value is 170 or less, preferably 165 or less, even more preferably 160 or less. In a more specific embodiment, the alloy of the present invention is characterized in that the M d30 value is 55 to 170, preferably 60 to 165, more preferably 65 to 160.

[0037] In a specific embodiment, the alloy composition of the present invention has the formula M d30M obtained by (°C)=551-462(%C+%N)-9.2%Si-8.1%Mn-13.7%Cr-29(%Ni+%Cu)-18.5%Mo-68%Nb d30 The value is characterized by being at least 55, preferably at least 60, and more preferably at least 65. In certain embodiments, the alloy of the present invention is calculated as described above. d30 The value is further characterized by being 170 or less, preferably 165 or less, and more preferably 160 or less. In more specific embodiments, the alloy of the present invention is calculated as described above. d30 The value is characterized by being 55 to 170, preferably 60 to 165, and more preferably 65 to 160.

[0038] The ferrite index is important for avoiding hot ductility problems during the hot rolling process, which affect the workability of austenitic stainless steel. It represents the amount of delta ferrite that solidifies during the casting process and may be present in the material during and after the hot rolling stage. The presence of this phase in the material upon shipment degrades formability and corrosion resistance. A lower ferrite index results in better workability.

[0039] Finally, the PRE-Mn (Pitting Corrosion Index - Mn) value is related to the pitting corrosion resistance of a material and is a function of its chemical composition. Mn negatively affects corrosion behavior, and since the alloy of this invention has a high content of this element, Mn is included in the PRE formula to account for this negative effect. A higher PRE-Mn value indicates higher pitting corrosion resistance.

[0040] In one embodiment, the composition of the present invention is Ni: 2.00~3.60% by weight, Mn: 6.0~7.0% by weight, Cr:15.0~16.5% by weight, N:0.085~0.180% by weight, Mo: 0.00~0.50% by weight, Nb: greater than 0.00% by weight and less than or equal to 0.40% by weight. Cu: 0.00~1.00% by weight, Si: 0.40 to 1.00 wt%, C: 0.060 to 0.095 wt%, S: 0.00 to 0.007 wt%, P: 0.00 to 0.045 wt%, Ti: exceeding 0.00 wt% and not exceeding 0.45 wt%, Fe: the balance, and unavoidable impurities, and contains

[0041] Throughout this description, the amounts of elements and their decimal places are shown in accordance with the tolerances specified in the standard EN 10088-2 (2015) for the chemical composition of stainless steel.

[0042] The specific ranges are important to achieve a good balance of desirable properties. The following alloy compositions A1 to A5 in the table are specific embodiments of the present invention, the values are expressed in wt%, and the balance is Fe and unavoidable impurities.

Table 2

[0043] In the present invention, the amounts of each element present in the alloy are expressed in weight percentages, wt%. In the present invention, ranges are expressed as including or not including the lower limit value and / or the upper limit value. Those skilled in the art will easily understand that a range, for example, 0.085 ≦ N ≦ 0.180 means that the amount of element N present in the alloy is 0.085 to 0.180, and it is considered that the lower limit value and the upper limit value are within such a range. Conversely, the symbol "<" means excluding the value written next to it. For example, in the range 0.00 < Ti < 0.40, the amount of Ti is more than 0.00 and less than 0.40, and thus the lower limit value and the upper limit value are excluded. Therefore, those skilled in the art will easily understand that the range of C of 0.070 to less than 0.095 is a range including the lower limit value but not including the upper limit value, that is, a range corresponding to 0.070 ≦ C < 0.095.

[0044] The alloy composition of the present invention is lower than grade EN-1.4372 InIt has a content. By reducing this element, a positive effect on the martensite heat treatment is brought about, promoting the formation of strain-induced martensite during the cold rolling process, and it has been observed that it brings good properties to the final steel. However, since it has been found that the formation of delta ferrite increases as the Ni content decreases, it is important to control it so that problems do not occur during hot rolling. If the content of this phase is high at high temperatures, problems with hot ductility (edge cracking and slivers) occur.

[0045] The amount of Ni in the alloy composition of the present invention is 2.00 ≦ Ni ≦ 3.60, preferably 2.00 ≦ Ni ≦ 3.40, more preferably 2.00 ≦ Ni ≦ 3.20, and even more preferably 2.10 ≦ Ni ≦ 3.20. In a preferred embodiment, the Ni content in Composition A1 is 2.00 < Ni < 3.60, preferably 2.00 < Ni < 3.40, more preferably 2.00 < Ni ≦ 3.20, and even more preferably 2.10 < Ni ≦ 3.20. In another embodiment, these Ni amounts are applied to Composition A2. In yet another embodiment, these Ni amounts are applied to Composition A3. In another embodiment, these Ni amounts are applied to Composition A4, and in yet another embodiment, these Ni amounts are applied to Composition A5. These amounts of Ni have been found to promote an appropriate martensite hot working treatment with desirable final properties without problems in industrial production.

[0046] Mn The reduction is also beneficial for the martensite hot working treatment and pitting corrosion resistance because this element has an adverse effect on the PRE-Mn value. However, since the formation of delta ferrite increases as the Mn content decreases, it is analyzed that it is necessary to control this reduction to avoid problems in hot rolling.

[0047] In the alloy composition of the present invention, the Mn content is 6.0 ≦ Mn ≦ 7.0, preferably 6.2 ≦ Mn ≦ 6.9, more preferably 6.2 ≦ Mn ≦ 6.8, and even more preferably 6.2 ≦ Mn ≦ 6.7. In a preferred embodiment, the Mn content in Composition A1 is 6.0 < Mn < 7.0, preferably 6.2 < Mn < 6.9, more preferably 6.2 < Mn < 6.8, and even more preferably 6.2 < Mn < 6.7. In another embodiment, these Mn amounts are applied to Composition A2. In yet another embodiment, these Mn amounts are applied to Composition A3. In another embodiment, these Mn amounts are applied to Composition A4, and in yet another embodiment, these Mn amounts are applied to Composition A5. These Mn values are beneficial for controlling the pitting corrosion resistance without causing hot rolling troubles, especially the properties of the final steel.

[0048] Cr In relation to the effect of Cr , reducing Cr is beneficial for martensitic processing heat treatment, and controlling the formation of delta ferrite has been studied, thereby avoiding an increase in the N and C contents harmful to the formation of martensite during cold rolling. However, the lower the Cr content, the lower the pitting corrosion resistance, so the level of this element is adjusted to maintain the pitting corrosion resistance at least equivalent to EN-1.4372.

[0049] The amount of Cr in the alloy composition of the present invention is 15.0 ≦ Cr ≦ 16.5, preferably 15.2 ≦ Cr ≦ 16.3, more preferably 15.2 ≦ Cr ≦ 16.2, and even more preferably 15.2 ≦ Cr ≦ 15.9. In a preferred embodiment, the Cr content in Composition A1 is 15.0 < Cr < 16.5, preferably 15.2 < Cr < 16.3, more preferably 15.2 < Cr < 16.2, and even more preferably 15.2 < Cr ≦ 15.9. In another embodiment, these Cr amounts are applied to Composition A2. In yet another embodiment, these Cr amounts are applied to Composition A3. In another embodiment, these Cr amounts are applied to Composition A4, and in yet another embodiment, these Cr amounts are applied to Composition A5.

