High-Strength, Lightweight Austenitic-Martensitic Steel and the Use Thereof

Abstract

The invention relates to a high-strength, lightweight austenitic-martensitic steel and the use thereof. The inventive lightweight steel is characterized by a chrome content of more than 0.5% and less than 18%, a silicon content of more than 1% and less than 4%, a manganese content of more than 2.5% and less than 30% and an aluminum content of more than 0.05 to 4% and lies within an alloy range that is determined by the coordinates of four points (Crequ=2; Niequ=2), (Crequ=2; Niequ=24), (Crequ=20; Niequ=10) and (Crequ=20; Niequ=6.5), whereby the chrome and nickel equivalent is calculated from the chemical composition of the steel using the relations (1) and (2): Crequ=% Crequ+% Mo+1.5% Si+0.5% W+0.9% Nb+4% Al+4% Ti+1.5% Nieqe=% Ni+30% C+18% N+0.5% Mn+0.3% Co+0.2% Cu−0.2% AI (2). The indications are in weight percent and remainder substantially consists of iron and other elements usually present in steel (P. S). The inventive steel can be cold-formed, and is suitable for use as a material for hot- and cold-rolled sheets, strips and tubes, for non-flat semifinished products and non-flat products and retaining elements, for crash-relevant components and reinforcing structural components in the automobile industry, for expendable parts and as a material for weatherproof, corrosion resisting and stainless parts.

Description

The improvement relates to a high-strength austenitic-martensitic lightweight steel alloyed with chromium, silicon, manganese, and aluminum and having a tensile strength greater than 800 to 1,200 MPa and an elongation at break greater than 25%, and relates to its use.

Steels with tensile strengths above 600 MPa are referred to as lightweight steels because the tensile strength per weight unit is higher than that of aluminum.

PRIOR ART

There are various possibilities for increasing the strength of multi-phase steels such as austenitic-martensitic steels. For example, the increase of the phase proportion of martensite and/or coldforming and/or precipitation hardening. In austenitic-martensitic steels, the 0.2% technical elastic limit, the tensile strength, and the hardness are increased in comparison to austenitic steels as a result of the martensite content. Rustproof austenitic-martensitic CrNi steels combine the advantages of the austenitic steels and of the preferably soft-martensitic steels.

The disadvantage of the aforementioned methods for increasing strength resides in that generally they entail a deterioration of the toughness properties and thus in general of the transformation properties. Austenitic steels with TRIP/TWIP effect (transformation-induced plasticity and twinning-induced plasticity) compensate this disadvantage in that one or several transformation-induced martensites or twinning are induced during coldforming. These effects cause a simultaneous increase of the tensile strength and of the elongation at break so that coldforming properties are improved and energy absorption capacity increases. For austenitic-martensitic steels, there are no solutions disclosed yet for eliminating this disadvantage and the loss of toughness as strength is increased.

High-alloy austenitic-martensitic steels are rustproof steels [1] or high-manganese steels and obviously also LIP steels (light induced plasticity) [2, 3, 4]. There is no information available yet in the literature in regard to LIP steels. Comprehensive test results in regard to the TRIP/TWIP effect and its effects on the mechanical properties and the energy absorption capacity are available only for high-manganese steels [2, 3]. These high-manganese steels contain no chromium and are thus not corrosion-resistant and weather-resistant or slow to corrode.

The high-manganese steels have 0.2% technical elastic limits of 200 to 450 MPA and tensile strengths of 780 MPa to 1,100 MPa and elongation at break between 39 and 47%. For example, a steel with 15% manganese and silicon content of 4 to 2% and aluminum content of 2 to 4% exhibits these properties [1, 2]. The alloying range in which the austenitic-martensitic steels with TRIP effect exist has been specified partially for high-manganese steels but not for rustproof steels [3].

For a targeted utilization of the TRIP effect it is necessary that the chemical composition of the steels with TRIP effect is adjusted with regard to chromium and nickel equivalents. For austenitic steels that have excellent coldforming properties this is disclosed in [7]. In this connection, the ferrite-forming effect of chromium, silicon, and aluminum is represented by the chromium equivalent and the austenitic stabilization effect of the elements manganese and nickel is represented by the nickel equivalent. In this connection, it was demonstrated that aluminum increases the Ms temperature and therefore has an effect on the nickel equivalent. In regard to the effect on Ms temperature, aluminum therefore behaves reversely to the other alloying elements and accompanying elements. Recent tests have shown that the effect of aluminum on the Ms temperature is weaker than disclosed in [7].

