FIELD OF THE INVENTION

The present invention relates generally to an austenitic stainless steel alloy of low nickel content having high tensile strength and elongation properties, a process for its manufacture and to articles manufactured therefrom.

BACKGROUND

Stainless steel is a steel alloy, with a minimum of 10.5% chromium content by mass and a maximum of 1.2% carbon by mass and characterized by its resistance to corrosion and mechanical properties. The chromium forms a microscopically thin inert surface film of chromium oxide by reaction with the oxygen in air and water. This passive layer prevents further corrosion by blocking oxygen diffusion to the steel surface and thus prevents corrosion from spreading into the bulk of the metal.

Stainless steels may be classified by their crystalline structure into four main types: austenitic, ferritic, martensitic and duplex (austenite-ferrite structure). Austenitic stainless steels possess austenite as their primary crystalline structure (face centered cubic (FCC)). This austenite crystalline structure is achieved by sufficient addition of the austenite stabilizing elements like nickel, manganese, carbon and nitrogen. Standard austenitic stainless steels comprise between 16 and 25% chromium, at least 8 % nickel and the rest iron, other alloying elements are also often included in order to get different properties. Austenitic steels have good formability and weldability, in general much better than the ferritic grades, excellent toughness (impact resistance) even to very low temperature and they are not magnetic in annealing conditions, although some degree of magnetism can be developed when they are cold worked, such as in a bolt or at a bent edge. Known for their good mechanical properties in a wide range of temperatures (from cryogenic to high temperatures) as well as good workability and resistance to corrosion, austenitic steels are the most widely used grade of stainless steel. They can easily be used to manufacture all manner of items.

The fast growing need for stainless steels around the world and the following high demand of alloying metals in the steel production has led to increases in metal prices. Especially nickel has become expensive. Therefore, various attempts have been made to substitute nickel in austenitic stainless steels by other alloying elements. However, these steels have proven unsuitable for certain articles which require cold working, including large reduction ratios.

Current austenitic stainless steels (ASS) exhibit both high strength and excellent ductility and thus meet many vehicle functional requirements. However, in general, ASS are expensive choices for many components due to their alloying content. There are two main subgroups of austenitic stainless steel.

The conventional stainless steels achieve their austenitic structure primarily by nickel addition. Until 2003, almost 70% of the produced stainless steel grades were traditional ASS which contain generally 8-10 wt% Ni to stabilize the austenite 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 which can be added into an 18% Cr stainless steel in order to change all the ferrite to 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, the rise of alloying elements’ prices, particularly Ni, and the extreme fluctuations of these prices have made stainless steel users extremely concerned about using the conventional ASS.

The low Ni ASS substitute the Ni by manganese (Mn) and nitrogen (N), which are also austenite formers, allowing manufacturers and ultimately users to be less susceptible to price volatility. Nitrogen is a gas and it can only be added in limited amounts before problems arise, such as the formations of chromium nitrides and gas porosity. A combination of Mn and N is normally not sufficient to change all the ferrite to austenite so some Ni is still added, although in a smaller amount compared to what would be used in a conventional ASS grade. Additionally, in the low Ni ASS the amount of Cr, which is a ferrite former, is reduced, to decrease the amount of austenite formers needed. However, properties of the low Ni ASS suffer from lower corrosion resistance, lower formability and ductility than conventional ASS grades, resulting in a much narrower range of applications.

On the other hand, the presence of higher amounts of N in comparison with the conventional ASS also results in higher strength and hardness. Grade EN-1.4372, for example, has a yield strength about 20% higher than grade EN-1.4301. However, the downside is that they are more difficult to form. Formability can be improved by the addition of copper which also has the benefit that it is an austenite former, but in this case strain hardening rate is reduced.

Some of the most common of the registered low Ni ASS grades are as follows, compared with the composition of grade EN-1.4301 :

EN: Euronorm.

In some industries, steels with very high strength and good ductility and formability are needed. At the same time, it is important that these steels allow a lightweight strategy (through thickness reduction without losing mechanical properties) and have low cost. Steels with yield strength levels in excess of 550 MPa are generally referred to as Advanced High Strength Steels (AHSS). These steels are also sometimes called “ultrahigh-strength steels” for tensile strengths exceeding 780 MPa.

The use of AHSS in cars is quickly expanding with more research, in order to meet the demands for increased safety and fuel efficiency through light-weighted vehicle structures. AHSS are primarily steels with a microstructure containing a phase other than ferrite, pearlite, or cementite - for example, martensite, bainite, austenite, and/or retained austenite in quantities enough to produce unique mechanical properties. The AHSS grades that are currently being applied or that are under increased investigation by the steel community are mainly Dual Phase (DP), Complex-Phase (CP), Ferritic-Bainitic (FB), Martensitic (MS), Transformation-Induced Plasticity (TRIP), Hot-Formed (HF), Twinning-Induced Plasticity (TWIP) and Quenching & Partitioning (Q&P).

The AHSS grades are designed to meet the functional performance demands of certain parts. Each type has unique microstructural features, alloying additions, processing requirements, advantages and limitations associated with its use. Recently there has been increased funding and research for the development of the “3rd Generation” of AHSS. These are steels with improved strength-ductility combinations, as the steels proposed in the preset invention. Metastable ASSs are one of the important grades of the ASSs in which austenite can be transformed to martensite during deformation. Hence, metastable grades exhibit higher tensile strength and better formability than those in which austenite is stable. The metastable ASSs are employed in various structural applications, such as railway and automotive structural components, due to the necessity of weight reduction and crash safety of automobiles. However, they possess relatively low yield strength, which limits their structural applications.

WO2014/135441 describes a stainless steel alloyed with manganese and chromium, without Ni, which is fully austenitic with a special hardening mechanism by cold working (cold rolling) followed by a heat treatment below the recrystallization temperature. This induces individual dislocations and mechanical twinning in the microstructures, improving the properties through Twinning-Induced Plasticity (TWIP), an effect that needs high amounts of Mn (above 20%). These high amounts of Mn can reduce the corrosion resistance of the steel.

US2009/0324441 discloses an austenitic steel cast characterized in that it has a content of Ni of 2-8%, Mn of 5-12%, Cr of 12-20%, N of 0.005-0.500%, Mo of 0.0-2.5%, Nb of 0.0-1.2%, Cu of 0-2%, Si of 0-4% and C of 0.01-0.15%. In addition, the alloy comprises as an essential component Al, between greater than 0 to 4%. The presence of Al and Si promotes martensite formation at room temperature and TRIP effect, increasing the tensile strength and elongation. The problem is that aluminum can make the steel brittle, in particular in bending operations, due to the formation of B2 crystalline structures.