[0050] To avoid problems in the melting plant and the hot rolling stage, N it is necessary to control the amount of. Furthermore, it has been observed that due to its reduction, the austenite phase becomes unstable and transforms into martensite during the cold rolling process, so it is an important element. However, its presence is important to control the formation of delta ferrite and maintain pitting corrosion resistance at least equivalent to EN-1.4372.

[0051] The amount of N in the alloy composition of the present invention is 0.085 ≦ N ≦ 0.180, preferably 0.100 ≦ N ≦ 0.180, more preferably 0.100 ≦ N ≦ 0.160, and even more preferably 0.110 ≦ N ≦ 0.150. In a preferred embodiment, the N content in Composition A1 is 0.085 < N < 0.180, preferably 0.100 < N < 0.180, more preferably 0.100 < N < 0.160, and even more preferably 0.110 < N < 0.150. In another embodiment, these amounts of N are applicable to Composition A2. In yet another embodiment, these amounts of N are applicable to Composition A3. In another embodiment, these amounts of N are applicable to Composition A4, and in yet another embodiment, these amounts of N are applicable to Composition A5.

[0052] Know The amount of is also analyzed, and it has been observed that the reduction of this element brings a beneficial effect on the martensite processing heat treatment and controls the formation of delta ferrite at high temperatures. However, this has an adverse effect on pitting corrosion resistance. The amount of Mo in the alloy composition of the present invention is 0.00 ≦ Mo ≦ 0.50, preferably 0.00 < Mo ≦ 0.50, more preferably 0.01 ≦ Mo ≦ 0.50, and even more preferably 0.01 ≦ Mo ≦ 0.40. In a preferred embodiment, the Mo content in Composition A1 is 0.00 < Mo < 0.50, preferably 0.01 < Mo < 0.50, more preferably 0.01 < Mo < 0.40. In yet another embodiment, these amounts of Mo are applicable to Composition A3. In another embodiment, these amounts of Mo are applicable to Composition A4, and in yet another embodiment, these amounts of Mo are applicable to Composition A5.

[0053] Nb In relation to the content, it has been studied that the reduction of this element has a beneficial effect on the martensite heat treatment. On the other hand, Nb carbides and carbonitrides are powerful elements for controlling the austenite grain size and are known to lead to an improvement in mechanical properties. In the alloy composition of the present invention, the amount of Nb is 0.00 < Nb ≦ 0.40, preferably 0.00 < Nb ≦ 0.30, more preferably 0.00 < Nb ≦ 0.20, and even more preferably 0.05 ≦ Nb ≦ 0.20. In a preferred embodiment, the Nb content in Composition A1 is 0.00 < Nb < 0.40, preferably 0.00 < Nb < 0.30, more preferably 0.00 < Nb < 0.20, and even more preferably 0.05 < Nb < 0.20. In another embodiment, these Nb amounts are applied to Composition A2. In yet another embodiment, these Nb amounts are applied to Composition A3. In another embodiment, these Nb amounts are applied to Composition A4, and in yet another embodiment, these Nb amounts are applied to Composition A5.

[0054] Cu The amount of... is an austenite former, so it affects the stability of the austenite phase and has an adverse effect on the formation from austenite to martensite. On the other hand, it can improve the ductility of the alloy. Also, it has been observed that the less the amount of Cu, the more the formation of delta ferrite at high temperatures. The amount of Cu in the alloy composition of the present invention is 0.00 ≦ Cu ≦ 1.00, preferably 0.00 ≦ Cu ≦ 0.70, more preferably 0.00 ≦ Cu ≦ 0.60, and even more preferably 0.40 ≦ Cu ≦ 0.60. In a preferred embodiment, the Cu content in Composition A1 is 0.00 < Cu < 1.00, preferably 0.00 < Cu < 0.70, more preferably 0.00 < Cu < 0.60, and even more preferably 0.40 < Cu < 0.60. In another embodiment, these Cu amounts are applied to Composition A2. In yet another embodiment, these Cu amounts are applied to Composition A3. In another embodiment, these Cu amounts are applied to Composition A4, and in yet another embodiment, these Cu amounts are applied to Composition A5.

[0055] ToReduction is known to be beneficial for controlling the precipitation of delta ferrite during the hot rolling process. The amount of Si in the alloy composition is 0.40 ≦ Si ≦ 1.00, preferably 0.50 ≦ Si ≦ 0.90, more preferably 0.50 ≦ Si ≦ 0.80, and even more preferably 0.50 ≦ Si ≦ 0.75. In a preferred embodiment, the Si content in Composition A1 is 0.40 < Si < 1.00, preferably 0.50 < Si < 0.90, more preferably 0.50 < Si < 0.80, and even more preferably 0.50 < Si < 0.75. In another embodiment, these Si amounts are applicable to Composition A2. In yet another embodiment, these Si amounts are applicable to Composition A3. In another embodiment, these Si amounts are applicable to Composition A4, and in yet another embodiment, these Si amounts are applicable to Composition A5.

[0056] To avoid high rolling loads, C the level of needs to be controlled. Reduction of the C level has an important beneficial effect on the formation from austenite to martensite, not only increasing the instability of austenite, but it has also been found that the lower the C content, the higher the formation of delta ferrite and the better the corrosion resistance. The amount of C in the alloy composition is 0.060 ≦ C ≦ 0.095, preferably 0.065 ≦ C ≦ 0.095, more preferably 0.070 ≦ C ≦ 0.095, and even more preferably 0.070 ≦ C < 0.095. In a preferred embodiment, the C content in Composition A1 is 0.060 < C < 0.095, preferably 0.065 < C < 0.095, more preferably 0.070 < C < 0.095. In another embodiment, these C amounts are applicable to Composition A2. In yet another embodiment, these C amounts are applicable to Composition A3. In another embodiment, these C amounts are applicable to Composition A4, and in yet another embodiment, these C amounts are applicable to Composition A5.

[0057] Since Ti is a stabilizing element that prevents the precipitation of chromium carbides, OfThe amount has a beneficial effect on pitting corrosion resistance. Titanium is also very effective as a micro-alloy in steel and affects the microstructure by forming nitrides (TiN) and carbides (TiC). While not wishing to be bound by any particular theory, this behavior is thought to be related to better grain size control and is likely related to changes in the nature and morphology of precipitates that may improve the mechanical properties of the steel.