Moreover, aluminum and silicon have a detectable positive effect on the passivation behavior of rustproof steels and on the rust layer formation in weather-resistant steels and corrosion-resistant steels. At the same time, these elements however can cause deterioration of the coldforming properties and the surface quality of the products. This is a disadvantage when relatively large aluminum-containing and silicon-containing oxide inclusions are preferably formed in steels.

The patents EP 1 0901 006 B1 [8], EP 1 006 706 B1 [9], and EP 0 031 800 B1 [10] disclose ultra high strength steels whose tensile strengths are above 2,200 MPa. These steels are originally austenitic steels that have been subjected to coldforming and subsequently have been subjected to an aging or precipitation hardening process. The high tensile strengths are then achieved in the thus treated material. This coldformed material is very brittle and can hardly be elongated. It is no longer designed for a further coldforming process.

For evaluating the coldforming properties of steels, the product of tensile strength and maximum elongation can be utilized as a characteristic value. The product of maximum elongation and tensile strength for austenitic-martensitic steels is in the range above 20,000 MPa % [3-5]. Despite relatively high tensile strength, the steels can still be coldformed relatively well. The steels still have residual energy absorption capacity. This means that in case of crash loading the austenitic martensitic steels still have a satisfactorily high elongation buffer [3-5].

By means of the stacking fault energy of the austenite that is dependent on the chemical composition of the austenite, the different strength-increasing mechanisms can be affected in principle [2, 6].

One condition for the formation of transformation-induced α′ martensite is that the micro-structure is comprised at least partially of austenite. Moreover, the austenite must be metastable in order to have a correspondingly high tendency for forming transformation-induced martensite. For these reasons, for the chemical composition of the steels an appropriate chromium equivalent and nickel equivalent are required. This means that in the chemical composition of the steels the ferrite-stabilizing and austenite-stabilizing elements must be adjusted relative to one another. For this reason, a modified chromium equivalent and a known nickel equivalent have been used in order to specify, as formulated in the claim, the range of existence of transformation-induced α′ phase formation. Under these conditions, the required chemical composition of the steel according to the invention can be determined.

LITERATURE

[1] Stahlschlüssel 2004, Verlag Stahlschlüssel Wegst GmbH

[2] Grässel, O., L. Krüger, G. Frommeyer, and L. W. Meyer, Intern. J. Plasticity 16 (2000)

[3] Frommeyer, G.: Published Application, DE 197 27 759 A1, pp. 1391-1409

[4] Schröder, T.: Technische Rundschau, 1/2 (2006), pp. 48-52

[5] Bode, R. a.o.: stahl und eisen 8 (2004), pp. 19 to 26

[6] Martinez, L. G. u.a.: Steal research 63 (1992) 5, pp. 221-223

[7] Weiβ, A., H. Gutte, and P. R. Scheller: DE 10 2005 024 029 A1

[8] Uehara, Toshihiro: Letters Patent EP 1 091 006 B1

[9] Hiramatsu, Naoto and Tomimura, Kouki: Letters Patent EP 1 106 706 B1 [9]

[10] Malmgren, Nils: Letters Patent EP 0 031 800 B1

The invention as defined in the independent claims therefore concerns the problem of providing austenitic-martensitic lightweight steels with excellent coldforming properties and with tensile strengths between 800 to 1,200 MPa and elongation at break greater than 25%.

This object is solved by the invention in accordance with the independent claims and advantageously the dependent claims.

The advantages achieved with the invention reside in particular in that with the lightweight steels according to the invention an improvement of the strength properties is achieved and, at the same time, the toughness properties remain at a relatively high level. These steels are characterized therefore by a good combination of high strength and, at the same time, good toughness properties. Accordingly, these steels can still be relatively well coldformed and have still a relatively high energy absorption capacity.

The invention will be explained in the following preferred embodiments.

The lightweight steels according to the invention can be divided into two different steel types. The first steel type comprises rustproof lightweight steels with TRIP effect and with chromium content in the limits of greater than 12.0 to 18%. The second steel type comprises lightweight steels with TRIP/TWIP effect and with chromium content of more than 0.5% and smaller than 12.0% that generally are weather-resistant and corrosion-resistant.