WO2016/027009 discloses a low nickel austenitic stainless steel characterized in that the steel contains in weight: 0.0-0.4% C, 0-3% Si, 3-20% Mn, 10-30% Cr, 0.0-4.5% Ni, 0-3% Mo, 0-3% Cu, 0.05-0.50% N, 0.0-0.5% Nb, 0.0-0.5% Ti, 0.0-0.5% V, the balance of Fe and inevitable impurities. After cold deformation and annealing below 1050 °C the grain size is lower than 10 micrometers.

US4814140A discloses an austenitic stainless steel alloy characterized by a tensile strength of about 900MPa, comprising 3.57% Ni, 5.94% Mn, 15.96% Cr, 0.16% N, 0.98% Si and 0.102% C, with alleged improved galling resistance. US4814140A is silent with regards to cold rolling. US4609577A discloses an alloy comprising 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, as a weld overlay to improve the metal-to-metal wear resistance and corrosion resistance of products such as steel mill rolls.

WO2012/160594A1 discloses an alloy aiming at suppressing the increase in magnetic permeability while maintaining a desired hardness. The alloy comprises 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, and is characterized by -50 < MdsoMn < -30. With the WO2012/160594A1 alloys it is not expected to achieve a good combination of tensile strength and total elongation.

Still, there is a need for new alternative AHSS steels with improved tensile strength-elongation combinations compared to present grades, with good formability properties, potential for more efficient joining capabilities, at lower costs, meeting the stringent demands of, for example, the automotive sector.

SUMMARY OF THE INVENTION

The present invention provides a low Ni austenitic stainless steel alloy composition with high tensile properties, combination of tensile strength and total elongation in the range of 1000MPa I 35-55% elongation to 1350MPa I 25-45% elongation, with good formability and good weldability behavior, allowing weight saving. Through an appropriate martensite thermomechanical treatment, which includes cold rolling to induce martensite transformation from metastable austenite and the reversion of the cold rolling induced martensite to austenite through a thermal treatment, these alloys provide an austenitic microstructure which improves the mechanical properties of the material. Moreover, after a stamping process they retain a good elongation that could be useful in the automotive industry to absorb more energy during crash events. The new alloy can be used for automotive applications, where complex shapes and high crash requirements are needed, such as the central tunnel, under seat beams, the side sill, etc.

Accordingly, in a first aspect, the invention is directed to an alloy composition comprising:

Ni: between 2.00 and 3.60 wt%; Mn: between 6.0 and 7.0 wt%;

Cr: between 15.0 and 16.5 wt%;

N: between 0.085 and 0.180 wt%;

Mo: between 0.00 and 0.50 wt%;

Nb: between 0.00 and 0.10 wt%;

Cu: between 0.00 and 1.00 wt%;

Si: between 0.50 and 1.00 wt%;

C: between 0.065 and 0.095 wt%;

Fe: to balance the composition and incidental impurities.

In a further aspect, the invention is directed to a process for the preparation of austenitic stainless steel from the alloy of the invention, comprising the following steps: a) hot rolling the above defined alloy at a temperature from 1200 °C to 1300 °C, such as from 1260 °C to 1285 °C; b) solution annealing of the alloy from step (a) at a temperature from 1000 °C to about 1200 °C, such as from 1080 °C to 1120 °C, for 70 to 170 seconds; c) cold rolling the resulting alloy from step (b) to obtain a thickness reduction higher than 50%; d) annealing the resulting alloy from step (c) at a temperature between 900 °C and 1200°C, such as between 950 °C and 1100°C, for 30 to 300 seconds, such as 30 to 200 seconds.

The inventors have found that this martensite thermomechanical treatment provides an austenitic microstructure which improves strength and ductility of the material.

The invention is also directed to an austenitic stainless steel obtainable from the process previously defined. Preferably, the austenitic stainless steel of the invention has a combination of tensile strength and total elongation in the range of 1000MPa 1 35-55% elongation to 1350MPa 125-45% elongation.

In another aspect, the invention relates to the use of the defined austenitic stainless steel in the automotive industry. DETAILED DESCRIPTION OF THE INVENTION

The invention provides new alloys that, after an appropriate martensite thermomechanical treatment, show an austenitic microstructure stainless steel, with good production and mechanical performance, pitting resistance and weldability performance. These alloys provide a good combination of tensile strength and total elongation, above 1000 MPa and more than 25% elongation, preferably in the range of 1000 MPa/ 35-55% elongation to 1350 MPa / 25-45% elongation. This allows a reduction in the thickness of the components and therefore the steels of the invention fulfill the lightweight demand and are useful for their industrial use.

In the context of the present invention, the total elongation is measured according to standard UNE-EN ISO 6892-1 :2017.

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which this disclosure belongs.

Alloy compositions

The alloy compositions of the present invention have been carefully designed to ensure the industrial fabrication of an austenitic stainless steel with low Ni content, exceptional combination of strength and elongation properties, without the detriment of the corrosion resistance, weldability and formability, which can be used in automotive parts with high mechanical and formability demand, at the same time as reducing their weight.

For the design of the composition, different parameters (SFE, Mdso, ferrite index and PRE-Mn factor) relevant to achieve a good manufacturing and performance of the new alloys were considered, using equations and experimental data that allowed to analyze many combinations of alloying elements and their content.

The parameters SFE (Stacking Fault Energy) and Mdso relate to the stability of the austenite to transform to martensite during a forming process. Particularly, the SFE refers to the movement of dislocations, the lower SFE the higher tendency of the austenite to transform to martensite when it is formed.

In the case of the Mdso, this parameter defines the temperature at which 50% of the austenite transforms to martensite after a 30% true tensile strain. A higher Mdso value means a lower austenite stability and thus more susceptible to the martensite formation. Nohara et al. proposed the following empirical equation to determine Mdso (Nohara K., Ono Y. and Ohashi N.: “Composition and grain size dependencies of strain-induced martensitic transformation in metastable austenitic stainless steels”, Tetsu-to-Hagane 63 (1977) 772-782), equation (I):

M _d 3o(°C) =

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

In equation (I), the % of each element is to be understood as weight percentage, wt%, therefore meaning that the % of each element in equation (I) is the herein disclosed amount for each element in each embodiment. The examples provide the Md3o value for three exemplary alloy compositions of the invention.

In an embodiment, in any of the herein disclosed embodiments, the alloy of the present invention is characterized by an Md3o value of at least 55, preferably at least 60, more preferably at least 65.

Therefore, the person of ordinary skill in the art will readily understand what are the amounts of each element in the alloy of the invention that yield an Md3o value of at least 55.

The alloy of the present invention may be further characterized by an Md3o value of not more than 170, preferably not more than 165, even more preferably not more than 160. In a more particular embodiment, the alloy of the present invention is characterized by an Md3o value of between 55 and 170, preferably of between 60 and 165, and more preferably of between 65 and 160.