[0058] The amount of Ti in the alloy composition is 0.00 < Ti ≤ 0.45, preferably 0.00 < Ti ≤ 0.40, more preferably 0.00 < Ti ≤ 0.30, more preferably 0.00 < Ti ≤ 0.10, even more preferably 0.00 < Ti ≤ 0.045. In a preferred embodiment, the Ti content in Composition A1 is 0.00 < Ti < 0.45, preferably 0.00 < Ti < 0.40, more preferably 0.00 < Ti < 0.30, more preferably 0.00 < Ti < 0.10, even more preferably 0.00 < Ti < 0.045, even more preferably 0.00 < Ti < 0.015. In another embodiment, these Ti amounts are applicable to Composition A2. In yet another embodiment, these Ti amounts are applicable to Composition A3. In another embodiment, these Ti amounts are applicable to Composition A4, and in yet another embodiment, these Ti amounts are applicable to Composition A5.

[0059] As explained, most alloying elements have an opposite effect on the material (i.e., they improve some properties but worsen others). The inventors have found that using the proposed range of elements achieves a good balance and results in an ASS with the remarkable properties as described above.

[0060] Regarding other elements, such as P and S, their influence on austenite stability is very small and they are usually present in normal amounts in EN-1.4372 and conventional ASS. The S content is important to avoid hot ductility problems and control corrosion resistance and weldability.

[0061] The amount of S in the alloy composition is 0.00 ≤ S ≤ 0.007, preferably 0.00 ≤ S ≤ 0.0065, more preferably 0.00 ≤ S ≤ 0.006, and even more preferably 0.00 ≤ S ≤ 0.005. In a preferred embodiment, the S content in composition A1 is 0.00 ≤ S < 0.007, preferably 0.00 ≤ S < 0.0065, preferably 0.00 ≤ S < 0.006, and even more preferably 0.00 ≤ S < 0.005. In another embodiment, these S amounts apply to composition A2. In yet another embodiment, these S amounts apply to composition A3. In yet another embodiment, these S amounts apply to composition A4, and in yet another embodiment, these S amounts apply to composition A5.

[0062] The amount of P in the alloy composition is 0.00 ≤ P ≤ 0.045, preferably 0.00 ≤ P ≤ 0.04, and more preferably 0.00 ≤ P ≤ 0.035. In a preferred embodiment, the P content in composition A1 is 0.00 ≤ P < 0.045, preferably 0.00 ≤ P < 0.04, and more preferably 0.00 ≤ P < 0.035. In another embodiment, these P amounts apply to composition A2. In yet another embodiment, these P amounts apply to composition A3. In yet another embodiment, these P amounts apply to composition A4, and in yet another embodiment, these P amounts apply to composition A5.

[0063] In certain embodiments, the alloy composition of the present invention has elemental amounts of options a) to l), a) Ni: greater than 2.00% by weight and less than 3.60% by weight, preferably less than 3.40% by weight. b) Mn: greater than 6.0% by weight and less than 7.0% by weight, preferably greater than 6.2% by weight and less than 6.9% by weight. c) Cr: greater than 15.0% by weight and less than 16.5% by weight, preferably greater than 15.2% by weight and less than 16.3% by weight. d) N: 0.085~0.180% by weight, preferably 0.100~0.180% by weight. e) Mo: greater than 0.00% by weight and less than 0.50% by weight, f) Nb: greater than 0.00% by weight and less than 0.40% by weight, g) Cu: greater than 0.00% by weight and less than 1.00% by weight, preferably less than 0.70% by weight. h) Si: greater than 0.40% by weight and less than 1.00% by weight, preferably greater than 0.50% by weight and less than 0.90% by weight. i) C: 0.060~0.095% by weight, preferably 0.065~0.095% by weight, j) S: Less than 0.007% by weight, k) P: less than 0.045% by weight, l) Ti: greater than 0.00% by weight and less than 0.45% by weight, preferably less than 0.40% by weight. It is selected independently from one of the following.

[0064] In a particular embodiment, the alloy composition of the present invention has an elemental amount of option a) to i), a) Ni: greater than 2.00% by weight and less than 3.40% by weight, preferably 3.20% by weight or less. b) Mn: greater than 6.2% by weight and less than 6.9% by weight, preferably less than 6.8% by weight. c) Cr: greater than 15.2% by weight and less than 16.3% by weight, preferably less than 16.2% by weight. d)N: 0.100~0.180% by weight, e) Nb: greater than 0.00% by weight and less than 0.40% by weight, preferably less than 0.30% by weight. f) Cu: greater than 0.00% by weight and less than 0.70% by weight, preferably less than 0.60% by weight. g) Si: greater than 0.50% by weight and less than 0.90% by weight, preferably less than 0.80% by weight. h) C: 0.065~0.095% by weight, preferably 0.070~0.095% by weight. i) Ti: greater than 0.00% by weight and less than 0.40% by weight, preferably less than 0.30% by weight. It is selected independently from one of the following.

[0065] In a particular embodiment, the alloy composition of the present invention has an elemental amount of option a) to i), a) Ni: more than 2.00% by weight and 3.20% by weight or less, preferably more than 2.10% by weight and 3.20% by weight or less. b) Mn: greater than 6.2% by weight and less than 6.8% by weight, preferably less than 6.7% by weight. c) Cr: greater than 15.2% by weight and less than 16.2% by weight, preferably 15.9% by weight or less. d) N: 0.100~0.180% by weight, preferably 0.100~0.160% by weight. e) Nb: greater than 0.00% by weight and less than 0.30% by weight, preferably less than 0.20% by weight. f) Cu: greater than 0.00% by weight and less than 0.60% by weight, preferably greater than 0.40% by weight and less than 0.60% by weight. g) Si: greater than 0.50% by weight and less than 0.80% by weight, preferably less than 0.75% by weight. h) C: 0.070~0.095% by weight, preferably less than 0.070~0.095% by weight. i) Ti: greater than 0.00% by weight and less than 0.30% by weight, preferably less than 0.10% by weight. It is selected independently from one of the following.

[0066] In a particular embodiment, the alloy composition of the present invention has an elemental amount of option a) to i), a) Ni: greater than 2.10% by weight and less than or equal to 3.20% by weight. b) Mn: greater than 6.2% by weight and less than 6.7% by weight, c) Cr: greater than 15.2% by weight and less than or equal to 15.9% by weight. d)N: 0.100~0.160% by weight, e) Nb: greater than 0.00% by weight and less than 0.20% by weight, f) Cu: greater than 0.40% by weight and less than 0.60% by weight, g)Si: greater than 0.50% by weight and less than 0.75% by weight, h) C: Less than 0.070~0.095% by weight, i) Ti: greater than 0.00% by weight and less than 0.10% by weight, It is selected independently from one of the following.

[0067] The alloy compositions in the table below represent other specific embodiments of the present invention, with values ​​expressed in weight percent, and the remainder being Fe and unavoidable impurities. [Table 3]

[0068] In one embodiment, the composition of the present invention contains Mo.

[0069] In one embodiment, the composition of the present invention contains Cu.

[0070] In certain embodiments, the alloy of the present invention contains S.

[0071] In certain embodiments, the alloy of the present invention contains P.

[0072] In another embodiment, the composition of the present invention comprises Mo and Cu.