EXAMPLE 1

Preferably the inventive high-strength lightweight steel with TRIP effect has a carbon content of 0.03%, a chromium content of 14.1%, a silicon content of 1.23%, a nickel content of 6.3%, a manganese content of 7.94%, an aluminum content of 0.051%, and a niobium content of 0.5%, the remainder being essentially iron. The micro-structure of the steel is comprised primarily of metastable austenite and martensite. The steel exhibits a TRIP effect at room temperature. A high hardening capacity is observed. The 0.2% technical elastic limit is approximately at 300 MPa and the tensile strength is at 890. The steel exhibits a maximum elongation of 45%.

EXAMPLE 2

Preferably, the inventive high-strength lightweight steel with TWIG/TRIP effect has a carbon content of 0.04%, a chromium content of 0.52%, a silicon content of 1.5%, a nickel content of 2.1%, a manganese content of 11.5%, and an aluminum content of 0.051%, the remainder being essentially iron. The micro-structure of the steel is comprised of metastable austenite and martensite. The steel exhibits a TRIP/TWIG effect. A relatively high hardening capacity is observed. The 0.2% technical elastic limit is at 310 MPa and the tensile strength is at 1170 MPa and maximum elongation is at 31%.

In this way, the production of high-strength rustproof steels is achieved that form a passive layer on the surface. On the other hand, it is possible to produce high-strength steels that usually are weather-resistant or corrosion-resistant.

Since these steels are alloyed with chromium, silicon, and aluminum and partially with nickel, they have increased resistance with regard to material loss through rust. A variety of these steels can therefore be viewed as weather-resistant or corrosion-resistant. In particular steels with chromium content of 10 to 12% have a distinct corrosion resistance.

The mechanical properties of the stainless steels according to the invention with chromium content greater 12 and less than 18% are comparable to the mechanical properties of rustproof soft-martensitic steels inasmuch as there is still residual austenite in the micro-structure. The rustproof steels according to the invention have in general in comparison to soft-martensitic steels low martensite and no ferrite proportions in the un-transformed initial micro-structure. Only as a result of a TRIP effect in the process of coldforming, the martensite proportion in the steels according to the invention will increase and reach values that are existing in soft-martensitic steels in general already in the un-transformed initial state. Therefore, in comparison to the soft-martensitic steels, the steels according to the invention generally have lower 0.2% technical elongation limits. At the same time, the steels will harden strongly in the process of mechanical loading and will reach almost the same or higher tensile strengths and high elongation at break. For this reason, these steels can still be coldformed well. Moreover, particularly in the rustproof CrNiMn steels according to the present invention, the nickel content can be lowered in comparison to the commercially available soft-martensitic CrNi steels. This provides a cost-effective production of these steels. The steel according to the invention can be differentiated from steels as they are disclosed in [7] by a lower nickel equivalent. Moreover, the micro-structure of the un-transformed initial state is comprised of martensite and austenite.

The advantage of the austenitic lightweight steels according to the invention with chromium content between 0.5 and 12% relative to high-strength chromium-free lightweight steels resides in their weather resistance and corrosion resistance. These properties are achieved in the case of a tightly adhering rust layer. The strength and toughness properties of this group of steels according to the invention in individual situations approach the excellent mechanical properties of high-manganese TRIP/TWIP steels. These steels according to the invention with rust layer formation can also still be coldformed and still have a relatively high energy absorption capacity.

The austenite in the steels according to the invention is metastable. By means of a mechanical treatment it is possible to affect the micro-structure of the austenite with regard to generating stacking faults, twinning, and transformation-induced martensite, preferably transformation-induced α′ martensite.

By employing alloy-technological measures, the formation of preferably transformation-induced α′ martensite in an austenitic-martensitic micro-structure is activated in the steel according to the invention. For this purpose, the nickel equivalent relative to the coldformable austenitic lightweight steels [7] is lowered. The steels according to the invention differ in this respect from the austenitic lightweight steels that can be coldformed well.

In the austenitic-martensitic steel according to the invention, the indicated property potential is however achieved in the process of mechanical loading as a result of transformation-induced martensite formation and without after treatment. In this way, the steels according to the present invention differ in principle from the ultra high strength steels as they are disclosed in [8, 9, 10]. The steel according to the invention can possibly have a chemical composition as observed in aluminum-containing CrNi steels [8, 10] as well as in those that contain Ti, Si, Nb, and V [9].

Manganese is alloyed in the steels according to the invention as an austenite former and as a substitution element for nickel.

Titanium and niobium improve moreover the formation of austenitic fine grain and cause a fine martensite structure. Accordingly, these elements have a positive effect on the mechanical properties. Moreover, niobium and titanium effect binding of carbon and cause thus an improvement of the corrosion properties.