In a particular embodiment, the alloy composition of the invention is characterized in that the Md3o value, obtained according to the equation Md3o(°C) = 551 -462(%C + %N) - 9.2%Si - 8.1%Mn - 13.7%Cr - 29(%Ni + %Cu) - 18.5%Mo - 68%Nb, is at least 55, preferably at least 60, more preferably at least 65. In a particular embodiment, the alloy of the present invention is further characterized by an Md3o value, calculated as explained above, of not more than 170, preferably not more than 165, even more preferably not more than 160. In a more particular embodiment, the alloy of the present invention is characterized by an Md3o value, calculated as explained above, of between 55 and 170, preferably of between 60 and 165, and more preferably of between 65 and 160.

The ferrite index is important to avoid hot ductility problems during hot rolling step that involves the fabricability of the austenitic stainless steel. It represents the quantity of delta ferrite that can solidify during the casting process and be present during the hot rolling stage and so, the material after this step. The presence of this phase in the material with as delivery condition reduces its properties in relation to the formability and corrosion resistance. The lower the ferrite index, the better the fabricability.

Finally, the PRE-Mn (Pitting Resistance Equivalent - Mn) value relates to the pitting corrosion resistance of the material and is function of the chemical composition. Mn has a negative effect on the corrosion behavior, the alloys of the invention have high content of this element, so Mn was included in the PRE equation to consider its harmful influence: the higher PRE-Mn the higher pitting corrosion resistance is expected.

In one embodiment, the compositions of the invention comprise:

Ni: between 2.00 and 3.60 wt%;

Mn: between 6.0 and 7.0 wt%;

Cr: between 15.0 and 16.5 wt%;

N: between 0.085 and 0.180 wt%;

Mo: between 0.00 and 0.50 wt%;

Nb: above 0.00 and not higher than 0.40 wt%;

Cu: between 0.00 and 1.00 wt%;

Si: between 0.40 and 1.00 wt%;

C: between 0.060 and 0.095 wt%;

S: between 0.00 and 0.007 wt%;

P: between 0.00 and 0.045 wt%;

Ti: above 0.00 and not higher than 0.45 wt%;

Fe: to balance the composition and incidental impurities.

Throughout this description the amounts and their decimals for the elements are given in accordance to the tolerance specified in the standard EN 10088-2 (2015) for Chemical Composition of Stainless Steel.

The specified ranges are important to achieve a good balance of the desired properties.

The alloy compositions A1-A5 in the following table are particular embodiments of the invention, the values being expressed as wt% and Fe balancing the composition and incidental impurities:

In the present invention, the amounts of each element present in the alloy are expressed as weight percentage, wt%. In the present invention, the ranges are expressed as either including or excluding the lower and/or upper limit values. The skilled person will readily understand that a range such as, for example 0.085<N<0.180, means that the amount of the element N present in the alloy is between 0.085 and 0.180, and that the lower and upper limit values are contemplated in such a range. Conversely, the symbol “<” means that the value expressed next to it is excluded. For example, in the range 0.00<Ti<0.40, the Ti amount is above 0.00 and lower than 0.40, thus excluding the lower and upper limit values. Therefore, the skilled person readily understands that a C range between 0.070 and below 0.095 is a range that includes the lower limit value but excludes the upper limit value, i.e. , a range equivalent to 0.070<C<0.095.

The alloy compositions of the invention have a Ni content which is lower than in the EN-1.4372 grade. It has been observed that the reduction of this element has a positive effect on the martensite thermo-mechanical treatment, promoting the strain induced martensite formation during the cold rolling process, and providing good properties to the final steel. However, it has been seen that the lower the Ni content the higher the delta ferrite formation, so it is important to control it to avoid problems during hot rolling. High content of this phase at high temperature produces hot ductility problems (edge cracking and slivers).

The amount of Ni in the alloy composition of the invention is 2.00<Ni<3.60, preferably 2.00<Ni<3.40, more preferably 2.00<Ni<3.20, even more preferably 2.10<Ni<3.20. In a preferred embodiment, the Ni content in composition A1 is such that 2.00<Ni<3.60, preferably 2.00<Ni<3.40, more preferably 2.00<Ni<3.20, even more preferably 2.10<Ni<3.20. In another embodiment, these Ni amounts apply to composition A2. In yet another embodiment, these Ni amounts apply to composition A3. In a further embodiment these Ni amounts apply to composition A4 and in yet another embodiment these Ni amounts apply to composition A5. It has been seen that these amounts of Ni promote a proper martensite thermo-mechanical treatment with the desired final properties without industrial production problems.

The reduction of Mn is also beneficial for the martensite thermo-mechanical treatment and for the pitting corrosion resistance due to the negative effect of this element to the PRE-Mn value, however it has been analyzed that this reduction must be controlled to avoid hot rolling problems because the lower the Mn content the higher delta ferrite formation.

The Mn content in the alloy composition of the invention is 6.0 < Mn < 7.0, preferably 6.2 < Mn < 6.9, more preferably 6.2 < Mn < 6.8, even more preferably 6.2 < Mn < 6.7. In a preferred embodiment, the Mn content in composition A1 is such that 6.0 < Mn < 7.0, preferably 6.2 < Mn < 6.9, more preferably 6.2 < Mn < 6.8, even more preferably 6.2 < Mn < 6.7. In another embodiment, these Mn amounts apply to composition A2. In yet another embodiment, these Mn amounts apply to composition A3. In a further embodiment these Mn amounts apply to composition A4 and in yet another embodiment these Mn amounts apply to composition A5. These Mn values are beneficial for the properties of the final steel, in particular to control the pitting corrosion resistance without hot rolling troubles.

In relation to the effect of Cr it has been studied that its reduction is positive for the martensite thermo-mechanical treatment and to control the delta ferrite formation, this avoids increasing N and C contents which are detrimental to the martensite formation during cold rolling, however the lower Cr content the lower pitting corrosion resistance, so the level of this element has been adjusted to keep the pitting corrosion resistance at least equal to the EN-1.4372. The amount of Cr in the alloy composition of the invention is 15.0 < Cr < 16.5, preferably 15.2 < Cr < 16.3, more preferably 15.2 < Cr < 16.2, even more preferably 15.2 < Cr < 15.9. In a preferred embodiment, the Cr content in composition A1 is such that 15.0 < Cr < 16.5, preferably 15.2 < Cr < 16.3, more preferably 15.2 < Cr < 16.2, even more preferably 15.2 < Cr < 15.9. In another embodiment, these Cr amounts apply to composition A2. In yet another embodiment, these Cr amounts apply to composition A3. In a further embodiment these Cr amounts apply to composition A4 and in yet another embodiment these Cr amounts apply to composition A5.

The amount of N should be controlled in order to avoid problems during the melting shop and the hot rolling stages. Further, it is an important element since it has been observed that its reduction results in the instability of the austenite phase to transform into martensite during the cold rolling step. However, its presence is important to control the delta ferrite formation and to keep the pitting corrosion resistance at least equivalent to the EN-1.4372.