[0073] In another embodiment, the composition of the present invention comprises Mo and S.

[0074] In another embodiment, the composition of the present invention comprises Mo and P.

[0075] In another embodiment, the composition of the present invention comprises S and P.

[0076] In another embodiment, the composition of the present invention comprises Mo and S.

[0077] In another embodiment, the composition of the present invention comprises Mo, Cu, and S.

[0078] In another embodiment, the composition of the present invention comprises Mo, Cu, and P.

[0079] In another embodiment, the composition of the present invention comprises Mo, Cu, S, and P.

[0080] The combination of the above embodiments is also considered to be part of the present invention.

[0081] In one embodiment, the composition of the alloy is as specified above, but the amount of Ni is 2.00 to 3.20% by weight, most preferably about 2.00 to 3.00% by weight, and the amount of Mn is 6.2 to 6.5% by weight.

[0082] In one embodiment, the composition of the alloy is as specified above, but the amount of Ni is 2.00 to 3.20% by weight, most preferably about 2.00 to 3.00% by weight, and the amount of Cr is 15.4 to 15.9% by weight.

[0083] In one embodiment, the composition of the alloy is as specified above, but the amount of Ni is 2.00 to 3.20% by weight, most preferably about 2.00 to about 3.00% by weight, and the amount of N is 0.100 to 0.160% by weight.

[0084] In one embodiment, the alloy composition is as specified above, but the amount of Ni is 2.00 to 3.20% by weight, most preferably about 2.00 to about 3.00% by weight, and the amount of Cu is 0.40 to 0.60% by weight.

[0085] In one embodiment, the composition of the alloy is as specified above, but the amount of Ni is 2.00 to 3.20% by weight, most preferably about 2.00 to about 3.00% by weight, and the amount of Si is 0.50 to 0.75% by weight.

[0086] In one embodiment, the composition of the alloy is as specified above, but the amount of Ni is 2.00 to 3.20% by weight, most preferably about 2.00 to about 3.00% by weight, and the amount of C is 0.070 to 0.090% by weight.

[0087] In a more preferred embodiment, the alloy composition is : 2.00~3.20% by weight, most preferably about 2.00~about 3.00% by weight, Mn: 6.2~6.5% by weight, Cr:15.4~15.9wt%, N:0.100~0.160% by weight, Mo: 0.00~0.50% by weight, Nb: 0.00~0.10% by weight, Cu:0.40~0.60wt%, Si:0.50~0.75wt%, C: 0.070~0.090% by weight, Fe: remainder, and unavoidable impurities, Includes.

[0088] In one embodiment, the stainless steel of the present invention is characterized by being selected from flat plates, elongated pieces, or powder products.

[0089] alloy casting The first step in the production of stainless steel is the selection of raw materials: stainless steel and carbon steel scrap, metallic manganese, and ferrochrome. A person skilled in the art will be able to select the raw materials necessary to obtain the desired composition, taking into account the compositions specified above. The raw materials are introduced into an arc electric furnace where they are melted by the action of graphite electrodes. When the steel is liquid, it is poured into a transfer ladle and moved to an AOD converter, where decarburization, reduction, desulfurization processes, and final adjustment of the chemical composition take place. Finally, the liquid metal is sent to a continuous casting machine, where it solidifies into slab form.

[0090] Hot rolling Following casting, the alloy of the present invention is subjected to a hot rolling process, and the thickness of the slab is reduced by several passes at high temperature through two rolling mills, one a roughing mill and the other a finishing mill.

[0091] The alloys of the present invention are preferably hot-rolled at a temperature of 1200°C to 1300°C, preferably 1240°C to 1300°C, preferably 1250°C to 1285°C, more preferably 1260°C to 1285°C, and even more preferably 1270°C to 1280°C. Most preferably, they are hot-rolled at a temperature of about 1275°C. This process is carried out in a walking beam furnace with a holding time of 45 to 80 minutes, preferably 50 to 70 minutes, and most preferably about 1 hour, within a leveling zone.

[0092] The temperature, time, and conditions (speed, pressure, etc.) for hot rolling may be adjusted by those skilled in the art depending on the width and thickness of the black coil.

[0093] Solution annealing After the hot rolling process, the stainless steel produced from the alloy of the present invention is subjected to a solution annealing process to restore its microstructure and obtain the correct mechanical properties.

[0094] This process is crucial for obtaining a microstructure characterized by an equiaxed austenite grain matrix, a completely recrystallized structure, and a reduction in residual delta ferrite (typically <1%).

[0095] Temperature and time are crucial conditions for this heat treatment.

[0096] The solution annealing temperature is approximately 1000°C to 1200°C, preferably approximately 1050°C to 1150°C, more preferably approximately 1080°C to 1120°C, even more preferably approximately 1090°C to 1110°C, and most preferably approximately 1100°C.

[0097] The duration of the solution annealing treatment is preferably 50 to 180 seconds, and more preferably 70 to 170 seconds, depending on the width and thickness of the steel strip.

[0098] Martensitic heat treatment Following solution annealing, the material undergoes martensitic processing heat treatment consisting of cold rolling and annealing steps.

[0099] This process yields the final austenitic microstructure, as well as the desired mechanical properties of tension and elongation necessary for the high-performance use of the steel of the present invention. The martensitic processing thermal process includes cold heavy rolling to induce martensitic transformation, followed by annealing of strain-induced martensite (SIM) to reverse transformation to austenite.

[0100] The volume fraction of SIM increases with increasing strain, and at a certain strain called saturation strain, martensite formation saturates. If the strain increases after saturation strain, martensite fragmentation occurs during deformation, leading to an increase in defects within SIM and an increase in nucleation sites during the austenite reverse transformation. Finally, during subsequent annealing, the martensite reverts to austenite, forming austenite grains.

[0101] Process 1: Cold rolling Cold rolling is carried out with equipment well known to those skilled in the art, and typically, thickness reduction is achieved by passing steel between the rolls of a Zenzimir rolling mill, for example, a reversible rolling mill having a roll stand consisting of 20 rolls. Several passes may be required to achieve the desired plasticity and thickness reduction.

[0102] Cold rolling preferably results in a thickness reduction of at least 50%, more preferably at least 65%, and even more preferably 65-75%. In this way, stainless steel with good mechanical properties is obtained, having a thickness in the range of 2.00-0.50 mm, more preferably 1.5-1.0 mm.

[0103] This process yields a material having a strain-induced martensite (SIM) volume fraction exceeding 75%, preferably exceeding 85%, and most preferably exceeding 95%. The martensite volume fraction can be determined by converting the magnetic measurement values ​​obtained with a ferrite scope.

[0104] Step 2: Final annealing After the cold rolling process, the stainless steel of the present invention undergoes an annealing process to complete the martensitic heat treatment.

[0105] Annealing is preferably carried out using equipment well known to those skilled in the art, such as a continuous process line of annealing and pickling.