When the austenite of the austenitic-martensitic steels transforms, induced by mechanical loading, into ε and/or α′ martensite inside, a TRIP effect is observed. As a result thereof, the plastic deformation capability and the tensile strength are increased. By twinning, these property changes can be enhanced even more. A high hardening potential is then observed. In contrast to the metastable austenitic steels with TRIP effect, austenitic-martensitic steels with TRIP effect have a higher 0.2% technical elastic limit and higher tensile strengths.

The steels according to the invention differ from the known austenitic TRIP/TWIP steels in that the TRIP effect is induced not in an austenitic initial micro-structure but in an austenitic-martensitic micro-structure. The tensile strengths of more than 800 MPa are thus mainly the result of the already existing annealed martensite and of the transformation martensite. Elongation at break of more than 25% is caused primarily by the TRIP effect and thus the formation of transformation martensite. Precipitation hardening or aging is not required in order to obtain the indicated mechanical properties.

In order to minimize the known negative effects of aluminum, metallurgical measures with regard to oxygen uptake of the melt and thus of the dissolved oxygen content as well as with regard to precipitation of such inclusions are required. The dissolved oxygen content in the melt should therefore not surpass a value of 0.003% in the steel according to the invention.

Aluminum is special with regard to its alloying effect. As a ferrite-stabilizing element it has an effect on the chromium equivalent as expressed in the relationship 1 of claim 1. The effective factor of aluminum on the nickel equivalent in the relationship 2 indicated in claim 1 has been set to −0.2.

Claims

1. High-strength austenitic-martensitic lightweight steel having a tensile strength greater than 800 to 1.200 MPa and an elongation at break greater than 25%, characterized in that the steel has a chromium content of greater than 0.5% and less than 18%, a silicon content of greater than 1% and less than 4%, a manganese content greater than 2.5% and less than 30%, and an aluminum content greater than 0.05% to 4%, and is within an alloying range that is determined by the coordinates of four points (Crequ=2; Niequ=2), (Crequ=2; Niequ=24), (Crequ=20; Niequ=10), and (Crequ=20; Niequ=6.5), wherein the chromium and nickel equivalents are calculated with the relationships 1 and 2 based on the chemical composition of the steel, wherein the values are to be applied in % by weight and wherein the remainder is essentially iron and other accompanying elements (P, S) of steel and is coldformable.

Crequ=% Cr+% Mo+1.5% Si+0.5% W+0.9% Nb+4% Al+4% Ti+1.5% V   (1)

Niequ=% Ni+30% Cr+18% N+0.5% Mn+0.3% Co+0.2% Cu−0.2% Al   (2)

2. Lightweight steel according to claim 1, characterized in that

the nickel content is from 0 to 10%,

the niobium content is from 0 to 1.2%,

the carbon content is from 0.01 to 0.2%,

the nitrogen content is from 0 to 0.1%,

the copper content is from 0 to 4%,

the cobalt content is from 0 to 1%,

the molybdenum content is from 0 to 4%,

the tungsten content is from 0 to 3%,

the titanium content is from 0 to 1%, and

the vanadium content is from 0 to 0.15%,

the oxygen content dissolved in the steel is less than 0.003%, and the remainder is essentially iron.

3. Lightweight steel according to claim 1, characterized in that the remainder is essentially iron.

the carbon content is 0.03%,

the chromium content is 14.1%,

the silicon content is 1.23%,

the nickel content is 6.3%,

the manganese content is 7.94%,

the aluminum content is 0.051%,

the niobium content is 0.5%, and

4. Lightweight steel according to claim 1, characterized in that the remainder is essentially iron.

the carbon content is 0.04%,

the chromium content is 0.52%,

the silicon content is 1.5%,

the nickel content is 2.1%,

the manganese content is 11.5%, and

the aluminum content is 0.051%, and

5. The lightweight steel according to claim 1 as a material for hot-rolled and/or cold-rolled sheet steel, bands, and pipes.

6. The lightweight steel according to claim 1 as a material for non-flat products, non-flat semi-finished products, wire, cold massive formed parts and fasting elements.

7. The lightweight steel according to claim 1 as a material for crash-loaded components and reinforcing structural components.

8. The lightweight steel according to claim 1 as a material for wear parts.

9. The lightweight steel according to claim 3 wherein the material is subjected to a heat treatment before being coldformed.

10. The lightweight steel according to claim 1 as a material for rustproof parts.

11. The lightweight steel according to claim 1 as a material for weather-resistant and corrosion-resistant parts.