The amount of N in the alloy composition of the invention is 0.085 < N < 0.180, preferably 0.100 < N < 0.180, more preferably 0.100 < N < 0.160, even more preferably 0.110 < N < 0.150. In a preferred embodiment, the N content in composition A1 is such that 0.085 < N < 0.180, preferably 0.100 < N < 0.180, more preferably 0.100 < N < 0.160, even more preferably 0.110 < N < 0.150. In another embodiment, these N amounts apply to composition A2. In yet another embodiment, these N amounts apply to composition A3. In a further embodiment these N amounts apply to composition A4 and in yet another embodiment these N amounts apply to composition A5.

The amount of Mo has been also analyzed and it has been observed that the reduction of this element has a positive effect on the martensite thermo-mechanical treatment and control the delta ferrite formation at high temperature. However, this has a detrimental effect on the pitting corrosion resistance. The amount of Mo in the alloy composition of the invention is 0.00 < Mo < 0.50, preferably 0.00 < Mo < 0.50, more preferably 0.01 < Mo < 0.50, even more preferably 0.01 < Mo < 0.40. In a preferred embodiment, the Mo content in composition A1 is such that 0.00 < Mo < 0.50, preferably 0.01 < Mo < 0.50, more preferably 0.01 < Mo < 0.40. In yet another embodiment, these Mo amounts apply to composition A3. In a further embodiment these Mo amounts apply to composition A4 and in yet another embodiment these Mo amounts apply to composition A5.

In relation to the Nb content, it has been studied that the reduction of this element has a positive impact in the martensite thermo-mechanical treatment. On the other hand, Nb carbides and carbonitrides are known to be powerful elements in controlling austenite grain size, leading to better mechanical properties. In the alloy composition of the invention, the amount of Nb is 0.00 < Nb < 0.40, preferably 0.00 < Nb < 0.30, more preferably 0.00 < Nb < 0.20, even more preferably 0.05 < Nb < 0.20. In a preferred embodiment, the Nb content in composition A1 is such that 0.00 < Nb < 0.40, preferably 0.00 < Nb < 0.30, more preferably 0.00 < Nb < 0.20, even more preferably 0.05 < Nb < 0.20. In another embodiment, these Nb amounts apply to composition A2. In yet another embodiment, these Nb amounts apply to composition A3. In a further embodiment these Nb amounts apply to composition A4 and in yet another embodiment these Nb amounts apply to composition A5.

The amount of Cu has an effect in the stability of the austenite phase, since it is an austenite former, so that it affects negatively the austenite to martensite formation. On the other hand it can improve ductility of the alloy. It was also observed that the lower Cu the higher delta ferrite formation at high temperature. The amount of Cu in the alloy compositions of the invention is 0.00 < Cu < 1.00, preferably 0.00 < Cu < 0.70, more preferably 0.00 < Cu < 0.60, even more preferably 0.40 < Cu < 0.60. In a preferred embodiment, the Cu content in composition A1 is such that 0.00 < Cu < 1.00, preferably 0.00 < Cu < 0.70, more preferably 0.00 < Cu < 0.60, even more preferably 0.40 < Cu < 0.60. In another embodiment, these Cu amounts apply to composition A2. In yet another embodiment, these Cu amounts apply to composition A3. In a further embodiment these Cu amounts apply to composition A4 and in yet another embodiment these Cu amounts apply to composition A5.

It has been seen that the reduction of Si is positive to control the delta ferrite precipitation 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, even more preferably 0.50 < Si < 0.75. In a preferred embodiment, the Si content in composition A1 is such that 0.40 < Si < 1.00, preferably 0.50 < Si < 0.90, more preferably 0.50 < Si < 0.80, even more preferably 0.50 < Si < 0.75. In another embodiment, these Si amounts apply to composition A2. In yet another embodiment, these Si amounts apply to composition A3. In a further embodiment these Si amounts apply to composition A4 and in yet another embodiment these Si amounts apply to composition A5.

The level of C must be controlled to avoid high rolling loads. It has been seen that the reduction in the level of C has an important positive effect in the austenite to martensite formation, increasing the instability of austenite, but also that the lower C content the higher delta ferrite formation and the higher 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, even more preferably 0.070 < C < 0.095. In a preferred embodiment, the C content in composition A1 is such that 0.060 < C < 0.095, preferably 0.065 < C < 0.095, more preferably 0.070 < C < 0.095. In another embodiment, these C amounts apply to composition A2. In yet another embodiment, these C amounts apply to composition A3. In a further embodiment these C amounts apply to composition A4 and in yet another embodiment these C amounts apply to composition A5.

The amount of Ti has a good effect on the pitting corrosion resistance because it is a stabilizing element that avoids the precipitation of chromium carbides. Titanium is also very effective as a micro alloy in steel, influencing the microstructure by the formation of nitrides (TiN) and carbides (TiC). Without wishing to be bound by a particular theory, this behavior is thought to be related to a better grain size control and probably to a modification of the nature and morphology of precipitates that could improve the mechanical properties of the steel.

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 such that 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, further preferably 0.00 < Ti < 0.015. In another embodiment, these Ti amounts apply to composition A2. In yet another embodiment, these Ti amounts apply to composition A3. In a further embodiment these Ti amounts apply to composition A4 and in yet another embodiment these Ti amounts apply to composition A5.

As explained, most of the alloying elements have opposite effects on the material (i.e. improving some properties, but worsening others). The inventors have found that with the proposed ranges of the elements a good balance can be achieved to provide an ASS which has remarkable properties as explained above.

As for other elements, such as P and S have very minor effect on the austenite stability and usually are present in amounts that are usual in the EN-1.4372 and conventional ASSs. The content of S is important to avoid hot ductility problems and to control the corrosion resistance and the weldability.

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, even more preferably 0.00 < S < 0.005. In a preferred embodiment, the S content in composition A1 is such that 0.00 < S < 0.007, preferably 0.00 < S < 0.0065, preferably 0.00 < S < 0.006, 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 a further embodiment these S amounts apply to composition A4 and in yet another embodiment these S amounts apply to composition A5.

The amount of P in the alloy composition is 0.00 < P < 0.045, preferably 0.00 < P

< 0.04, more preferably 0.00 < P < 0.035. In a preferred embodiment, the P content in composition A1 is such that 0.00 < P < 0.045, preferably 0.00 < P < 0.04, 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 a further embodiment these P amounts apply to composition A4 and in yet another embodiment these P amounts apply to composition A5.