[0106] The annealing process temperature is 900°C to 1200°C, preferably 950°C to 1150°C. More preferably, the annealing process temperature is 950°C to 1100°C. Even more preferably, the annealing process temperature is 950°C to 1075°C. More preferably, the annealing treatment is carried out at a temperature of 950°C to 1050°C. Generally, as the annealing temperature decreases, the hardness increases and the grain size decreases. Therefore, a temperature of about 950°C is most preferred and also offers high energy efficiency.

[0107] Annealing in martensitic heat treatment is performed for a duration of 30 to 300 seconds, for example, 30 to 200 seconds, depending on the thickness of the coil. Shorter times result in higher energy efficiency.

[0108] In one embodiment, depending on the thickness of the coil, an annealing process at a temperature of approximately 950°C and for a time of 50 to 300 seconds is preferred.

[0109] In another embodiment, depending on the thickness of the coil, an annealing process at a temperature of approximately 950°C and for a time of 50 to 200 seconds is preferred.

[0110] In another embodiment, depending on the thickness of the coil, an annealing process at a temperature of approximately 1000°C and for a time of 40 to 175 seconds is preferred.

[0111] In another embodiment, depending on the thickness of the coil, an annealing process at a temperature of approximately 1075°C and for a time of 30 to 150 seconds is preferred.

[0112] These temperatures and times result in excellent performance in terms of tensile strength / total elongation, particle size, and cold formability.

[0113] Those skilled in the art may adjust and select annealing conditions according to the size and thickness of the coil. Thicker coils require higher temperatures and / or longer annealing times.

[0114] The final annealing process yields an austenite microstructure that includes perfectly recrystallized austenite with equiaxed grains and no martensite.

[0115] Properties of the obtained austenitic stainless steel The austenitic stainless steel of the present invention, obtained from the alloy of the present invention by applying the martensitic heat treatment specified above, has remarkable properties.

[0116] In terms of microstructure, it exhibits an austenite microstructure and is a steel that is slightly finer than the standard EN-1.4372. In certain embodiments, the stainless steel of the present invention has a grain size of at least ASTM 12.

[0117] The tensile strength / total elongation balance, which is crucial for the industrial applications of these materials, is in the range of 1000 MPa / 35-55% elongation to 1350 MPa / 25-45% elongation. In other words, the materials of the present invention can achieve tensile strength values ​​in the range of 1000-1350 MPa, total elongation in the range of 35-55% for a tensile strength of 1000 MPa, and total elongation in the range of 25-45% for a tensile strength of 1350 MPa.

[0118] This is significantly higher than the standard steel EN-1.4372, which generally has a tensile strength of 680-880 MPa. The steel of the present invention also provides a high yield strength value of generally over 550 MPa, which exceeds the value provided by the standard EN-1.4372.

[0119] In this description, the yield strength, tensile strength, and total elongation values ​​correspond to the results of tensile tests conducted in accordance with the standard UNE-EN ISO 6892-1:2017.

[0120] Furthermore, the bending behavior of the new alloy is good, and no cracks occur during bending, similar to the standard EN-1.4372.

[0121] Regarding stamping, the new alloy exhibits excellent performance, showing high tensile strength and high elongation after stamping, which is very helpful in absorbing more energy in the event of a collision.

[0122] The new alloys are also suitable for welding. They are superior to conventional carbon steels, and the weld microstructure is free of defects. The hardness of the weld is similar to that of the base material, and they have a favorable high tensile strength. Therefore, the welding performance is suitable for automotive use. In fact, the cross-tensile weld strength is much higher than that achievable with high-tensile carbon steel. The new alloys can be easily welded to carbon steel, yielding good results. And one of the very important advantages of the new alloys is that, unlike carbon steel, they do not require a zinc coating. This means that industrial welding processes, such as spot welding, laser welding, and MIG / MAG welding, can be applied with greater consistency and quality, as the zinc layer on carbon steel causes porosity and spatter in laser and MIG / MAG welding, as well as rapid electrode degradation in resistance spot welding.

[0123] Virtual crash simulations demonstrate that the new alloy guarantees superior performance compared to standard steel. The new material allows for improved structural performance, enabling a reduction in part thickness and thus enhancing the passive safety of vehicles.

[0124] In short, the ASS of the present invention possesses excellent properties despite the reduced amount of Ni. They are easy to form, bend, or stamp, and are also excellent in welding. Their tensile strength and elongation properties allow for reduced component thickness and enable collision resistance and energy absorption. Therefore, they are very suitable for the vehicle industry, especially the automotive industry.

[0125] Use of austenitic stainless steel The novel ASS of the present invention has many potential applications. One of its most important advantages is that it provides high tensile strength and elongation, enabling significant weight reduction. The combination of high tensile strength and good ductility makes the alloy of the present invention suitable for use in transportation, consumer goods, and construction sectors.

[0126] This new alloy can be used in applications requiring complex shapes and collision requirements, such as floor tunnels, side sills, underseat beams, dash panels, and all components involved in collisions that are bolted to the white body (BIW), i.e., front crash beams + crash boxes, door crash beams, etc.

[0127] Another specific embodiment Embodiment 1 Ni: 2.00~3.60% by weight, Mn: 6.0~7.0% by weight, Cr:15.0~16.5% by weight, N:0.085~0.180% by weight, Mo: 0.00~0.50% by weight, Nb: 0.00~0.10% by weight, Cu: 0.00~1.00% by weight, Si: 0.50~1.00% by weight, C: 0.065~0.095% by weight, S: Less than 0.005% by weight, P: Less than 0.045% by weight, Ti: greater than 0.00% by weight and less than 0.045% by weight, Fe: remainder, and unavoidable impurities, An alloy composition containing the following:

[0128] Embodiment 2 The amounts of the elements are given in options a) to g). a) Ni 2.00~3.20% by weight, preferably about 2.00~about 3.00% by weight, b) Mn: 6.2~6.8% by weight, c) Cr: 15.2~16.0% by weight, d)N: 0.100~0.180% by weight, e) Cu: 0.00~0.60% by weight, f)Si:0.50~0.80wt%, g)C:0.070~0.095% by weight, An alloy composition according to Embodiment 1, independently selected from any one of the following.

[0129] Embodiment 3 Ni: 2.00~3.20% by weight, most preferably about 2.00~about 3.00% by weight. Mn: 6.2~6.5% by weight, Cr:15.4~15.9wt%, N:0.100~0.160% by weight, Mo: 0.00~0.50% by weight, Nb: 0.00~0.10% by weight, Cu:0.40~0.60wt%, Si:0.50~0.75wt%, C: 0.070~0.090% by weight, Fe: remainder, and unavoidable impurities, The alloy composition according to Embodiment 1 or 2, comprising:

[0130] Embodiment 4 The following steps, a) A step of melting and casting an alloy composition as defined in any one of Embodiments 1 to 3, b) The process of hot-rolling the alloy from step a), c) The process of solution annealing the alloy from step b), d) A process to perform martensitic processing heat treatment on the alloy from step c), including a cold rolling process and a final annealing process, A method for producing austenitic stainless steel, including [the specified component].

[0131] Embodiment 5 The method according to Embodiment 4, wherein the hot rolling is carried out at a temperature of 1260°C to 1285°C, more preferably 1270°C to 1280°C.

[0132] Embodiment 6 The method according to Embodiment 4 or 5, wherein the solution annealing is performed at a temperature of 1080°C to about 1120°C, more preferably at a temperature of about 1090°C to about 1110°C.

[0133] Embodiment 7 The method according to any one of Embodiments 4 to 6, wherein the martensitic heat treatment in step d) includes a cold rolling step that reduces the thickness by 50% or more, preferably 65% ​​or more.

[0134] Embodiment 8 The method according to any one of Embodiments 4 to 7, wherein the martensitic heat treatment in step d) includes an annealing step at a temperature of 950°C to 1100°C, preferably 950°C to 1075°C, more preferably 950°C to 1050°C.

[0135] Embodiment 9 The method according to Embodiment 8, wherein the annealing process of the martensitic heat treatment is performed for a time of 30 to 200 seconds, depending on the thickness of the steel.

[0136] Embodiment 10 Austenitic stainless steel obtained by the method described in any one of Embodiments 4 to 9.

[0137] Embodiment 11 An austenitic stainless steel comprising the alloy composition described in any one of Embodiments 1 to 4.

[0138] Embodiment 12 Austenitic stainless steel of embodiment 10 or 11 having a tensile strength / total elongation of 1000 MPa / 35-55% to 1350 MPa / 25-45% when measured according to the standard UNE-EN ISO 6892-1:2017.

[0139] Embodiment 13 Use of austenitic stainless steel according to any one of embodiments 10 to 12 in the manufacture of vehicle parts.

[0140] Embodiment 14 The use according to Embodiment 13, wherein the vehicle is an automobile. [Examples]

[0141] The present invention will be illustrated here by examples and exemplary embodiments that are helpful in illustrating the invention. However, it should be understood that the present invention is not limited in any way to the following examples.

[0142] Considering the parameters and considerations included in this description, three compositions were specified that theoretically satisfy the requirements for a new low-Ni ASS with a good balance of tension and elongation (hereinafter referred to as "Alloy 1," "Alloy 2," and "Alloy 3"). These alloys were cast and tested experimentally alongside an EN-1.4372 grade alloy used as a reference for comparison (hereinafter referred to as the reference alloy).

[0143] Material manufacturing The composition was cast as a 35 kg ingot in a Pfeiffer-Balzers VSG-030 vacuum induction furnace. This type of furnace can generate heat under vacuum or inert gas atmosphere and is equipped with a melting / solidifying chamber, power unit, and vacuum system. The raw materials (scrap and ferroalloys) were calculated according to the chemical composition intended for melting and filled into a crucible within the induction coil. Heating and melting of these raw materials is achieved by the electric current generated by the magnetic field of the induction coil.

[0144] The raw materials for melting 35 kg were the base material and ferroalloy. As the base material, standard EN-1.4372 alloy manufactured by ACERINOX was selected (see Table 1), and approximately 23.5 kg of this alloy was typically used in each melting process. [Table 4]

[0145] For alloys 1, 2, and 3, the amount of ferroalloy required for each new composition was calculated, taking into account the chemical composition of the base material, the desired chemical composition, and the efficiency of the ferroalloy. Table 2 shows the weight of the molten raw materials for each of the three new chemical compositions. In the case of the reference alloy, only the base material was melted without the addition of ferroalloy. [Table 5]

[0146] The chemical composition of the ingot was analyzed by X-ray fluorescence analysis and by Leco analyzer for C, N, and S elements (Table 3). [Table 6]

[0147] Hot rolling The next step after ingot production was laboratory-scale processing heat treatment of the material to replicate the normal hot rolling process used in industrial production. This type of treatment was performed by forging using a 30-horsepower drop hammer, Titan Saab 270. Samples were cut from each ingot. The thickness of the samples was selected so that there would be a total reduction of 75% during forging and then a reduction of approximately 70% during cold rolling. The forging conditions applied were: • Heat-treat Carbolite RHF15 / 10 in a laboratory oven at 1240°C for 15 minutes. • Forging in two strokes, followed by an intermediate treatment at 1050°C for 1 minute to restore the temperature of the forged specimen. • Water quenching, That was the case.

[0148] Solution annealing After the forging process, the microstructure was restored by solution annealing in a Carbolite RHF 15 / 10 experimental oven. The conditions (temperature and time) for this heat treatment were specified to obtain a microstructure equivalent to that of standard EN-1.4372 after industrial solution annealing. This microstructure is characterized by a matrix of nearly completely recrystallized equiaxed austenite grains and minimal residual delta ferrite (typically <1%). The typical grain size of industrial materials is around ASTM 8.0. The remaining hot forging microstructure is also present. The thermal conditions for solution annealing applied to forged samples of all alloys were a heating temperature of 1100°C and a time of 80 seconds.

[0149] cold rolling The next step was laboratory-scale cold rolling of the samples using a Norton Duo mill. This mill consists of two rolls, the distance between which is controlled by a flywheel. All samples were passed several times to reach the final thickness. The magnetism of the samples before and after the cold rolling process was measured using a Fischercope MMS ferrite meter. Table 4 shows the average values ​​of the magnetic values ​​before and after cold rolling, the applied reduction rate, and the final thickness of the samples, along with the martensite volume fraction obtained by multiplying the magnetic values ​​by a correction factor of 1.7 (this is a common method for these steel grades). [Table 7]

[0150] Final annealing Finally, in the final step of the martensite processing heat treatment, the cold-rolled sample was annealed to convert the martensite back to austenite. To properly control the thermal cycle, the annealing process was performed using a Gleeble 3800 machine. The Gleeble system allows for precise control of the heat and mechanical parameters applied to the sample, simulating a variety of thermal / mechanical treatments.

[0151] Three different annealing treatments were defined by varying the heating temperature and time. Once the sample reached the target temperature, continuous rapid cooling was performed to simulate water quenching. Table 5 shows the annealing treatments applied to each alloy. [Table 8]

[0152] Characterization of a new alloy Samples annealed in a Gleeble machine were characterized to analyze the performance of each alloy / treatment. The main characterization items were tensile testing, microstructural analysis, and grain size identification. Furthermore, considering that higher magnetic values ​​correlate with a higher TRIP effect, magnetic measurements were performed on tensile samples to analyze the TRIP effect.

[0153] Tensile testing of the annealed specimens was performed at room temperature using an Instron 5585H machine, with a gauge length of 12.5 mm, in accordance with the European standard UNE-EN ISO 6892-1:2017. The elongation values ​​were then calculated according to standard ISO 2566-2:2000, according to standard A. 50 and A 80 The values ​​were converted to those equivalent to those of the test specimen. For microstructural analysis, the samples were metallographically prepared by surface polishing and etching with oxalic acid. Finally, the magnetism of the tensile samples after testing was measured with a ferrite meter.