In a particular embodiment, the alloy composition of the invention is such that the amounts of the elements are independently selected from any of the alternatives a) to I): a) Ni: above 2.00 and below 3.60 wt%, preferably below 3.40 wt%; b) Mn: above 6.0 and below 7.0 wt%, preferably above 6.2 and below 6.9 wt%; c) Cr: above 15.0 and below 16.5 wt%, preferably above 15.2 and below 16.3 wt%; d) N: between 0.085 and 0.180 wt%, preferably between 0.100 and 0.180 wt%; e) Mo: above 0.00 and below 0.50 wt%; f) Nb: above 0.00 and below 0.40 wt% g) Cu: above 0.00 and below 1 .00 wt%, preferably below 0.70 wt%; h) Si: above 0.40 and below 1.00 wt%, preferably above 0.50 and below 0.90 wt%; i) C: between 0.060 and 0.095 wt%, preferably between 0.065 and 0.095 wt%; j) S: below 0.007 wt%; k) P: below 0.045 wt%; l) Ti: above 0.00 and below 0.45 wt%, preferably below 0.40 wt%. In a particular embodiment, the alloy composition of the invention is such that the amounts of the elements are independently selected from any of the alternatives a) to i): a) Ni: above 2.00 and below 3.40 wt%, preferably not greater than 3.20 wt%; b) Mn: above 6.2 and below 6.9 wt%, preferably below 6.8 wt%; c) Cr: above 15.2 and below 16.3 wt%, preferably below 16.2 wt%; d) N: between 0.100 and 0.180 wt%; e) Nb: above 0.00 and below 0.40 wt%, preferably below 0.30 wt%; f) Cu: above 0.00 and below 0.70 wt%, preferably below 0.60 wt%; g) Si: above 0.50 and below 0.90 wt%, preferably below 0.80 wt%; h) C: between 0.065 and 0.095 wt%, preferably between 0.070 and 0.095 wt%; i) Ti: above 0.00 and below 0.40 wt%, preferably below 0.30 wt%.

In a particular embodiment, the alloy composition of the invention is such that the amounts of the elements are independently selected from any of the alternatives a) to i): a) Ni: above 2.00 and not higher than 3.20 wt%, preferably above 2.10 and not higher than 3.20 wt%; b) Mn: above 6.2 and below 6.8 wt%, preferably below 6.7 wt%; c) Cr: above 15.2 and below 16.2 wt%, preferably not higher than 15.9 wt%; d) N: between 0.100 and 0.180 wt%, preferably between 0.100 and 0.160 wt%; e) Nb: above 0.00 and below 0.30 wt%, preferably below 0.20 wt%; f) Cu: above 0.00 and below 0.60 wt%, preferably above 0.40 and below 0.60 wt%; g) Si: above 0.50 and below 0.80 wt%, preferably below 0.75 wt%; h) C: between 0.070 and 0.095 wt%, preferably between 0.070 and below 0.095 wt%; i) Ti: above 0.00 and below 0.30 wt%, preferably below 0.10 wt%.

In a particular embodiment, the alloy composition of the invention is such that the amounts of the elements are independently selected from any of the alternatives a) to i): a) Ni: above 2.10 and not higher than 3.20 wt%; b) Mn: above 6.2 and below 6.7 wt%; c) Cr: above 15.2 and not higher than 15.9 wt%; d) N: between 0.100 and 0.160 wt%; e) Nb: above 0.00 and below 0.20 wt%; f) Cu: above 0.40 and below 0.60 wt%; g) Si: above 0.50 and below 0.75 wt%; h) C: between 0.070 and below 0.095 wt%; i) Ti: above 0.00 and below 0.10 wt%. The alloy compositions in the following table are further particular embodiments of the invention, the values being expressed as wt% and Fe balancing the composition and incidental impurities:

In one embodiment, the composition of the invention comprises Mo. In one embodiment, the composition of the invention comprises Cu.

In a particular embodiment, the alloy of the present invention comprises S.

In a particular embodiment, the alloy of the present invention comprises P.

In another embodiment, the composition of the invention comprises Mo and Cu.

In another embodiment, the composition of the invention comprises Mo and S.

In another embodiment, the composition of the invention comprises Mo and P.

In another embodiment, the composition of the invention comprises S and P.

In another embodiment, the composition of the invention comprises Mo and S.

In another embodiment, the composition of the invention comprises Mo, Cu and

S.

In another embodiment, the composition of the invention comprises Mo, Cu and P.

In another embodiment, the composition of the invention comprises Mo, Cu, S and P.

Combination of the above embodiments is also contemplated as part of the invention.

In an embodiment, the composition of the alloy is as defined above, but wherein the amount of Ni is between 2.00 and 3.20 wt%; most preferably between about 2.00 and 3.00 wt%; and the amount of Mn is between 6.2 and 6.5 wt%.

In an embodiment, the composition of the alloy is as defined above, but wherein the amount of Ni is between 2.00 and 3.20 wt%; most preferably between about 2.00 and 3.00 wt%; and the amount of Cr is between 15.4 and 15.9 wt%.

In an embodiment, the composition of the alloy is as defined above, but wherein the amount of Ni is between 2.00 and 3.20 wt%; most preferably between about 2.00 and about 3.00 wt%; and the amount of N is between 0.100 and 0.160 wt%.

In an embodiment, the composition of the alloy is as defined above, but wherein the amount of Ni is between 2.00 and 3.20 wt%; most preferably between about 2.00 and about 3.00 wt%; and the amount of Cu is between 0.40 and 0.60 wt%.

In an embodiment, the composition of the alloy is as defined above, but wherein the amount of Ni is between 2.00 and 3.20 wt%; most preferably between about 2.00 and about 3.00 wt%; and the amount of Si is between 0.50 and 0.75 wt%.

In an embodiment, the composition of the alloy is as defined above, but wherein the amount of Ni is between 2.00 and 3.20 wt%; most preferably between about 2.00 and about 3.00 wt%; and the amount C is between 0.070 and 0.090 wt%.

In a more preferred embodiment, the alloy composition comprises:

Ni: between 2.00 and 3.20 wt %; most preferably between about 2.00 and about 3.00 wt %;

Mn: between 6.2 and 6.5 wt%;

Cr: between 15.4 and 15.9 wt%;

N: between 0.100 and 0.160 wt%;

Mo: between 0.00 and 0.50 wt%;

Nb: between 0.00 to 0.10 wt%;

Cu: between 0.40 and 0.60 wt%;

Si: between 0.50 and 0.75 wt%;

C: between 0.070 and 0.090 wt%

Fe: to balance the composition and incidental impurities.

In one embodiment, the stainless steel of the invention is characterized in that it is selected from flat, long or powder products.

Casting of the alloys

The first step in the manufacture of stainless steel is the selection of the raw materials, scrap of stainless steel and carbon steel, metal Mn and ferro-chromium. The skilled person, in view of the compositions defined above, will be able to select the raw materials necessary to achieve the desired compositions. The raw materials are introduced in an arc electric furnace where are melted by the action of graphite electrodes. When the steel is liquid, it is poured to a transfer ladle and moved to the AOD converter where the decarburation, reduction, desulfurization processes and final adjustment of the chemical composition take place. Finally, the liquid metal passes to the continuous casting machine where it is solidified in slab format. Hot Rolling

Following casting, the alloys of the invention are subject to hot rolling step where the thickness of the slab is reduced at high temperature through several passes in two mills, one rougher and another finisher.