[0154] Table 6 summarizes the main results obtained for each combination of alloy and annealing treatment. Table 6 shows all combinations, i.e., the applied cycle, tensile test results (YS-yield strength, TS-tensile strength, TEL-A12.5 total elongation, total elongation converted to A50 and A80), grain size (GS), and magnetic measurement (Mag.) of the tensile specimen. Furthermore, all specimens showed a recrystallized austenite microstructure. [Table 9]

[0155] In terms of yield strength and tensile strength, the alloys related to the present invention show considerable improvement compared to the reference alloy (Ref. alloy), and also achieve high elongation values. Furthermore, there is a correlation between the tensile strength value and the magnetic value of the sample after tensile testing (TRIP effect), and therefore, alloy 1 has the highest magnetic value (above 33) and the highest tensile strength, followed by alloy 2 (approximately 28) and alloy 3 (approximately 26), and these values ​​are also considerably higher than the magnetic value of the reference steel (Ref. alloy) (approximately 14).

[0156] Finally, it was observed that the tensile strength value increased with increasing the magnetic value of the sample after cold rolling (Table 4), which corresponds to the volume fraction of SIM. Therefore, as described in the references, increasing the thickness reduction rate increases the volume fraction of strain-induced martensite (SIM) during cold rolling, and thus it is expected that increasing the thickness reduction rate applied during cold rolling will improve the resulting tensile strength value.

[0157] In addition to this characterization of the three new alloys, the mechanical properties of alloy 2 after stamping were analyzed. For this purpose, first, omega samples were prepared by stamping a sheet of alloy 2 annealed in treatment 3 of Table 5 using a hydraulic press. For use as a reference, an industrial sheet of standard EN-1.4372 was also omega stamped using the same procedure. Next, sub-sized tensile test specimens were machined from the top and side surfaces of the stamped specimens. These specimens were the same as those used in the tensile tests described above (gauge length 12.5 mm), and the tensile tests were performed in the same manner (standard UNE-EN ISO 6892-1:2017). The results in Table 7 show that alloy 2 exhibits higher yield strength and tensile strength than the standard industrial EN-1.4372 alloy while maintaining a high elongation value. This elongation value can play an important role in absorbing energy during vehicle collisions. The difference in tensile properties between the side and top surfaces of alloy 2 is more pronounced, suggesting that significant work hardening occurred on the side surfaces as a result of stamping. [Table 10]

[0158] Here, the collision behavior of a new alloy was compared to that of several currently used carbon steel grades, such as Dual Phase 800, using a virtual simulation with the FE model (LS-Dyna). A portion of the side of a passenger car body was modeled and subjected to a collision with a deformable barrier, simulating a simplified lateral collision scenario. The maximum penetration of the new alloy was found to be lower than that of the analyzed reference carbon steel grades.

[0159] Resistance spot welding was also evaluated because it is a primary joining process used in the manufacture of steel car bodies in accordance with standard SEP1220-2 (Testing and Documentation Guideline for the Joinability of thin sheet of steel - Part 2: Resistance Spot Welding).

[0160] New alloys 1-3 and the reference alloy were evaluated with respect to shear weld strength (TSS) and cross-tensile weld strength (CTS), welding process window (range of current usable as a setting parameter), as well as weld microstructure and hardness.

[0161] The welding current range for the new alloy was found to be 0.8-1.4kA, which is comparable to that of conventional carbon steel. Furthermore, the weld microstructure was free of defects, and the hardness of the weld was similar to that of the base material, which contributes to high tensile strength. Stainless steel materials can be easily welded to carbon steel, yielding good results.

[0162] Furthermore, it was found that the cross-tensile strength (CTS) is far higher than that achievable with high-tensile carbon steel.

Claims

1. Ni: 2.00 to 3.60% by weight, Mn: 6.0 to 7.0% by weight, Cr: 15.0 to 16.5% by weight, N: 0.085 to 0.180% by weight, Mo: 0.00 to 0.50% by weight, Nb: greater than 0.00% by weight and less than or equal to 0.40% by weight. Cu: 0.00 to 1.00% by weight, Si: 0.40 to 1.00% by weight, C: 0.060 to 0.095% by weight, S: 0.00 to 0.007% by weight, P: 0.00 to 0.045% by weight, Ti: greater than 0.00% by weight and less than or equal to 0.45% by weight. Fe: remainder and unavoidable impurities, It consists of, Equation M ｄ３０ (°C) = 551 - 462 (%C + %N) - 9.2%Si - 8.1%Mn - 13.7%Cr - 29 (%Ni + %Cu) - 18.5%Mo - 68%Nb M obtained by this formula ｄ３０ An alloy composition characterized in that the value of is at least 55.

2. The amount of each element is, (options a) to l): a) Ni: greater than 2.00% by weight and less than 3.60% by weight, b) Mn: greater than 6.0% by weight and less than 7.0% by weight, c) Cr: greater than 15.0% by weight and less than 16.5% by weight, d) N: 0.085 to 0.180% by weight, e) Mo: greater than 0.00% by weight and less than 0.50% by weight, f) Nb: greater than 0.00% by weight and less than 0.40% by weight, g) Cu: greater than 0.00% by weight and less than 1.00% by weight, h) Si: greater than 0.40% by weight and less than 1.00% by weight, i) C: 0.060 to 0.095% by weight, j) S: Less than 0.007% by weight, k) P: less than 0.045% by weight, l) Ti: greater than 0.00% by weight and less than 0.45% by weight, The alloy composition according to claim 1, which is independently selected from any one of the following.

3. Ni: greater than 2.00% by weight and less than 3.60% by weight, Mn: greater than 6.0% by weight and less than 7.0% by weight, Cr: greater than 15.0% by weight and less than 16.5% by weight. N: 0.085 to 0.180% by weight, Mo: greater than 0.00% by weight and less than 0.50% by weight, Nb: greater than 0.00% by weight and less than 0.40% by weight, Cu: greater than 0.00% by weight and less than 1.00% by weight, Si: greater than 0.40% by weight and less than 1.00% by weight, C: 0.060 to 0.095% by weight, S: Less than 0.007% by weight, P: Less than 0.045% by weight, Ti: greater than 0.00% by weight and less than 0.45% by weight, Fe: remainder and unavoidable impurities, The alloy composition according to claim 1, comprising the above.

4. Equation M ｄ３０ The M obtained by (°C) = 551 - 462 (%C + %N) - 9.2%Si - 8.1%Mn - 13.7%Cr - 29 (%Ni + %Cu) - 18.5%Mo - 68%Nb ｄ３０ The alloy composition according to claim 1, characterized in that the value of is at least 60.