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

The temperatures, time and conditions (speed, pressure, etc.) of the hot rolling will be adjusted by the skilled person depending on the width and thickness of the black coil.

Solution annealing

After the hot rolling step, the stainless steel made with the alloys of the invention is subjected to a solution annealing process to recover the microstructure and get the correct mechanical properties.

This step is important to obtain a microstructure characterized by a matrix of equiaxial austenitic grains, completely recrystallized structure and reducing residual delta ferrite (normally < 1 %).

The conditions of this thermal treatment, temperature and time, are important.

The temperature of the solution annealing is between about 1000 °C to about 1200 °C, preferably between about 1050 °C to about 1150 °C, more preferably between about 1080 °C to about 1120 °C, even more preferably between about 1090 °C to about 1110 °C, most preferably of about 1100 °C.

The time for the solution annealing treatment is preferably between 50 and 180 seconds, preferably between 70 and 170 seconds, depending on the width and thickness of the strip.

Martensite Thermomechanical treatment Following the solution annealing, the material is subjected to a martensite thermomechanical treatment, comprising a cold rolling step and an annealing step.

This treatment provides the final austenitic microstructure and the desired mechanical properties of strength and elongation necessary for the high performance uses of the steel of the invention. The martensite thermomechanical process comprises a heavy cold rolling to induce the martensitic transformation, followed by an annealing for the strain-induced martensite (SIM) to reversely transform into austenite.

The volume fraction of SIM increases with increasing strain and at a definite strain, called saturating strain, martensite formation becomes saturated. With increasing strain after the saturation strain, fragmentation of martensite occurs during deformation, leading to an increase in defects inside SIM and an increase in the nucleation site during austenite reversion. Finally, the martensite is reverted to austenite during subsequent annealing, leading to the formation of austenite grains.

Step 1: Cold rolling

The cold rolling is carried out in equipment well known by the skilled person, usually thickness reduction is achieved by passing the steel between rolls such as in a Sendzimir mill which is a reversing mill and has a roll stand compounded by 20 rolls. Several passes may be needed to achieve the desired plastic effect and thickness reduction.

The cold rolling preferably provides at least a 50 % thickness reduction, more preferably at least 65 % thickness reduction, even more preferably between 65 and 75 % reduction. This way, stainless steel having a thickness in the range between 2.00 and 0.50 mm, more preferably between 1.5 and 1.0 mm, and having good mechanical properties can be achieved.

This process provides a material with strain induced martensite (SIM) volume fraction above 75%, preferably above 85%, most preferably above 95%. The martensite volume fraction can be obtained by converting the values from magnetic measures obtained with ferritoscope.

Step 2: Final annealing

After the cold rolling step, the stainless steel of the invention is subjected to an annealing step, to complete the martensite thermomechanical treatment.

The annealing is preferably carried out using equipment well known by the skilled person, such as in an annealing and pickling continuous process line.

The temperature of the annealing process is between 900 °C and 1200 °C, preferably between 950 °C and 1150 °C. More preferably, the temperature of the annealing process is between 950 °C and 1100 °C. Even more preferably, the temperature of the annealing process is between 950 °C and 1075 °C. More preferably the annealing treatment is carried out at a temperature between 950 °C and 1050 °C. In general, when the annealing temperature decreases, the hardness increases, and the grain size decreases. Therefore, a temperature of about 950 °C is mostly preferred, which is also energy efficient.

The annealing in the martensite thermomechanical process is carried out for a time between 30 seconds and 300 seconds, such as between 30 seconds and 200 seconds depending on the thickness of the coil. The shorter the time, the more energy efficient it will be.

In one embodiment, an annealing process at a temperature of about 950 °C and time between 50 and 300 seconds is preferred, depending on the thickness of the coil.

In another embodiment, an annealing process at a temperature of about 950 °C and time between 50 and 200 seconds is preferred, depending on the thickness of the coil.

In another embodiment, an annealing process at a temperature of about 1000 °C and time between 40 and 175 seconds is preferred, depending on the thickness of the coil.

In another embodiment, an annealing process at a temperature of about 1075 °C and time between 30 and 150 seconds is preferred, depending on the thickness of the coil.

These temperatures and times provide good performances in terms of tensile strength I total elongation, grain size and cold formability.

The skilled person will adjust and select the annealing conditions depending on the size and thickness of the coil. The thicker the coil, the higher temperature and/or time values will be needed.

The final annealing process provides an austenitic microstructure where no martensite is seen and comprising full recrystallized austenite with equiaxial grains. Properties of the resultant austenitic stainless steel

The austenitic stainless steel of the invention, obtainable from the alloys of the invention by applying a martensite thermomechanical treatment as defined above has remarkable properties.

In terms of microstructure, it shows an austenitic microstructure, steel slightly finer than the reference EN-1 .4372. In a particular embodiment, the stainless steel of the invention has a grain size of at least ASTM 12.

The balance of tensile strength I total elongation which is very important for the industrial application of these materials ranges from 1000MPa I 35-55% elongation to 1350MPa 125-45% elongation. In other words, the material of the present invention can achieve a tensile strength value in the range of 1000-1350MPa, with a total elongation in the range of 35-55% for a tensile strength of 1000MPa and a total elongation in the range of 25-45% for a tensile strength of 1350MPa.

This is well above the reference steel EN-1.4372 having, in general, a tensile strength between 680 and 880 MPa. The steel of the invention also provides high values of yield strength, in general more than 550 MPa, being above the values provided by the reference EN-1.4372.

In the present description, the yield strength, tensile strength and total elongation values correspond to the results of tensile tests performed according to the standard UNE-EN ISO 6892-1 :2017.

Additionally, the bending behavior of the new alloys is good with no cracks at bending, similar to the reference EN-1.4372.

In terms of stamping, the new alloys also perform well, and after a stamping operation they show a high tensile strength and high elongation, which is very useful during crash events to absorb more energy.

The new alloys are also good for welding operations. They compare well with conventional carbon steels and the weld microstructures are defect free. The weld zone hardness is similar to parent material, favoring high tensile strength. Therefore, the welding performance is acceptable to automotive uses. In fact, the weld strength in cross tension is much higher than the one that can be achieved with high strength carbon steels. The new alloys weld easily to carbon steels with good results. And one very significant advantage of the new alloys is that they do not require a zinc coating, unlike carbon steels. 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 steels is responsible for porosity and spatter in laser and MIG/MAG welding, and rapid electrode degradation in resistance spot welding.

In virtual crash simulations, the new alloys guarantee a good performance with respect to reference steels. The new materials can allow an increase of structural performance, which can be used to reduce the thicknesses of the parts, and also to improve the vehicle passive safety.

In summary, the ASS of the invention has good properties despite a reduction in the amount of Ni. They are easily formed, bended or stamped, and good in welding. Due to their tensile strength and elongation properties they can allow reducing the thicknesses of components and withstand crashes and absorb energy. Therefore, they are very suitable for the vehicle industry, in particular the automotive industry.

Use of the austenitic stainless steel

The new ASS of the invention can have many applications. One very important advantage is that they provide high strength and elongation, and they allow an important weight saving. A combination of high tensile strength and good ductility allows the alloys of the invention to be used in the transport, consumer goods and construction sectors.

The new alloys can be used for applications where complex shapes and crash requirements are needed, such as in a car the central tunnel, the side sill, the under seat beams, dash panel, all the components that have a role in crash and are screwed to the Body in White (BIW): Frontal crash beam + crash box, door crash beam, etc.

FURTHER PARTICULAR EMBODIMENTS

Embodiment 1. An alloy composition, comprising:

Ni: between 2.00 and 3.60 wt%;

Mn: between 6.0 and 7.0 wt%;

Cr: between 15.0 and 16.5 wt%;

N: between 0.085 and 0.180 wt%;

Mo: between 0.00 and 0.50 wt%; Nb: between 0.00 and 0.10 wt%;

Cu: between 0.00 and 1.00 wt%;

Si: between 0.50 and 1.00 wt%;

C: between 0.065 and 0.095 wt%;

S: below 0.005 wt%;

P: below 0.045 wt%;

Ti: above 0.00 and below 0.045 wt%;

Fe: to balance the composition and incidental impurities.

Embodiment 2. The alloy composition according to embodiment 1 wherein the amounts of the elements is independently selected from any of the alternatives a) to g): a) Ni: between 2.00 and 3.20 wt%, preferably between about 2.00 and about 3.00 wt%; b) Mn: between 6.2 and 6.8 wt%; c) Cr: between 15.2 and 16.0 wt%; d) N: between 0.100 and 0.180 wt%; e) Cu: between 0.00 and 0.60 wt%; f) Si: between 0.50 and 0.80 wt%; g) C: between 0.070 and 0.095 wt%.

Embodiment 3. The alloy composition according to embodiment 1 or 2, comprising:

Ni: between 2.00 and 3.20 wt%; most preferably between about 2.00 and about 3.00 wt%;

Mn: between 6.2 and 6.5 wt%

Cr: between 15.4 and 15.9 wt%;

N: between 0.100 and 0.160 wt%;

Mo: between 0.00 and 0.50 wt%;

Nb: between 0.00 to 0.10 wt%; Cu: between 0.40 and 0.60 wt%;

Si: between 0.50 and 0.75 wt%;

C: between 0.070 and 0.090 wt%

Fe: to balance the composition and incidental impurities.

Embodiment 4. A method for producing austenitic stainless steel, comprising the following steps: a) Melting and casting an alloy composition as defined in any one of embodiments 1 to 3; b) hot rolling the alloy from step a); c) Solution annealing the alloy from step b); and d) Subjecting the alloy from step c) to a martensite thermomechanical treatment comprising a cold rolling step and a final annealing step.

Embodiment 5. The method according to embodiment 4 wherein the hot rolling is carried out at a temperature between 1260 °C and 1285 °C, more preferably at a temperature between 1270 °C and 1280 °C.

Embodiment 6. The method according to embodiment 4 or 5 wherein the solution annealing is carried out at a temperature of 1080 °C to about 1120 °C, more preferably at a temperature between about 1090 °C to about 1110 °C.

Embodiment 7. The method according to anyone of embodiments 4 to 6 wherein the martensite thermomechanical treatment of step d) comprises a cold rolling step to reduce the thickness by 50% or more, preferably by 65 % or more.

Embodiment 8. The method according to anyone of embodiments 4 to 7 wherein the martensite thermomechanical treatment of step d) comprises an annealing step at a temperature between 950 °C and 1100 °C, preferably between 950 °C and 1075 °C, more preferably between 950 °C and 1050 °C. Embodiment 9. The method of embodiment 8, wherein the annealing step of the martensite thermomechanical treatment is carried out for a time between 30 seconds and 200 seconds, depending on the thickness of the steel.

Embodiment 10. An austenitic stainless steel obtainable from the process according to anyone of embodiments 4-9.

Embodiment 11 . An austenitic stainless steel comprising the alloy composition of anyone of embodiments 1-4.

Embodiment 12. The austenitic stainless steel of embodiment 10 or 11 having a tensile strength/ total elongation from 1000 MPa / 35-55% to 1350 MPa / 25-45% as measured according to the standard LINE-EN ISO 6892-1 :2017.

Embodiment 13. Use of the austenitic stainless steel of anyone of embodiments 10 to 12 in the manufacture of vehicle parts.

Embodiment 14. Use according to embodiment 13 wherein the vehicle is a car.

EXAMPLES

The present invention will now be described by way of examples which serve to illustrate the invention and testing of illustrative embodiments. However, it is understood that the present invention is not limited in any way to the examples below.

Considering the parameters and discussions included in the present description, three compositions were defined that theoretically could meet the requirements of the new low Ni ASS with a good balance of strength and elongation (compositions hereinafter referred to as “Alloy 1”, “Alloy 2” and “Alloy 3”). These alloys together with an EN-1.4372 grade alloy (sample hereinafter referred to as Ref. Alloy), which was used as reference for comparison reasons, were casted and experimentally tested. Material production

The compositions were casted as 35 kg ingots in a vacuum induction furnace Pfeiffer- Balzers VSG-030. This type of furnace allows the production of heats under vacuum or inert gas atmosphere, and comprises a fusion/solidification chamber, a power unit and a vacuum system. The raw materials (scrap and ferroalloys) were calculated as function of the heat aimed chemical composition and charged inside a crucible, which was inside an induction coil. The heating and melting of these raw materials were produced by the electrical current generated by the magnetic field of the induction coil.

The raw materials to produce the heats of 35 kg of weight were base material and ferroalloys. As base material, a standard EN-1.4372 alloy produced by ACERINOX was chosen (see Table 1), normally around 23.5 kg of this alloy was used in each heat.

Table 1 Composition of the EN-1.4372 grade used as base material for the ingots (wt%)

For Alloy 1 , Alloy 2 and Alloy 3, the ferroalloys needed for each new composition were calculated considering the chemical composition of the base material, the aimed chemical composition and the efficiency of the ferroalloys. Table 2 shows the weight of raw materials melted for each one of the new 3 chemical compositions. For the reference alloy, only base material was melted without any addition of ferroalloys.

Table 2 Weight of raw materials (kg) for each produced chemical composition

The chemical compositions of the ingots were analyzed by X-ray fluorescence spectrometry and Leco analyzers for C, N and S elements (Table 3). Table 3 Chemical composition of the casted alloys

Hot Rolling

After producing the ingots, the next stage was the thermomechanical treatment of the material at laboratory scale, to reproduce the usual hot rolling made in the industrial production. Such type of treatment was carried out through a forging process, using a 30 hp drop hammer Titan Saab 270. A sample was cut from each ingot. The thickness of the sample was chosen to apply a total reduction of 75% during the forging process and then to be able to apply a reduction level about 70% during cold rolling. The applied forging conditions were:

• Soaking treatment at 1240 °C for 15 min in a Carbolite RHF 15/10 laboratory oven.