5. Said M ｄ３０ The alloy composition according to claim 4, characterized in that the value of is at least 65.

6. The amount of the aforementioned element is, options a) to i): a) Ni: greater than 2.00% by weight and less than 3.40% by weight, b) Mn: greater than 6.2% by weight and less than 6.9% by weight, c) Cr: greater than 15.2% by weight and less than 16.3% by weight, d) N: 0.100 to 0.180% by weight, e) Nb: greater than 0.00% by weight and less than 0.40% by weight, f) Cu: greater than 0.00% by weight and less than 0.70% by weight, g) Si: greater than 0.50% by weight and less than 0.90% by weight, h) C: 0.065 to 0.095% by weight, i) Ti: greater than 0.00% by weight and less than 0.40% by weight, The alloy composition according to claim 1, which is independently selected from any one of the following.

7. Ni: greater than 2.00% by weight and less than 3.40% by weight Mn: greater than 6.2% by weight and less than 6.9% by weight, Cr: greater than 15.2% by weight and less than 16.3% by weight, N: 0.100 to 0.180% by weight, Mo: greater than 0.00% by weight and less than 0.50% by weight, Nb: greater than 0.00% by weight and less than 0.40% by weight, Cu: greater than 0.00% by weight and less than 0.70% by weight, Si: greater than 0.50% by weight and less than 0.90% by weight, C: 0.065 to 0.095% by weight, S: Less than 0.007% by weight, P: Less than 0.045% by weight, Ti: greater than 0.00% by weight and less than 0.40% by weight, Fe: remainder and unavoidable impurities, The alloy composition according to claim 1, comprising the above.

8. The amount of the aforementioned element is, options a) to i): a) Ni: greater than 2.00% by weight and less than or equal to 3.20% by weight, b) Mn: greater than 6.2% by weight and less than 6.8% by weight, c) Cr: greater than 15.2% by weight and less than 16.2% by weight, d) N: 0.100 to 0.180% by weight, e) Nb: greater than 0.00% by weight and less than 0.30% by weight, f) Cu: greater than 0.00% by weight and less than 0.60% by weight, g) Si: greater than 0.50% by weight and less than 0.80% by weight, h) C: 0.070 to 0.095% by weight, i) Ti: greater than 0.00% by weight and less than 0.30% by weight, The alloy composition according to claim 1, which is independently selected from any one of the following.

9. Ni: greater than 2.00% by weight and less than or equal to 3.20% by weight. Mn: greater than 6.2% by weight and less than 6.8% by weight, Cr: greater than 15.2% by weight and less than 16.2% by weight, N: 0.100 to 0.180% by weight, Mo: greater than 0.00% by weight and less than 0.50% by weight, Nb: greater than 0.00% by weight and less than 0.30% by weight, Cu: greater than 0.00% by weight and less than 0.60% by weight, Si: greater than 0.50% by weight and less than 0.80% by weight, C: 0.070 to 0.095% by weight, S: Less than 0.007% by weight, P: Less than 0.045% by weight, Ti: greater than 0.00% by weight and less than 0.30% by weight, Fe: remainder and unavoidable impurities, The alloy composition according to claim 1, comprising the above.

10. The amount of the aforementioned element is, from option a) to i): a) Ni: greater than 2.10% by weight and less than or equal to 3.20% by weight, b) Mn: greater than 6.2% by weight and less than 6.7% by weight, c) Cr: greater than 15.2% by weight and less than or equal to 15.9% by weight, d) N: 0.100 to 0.160% by weight, e) Nb: greater than 0.00% by weight and less than 0.20% by weight, f) Cu: greater than 0.40% by weight and less than 0.60% by weight, g) Si: greater than 0.50% by weight and less than 0.75% by weight, h) C: less than 0.070 to 0.095% by weight, i) Ti: greater than 0.00% by weight and less than 0.10% by weight, The alloy composition according to claim 1, which is independently selected from any one of the following.

11. Ni: greater than 2.10% by weight and less than or equal to 3.20% by weight. Mn: greater than 6.2% by weight and less than 6.7% by weight, Cr: greater than 15.2% by weight and less than or equal to 15.9% by weight. N: 0.100 to 0.160% by weight, Mo: greater than 0.00% by weight and less than 0.50% by weight, Nb: greater than 0.00% by weight and less than 0.20% by weight, Cu: greater than 0.40% by weight and less than 0.60% by weight, Si: greater than 0.50% by weight and less than 0.75% by weight, C: Less than 0.070-0.095% by weight, S: Less than 0.007% by weight, P: Less than 0.045% by weight, Ti: greater than 0.00% by weight and less than 0.10% by weight, Fe: remainder and unavoidable impurities, The alloy composition according to claim 1, comprising the above.

12. The alloy composition according to claim 1, wherein the amount of S is greater than 0.00% by weight and less than 0.007% by weight.

13. The alloy composition according to claim 1, wherein the amount of P is greater than 0.00% by weight and less than 0.045% by weight.

14. The following steps: a) A step of melting and casting the alloy composition specified in claim 1, b) The process of hot-rolling the alloy from step a), c) The step of solution annealing the alloy in step b), d) The alloy from step c) is subjected to a martensitic processing heat treatment including a cold rolling step and a final annealing step, A method for producing austenitic stainless steel, including [the specified component].

15. The method according to claim 14, wherein the hot rolling is performed at a temperature of 1200°C to 1300°C.

16. The method according to claim 14, wherein the solution annealing is performed at a temperature of 1000°C to 1200°C.

17. The method according to claim 14, wherein the martensitic heat treatment in step d) includes a cold rolling step that reduces the thickness by 50% or more.

18. The method according to claim 14, wherein the martensitic heat treatment in step d) includes an annealing step at a temperature of 900°C to 1200°C.

19. The method according to claim 14, wherein the annealing step of the martensitic heat treatment is performed for a time of 30 to 300 seconds, depending on the thickness of the steel.

20. The following steps: a) A step of melting and casting the alloy composition specified in claim 1, b) A step of hot rolling the alloy from step a) at a temperature of 1200°C to 1300°C, c) A step of solution annealing the alloy from step b) at a temperature of 1000°C to 1200°C, d) The alloy in step c), - In order to reduce the thickness by 50% or more, a martensitic processing heat treatment including a cold rolling process, - Depending on the thickness of the steel, the final annealing process is performed at a temperature of 900°C to 1200°C for a period of 30 seconds to 300 seconds. The process of performing, The method according to claim 14, including the method described in claim 14.

21. The alloy composition according to any one of claims 1 to 13, which is in the form of an austenitic steel.

22. The austenitic stainless steel according to claim 21, characterized by a tensile strength value in the range of 1000 to 1350 MPa, a total elongation in the range of 35 to 55% for a tensile strength of 1000 MPa, and a total elongation in the range of 25 to 45% for a tensile strength of 1350 MPa, as measured according to the UNE-EN ISO 6892-1:2017 standard.

23. The austenitic stainless steel according to claim 21, characterized in that it is selected from flat plates, elongated pieces, or powder products.

24. Use of austenitic stainless steel according to any one of claims 21 to 23 in the fields of automotive, transportation, consumer goods, and construction.

25. The use according to claim 24 in the manufacture of vehicles, household or building components.

26. The use according to claim 25, wherein the vehicle is an automobile.

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