• Forging in two strokes with an intermediate treatment at 1050 °C for 1 minute to recover temperature in the forged sample.

• Water quenching

Solution annealing

After the forging stage, a solution annealing treatment was applied on a Carbolite RHF 15/10 laboratory oven to recover the microstructure. The conditions of this thermal treatment (temperature and time) were defined to get a microstructure equivalent to the reference EN-1.4372 after the industrial solution annealing treatment. This microstructure is characterized by a matrix of equiaxial austenitic grains, quasi totally recrystallized and by the lowest residual delta ferrite (normally < 1 %). The typical grain size of the industrial material is around ASTM 8.0. Some rest of hot forging texture is also present. The thermal conditions of the solution annealing applied to the forged samples of all the alloys were: a heating temperature of 1100°C and a time of 80 seconds. Cold Rolling

The next stage was the cold rolling of the samples at laboratory scale using a Norton duo mill. This mill is formed by two rolls whose distance is controlled by a flywheel. Several passes were applied to every sample for reaching the final thickness. The magnetism of the samples before and after the cold rolling process was measured with a ferritometer Fischercope MMS. Table 4 shows the average values of the reduction applied, the final thickness of the samples together with the magnetism values before and after the cold rolling and the martensite volume fraction, which was obtained by multiplying the magnetism values by a correction factor of 1.7 (this is a common practice for these steel grades).

Table 4 Magnetism values of the samples before and after the cold rolling together with information of the reduction applied

Final Annealing

Finally, at last stage of the thermomechanical treatment of the martensite, the cold rolled samples were annealed to allow the reversion of martensite to austenite. In order to have a good control of the thermal cycle, the annealing treatments were applied on a Gleeble 3800 machine. The Gleeble system can simulate a wide variety of thermal/mechanical treatments with a precise control of the thermal and mechanical parameters applied to the sample.

Three different annealing treatments were defined by varying the heating temperature and time. Once the sample reached the target temperature, a continuous fast cooling was applied to simulate the water quenching. Table 5 shows the annealing treatments applied to each alloy. Table 5 Conditions of the annealing treatments applied to each alloy

Characterization of the new alloys

The samples annealed in the Gleeble machine were characterized to analyze the performance of each alloy I treatment. The main characterization activities were tensile tests, microstructural analysis and grain size identification. Besides, magnetism measurements of the tensile samples were performed to analyze the TRIP effect, considering that the higher magnetism values the higher TRIP effect.

The tensile tests of the annealed samples were carried out according to the European standard LINE-EN ISO 6892-1 :2017 at room temperature, with an Instron 5585H machine and using sub-size samples with a gauge length of 12.5mm. Then, the elongation values were converted to the equivalent from standard Asoand Aso specimens according to the standard ISO 2566-2:2000. For the microstructural analysis, the samples were metallographically prepared by means of a surface polishing and etching with oxalic acid. Finally, the magnetism of the tensile samples after the tests was measured with a ferritometer.

The main results obtained for each combination of alloy and annealing treatment are summarized in table 6, which shows for every combination: the cycle applied, the tensile test results (YS-yield strength, TS-tensile strength, TEL-total elongation for A12.5 and converted to Aso and Aso), the grain size (GS) and the magnetism measurements of the tensile samples (Mag.). Besides, all the samples showed a recrystallized austenite microstructure.

Table 6 Properties obtained for each alloy and annealing treatment applied in the Gleeble

In terms of yield and tensile strength, the alloys related to the invention show a considerable improvement with respect to the reference alloy Ref. Alloy, providing also high values of elongation. There is also an increasing relationship of the tensile strength values with the magnetism values of the samples after the tensile tests (TRIP effect), thus the alloy 1 has the highest magnetism values (more than 33) and highest tensile strength, followed by alloy 2 (around 28) and alloy 3 (around 26), being also those values quite higher than the magnetism of the reference steel Ref. Alloy (around 14).

Finally, it is observed that the tensile strength values increase with the increment of the magnetism values of the samples after the cold rolling (table 4), which corresponds to the volume fraction of SIM. Therefore, it is expected that the tensile strength values obtained could be improved by increasing the reduction level applied during the cold rolling since, as it is documented in the bibliography, an increase in the reduction percentage results in an increased volume fraction of strain induced martensite (SIM) during cold rolling.

In addition to this characterization of the three new alloys, the mechanical properties after stamping were analyzed for alloy 2. For this purpose, first a sheet of alloy 2, annealed with the treatment 3 of table 5, was stamped with a hydraulic press to produce an omega sample. In order to be used as reference, an industrial sheet of the reference EN-1.4372 was also omega stamped, following the same procedure. Then, sub-size tensile test specimens were machined out from the top and the side faces of stamped samples. These specimens were like those used for the tensile tests described above (gauge length of 12.5 mm) and the tensile tests were performed in the same way (standard LINE-EN ISO 6892-1 :2017). The results in table 7 show that alloy 2 reveals high yield and tensile strengths, surpassing those of the reference industrial EN-1.4372 alloy, while retaining a high elongation value. This elongation value can play an important role during crashing of a vehicle, absorbing the energy. The difference between tensile properties on the side and top faces for the alloy 2 was more pronounced; significant work hardening appears to have occurred on the side face as a result of the stamping.

Table 7 Tensile test results from omega stamped samples of the alloy 2 and the

EN-1.4372

In this line, the crash behavior of the new alloys was compared with that of some carbon steel grades presently used, such as the Dual Phase 800, by means of a virtual simulation with a FE model (LS-Dyna). A portion of a body side of a passenger car was modelled and impacted with a deformable barrier, which simulated a simplified lateral impact scenario. It was found that the maximum intrusion for the new alloys was lower than for the reference carbon steel grades analyzed.

Resistance spot welding was also assessed, since it is the main joining process used to manufacture steel car bodies, following standard SEP1220-2 (Testing and Documentation Guideline for the Joinability of thin sheet of steel - Part 2: Resistance Spot Welding).

The new alloys 1-3 and reference Ref. Alloy were assessed in terms of weld strength in shear (TSS) and cross Tension (CTS), welding process window (range of usable currents as set parameters) and weld microstructure and hardness.

It was found that the new alloys had welding current ranges from 0.8 to 1 .4 kA, which compares well with conventional carbon steels. Additionally, the weld microstructures were defect free, and the weld zone hardness was similar to that of the parent material, which favours high tensile strength. The stainless steel materials welded easily to carbon steels with good results.

It was also found that the weld strength in cross tension (CTS) was much higher than what can be achieved with high strength carbon steels.