WO2008099336A1

WO2008099336A1 - Austenitic stainless steel

Info

Publication number: WO2008099336A1
Application number: PCT/IB2008/050499
Authority: WO
Inventors: Federico Ruffini; Giuseppe Abbruzzese; Gianvincenzo Salamone
Original assignee: Thyssenkrupp Acciali Speciali Terni S.P.A.
Priority date: 2007-02-12
Filing date: 2008-02-12
Publication date: 2008-08-21
Also published as: DE602008003193D1; EP2109692A1; EP2109692B1; ITRM20070069A1; ATE486148T1

Abstract

An austenitic stainless steel containing the following alloy elements expressed in percent by weight: C 0.02-0.10; Cr 17.6-19.0; Ni 3.5-4.5; Mn 10.5-12.5; Cu 1.5-3.0; Si 0.15-0.60; N 0.25-0.40; S ≤ 0.01; P ≤ 0.03, and optionally Mo 0.2-1.0 and B 0.001-0.003, the remainder being, unavoidable impurities apart, iron. The invention also relates to a process for preparing rolled steel sections having the above-indicated chemical composition and the rolled sections thus obtainable. In addition to reduced production costs, the steel of the invention exhibits mechanical properties higher than those of conventional AISI 3XX steel products and, production costs being substantially equal, analogous formability and corrosion resistance higher than that of conventional AISI 2XX steel products.

Description

AUSTENITIC STAINLESS STEEL

DESCRIPTION

The present invention refers to the field of low- nickel nickel-chromium-manganese-copper-nitrogen steels. As it is known, 3XX series austenitic stainless steels, like, e.g., AISI 304 having a substantially austenitic microstructure at room temperature, are highly appreciated for their high ductility and moldability, their corrosion resistance and workability. However, these steels exhibit a mechanical strength not sufficient for structural uses finalized to the making of light-weight structures, like e.g. those of the automotive field (such as chassis, shock absorbers, suspensions) . Another relevant drawback of these alloys is their high cost, owing to the remarkable amounts of nickel required to stabilize the austenitic phase at room temperature.

Therefore, in the relevant field there is the need to decrease the amount of nickel in order to reduce the costs of these alloys, and improve their corrosion resistance and mechanical strength, assuring a high cold formability.

Attempts at partially replacing nickel with elements such as carbon, manganese, copper or nitrogen, highlighted the drawback that nickel reduction entails formation of delta-ferrite and, during cold working, formation of an excessive amount of martensite, which may entail undesired effects such as delayed cracking. In US 3,615,365 an austenitic stainless steel is described, composed mainly of manganese, chromium and nickel, containing small yet significant amounts of carbon, silicon, copper and nitrogen. Alloy elements are combined in critical proportions so as to assure a good hot workability thanks to the full control of the amounts of delta-ferrite present and of resulting martensite (both below 10%) . The alloy has the following composition: up to about 0.12% carbon, from about 5 to about 8.5% manganese, up to about 2% silicon, from about 15 to about 17.5% chromium, from about 3.5 to about 6.5% nickel, from about 0.75 to about 2.5% copper, up to about 0.05% nitrogen, the remainder being, occasional impurities apart, iron.

This alloy, as for manganese content, exhibits a lower limit of 5%, since a content lower than this value might require a higher content of expensive nickel, or excessive amounts of copper, making the alloy hot-short. Moreover, for manganese an upper limit of 8.5% is recommended, taking into account that between manganese and copper there should be maintained a proportion suitable to prevent hot-shortness induced by too high Cu contents.

According to US 3,615,365, higher nickel or manganese contents should be required, unless at least 0.75% copper is added in order to stabilize the alloy against martensite formation during deformation. However, by adding more than 2.5% copper the above-mentioned hot- shortness drawback occurs.

According to this document, nickel reduction below the lower limit of the range indicated therein should be compensated for, by increasing the amount of manganese and/or copper to prevent formation of martensite and of an excess of delta-ferrite . Concomitantly, Mn and Cu contents should be balanced to the upper limit of the ranges indicated for these alloy elements, i.e., to about 8% Mn and 2-2.5% Cu, in order to assure sufficient stability of the austenitic phase and a good combination of mechanical and corrosion resistance properties.

From this context derives the technical prejudice that high (>8%) Mn contents entail negative effects on corrosion resistance, particularly of pitting or crevice type .

Unexpectedly, it has now been observed that in the austenitic stainless steels of the invention, for low nickel contents, ranging from 3.5 to 4.2%, Mn percents may range from 10.5 to 11.5%, therefore being higher than those envisaged in the above-mentioned prior art, without prejudice of mechanical and corrosion resistance properties and with the advantage of increasing nitrogen solubility, thereby contributing to prevent the appearance of porosities, even in the presence of higher contents of this element.

Given the marked contribution to austenitic phase stabilization provided by nitrogen, which is maintained at high levels thanks to the presence of high Mn contents, there may be increased also the content of Cr, a notoriously alphagenic element, from 15-17.5% to 17.6- 18.3%, with the advantageous effect of improving the corrosion resistance.

Austenitic phase stabilization is also achieved in the alloy, thanks to the high N content (0.25-0.40%). This high nitrogen content entails an improvement of the corrosion resistance (in particular by pitting) and of the mechanical properties, by effect of the strengthening induced by its interstitial solid solution. The high concentration of nitrogen, thanks to its enhanced solubility, by effect of the increase of the Mg percent, prevents formation of porosities, in particular superficial ones.

Moreover, there is a second class of Cr-Mn-Ni-based 2XX series austenitic stainless steels that, with a reduced Ni content, partially solve the problem of reducing the costs of alloy elements, and in particular of Ni.

2XX series steels, though possessing a mechanical strength higher than that of series 3XX steels, exhibit however reduced corrosion resistance and lower formability. All of the hereto-shown drawbacks of the state of the art can be overcome by the steel of the present invention, which anyhow exhibits a delta-ferrite content lower than 10% and a martensite content, resulting from the working, lower than 10%.

Subject-matter of the present invention is a low- nickel austenitic stainless steel, containing the following components expressed in percent by weight: C 0.02-0.10; Cr 17.6-19.0; Ni 3.5-4.5; Mn 10.5-12.5; Cu 1.5-3.0; Si 0.15-0.60; N 0.25-0.40; S < 0.01; P < 0.03, and optionally Mo 0.2-1.0 and B 0.001-0.003, the remainder being substantially, unavoidable impurities apart, iron.

For applications requiring improved toughness, preferably the lower limit of the nickel range is 3.8%.

The function of the individual elements in the alloy according to the present invention is reported hereinafter.

Carbon, though being a strong stabilizer for the austenitic phase, cannot be used in high (>0.1%) contents in order not to cause an excessive decrease of intergranular corrosion resistance and not to create weldability problems.

Chromium fosters corrosion resistance and enhances nitrogen solubility, preventing the appearance of porosities. Chromium content cannot be higher than 19% in order to avoid effects of its alphagenic character and tendency to form undesired intermetallic phases.

Nickel is the primary austenitizing element. A reduction of its content in the alloy, in order to meet economic and strategic needs, should foresee the introduction of replacing elements that may compensate for the consequent reduction in the gammagenic character of the alloy.

Manganese has a marked stabilizing effect on the austenitic phase and considerably enhances nitrogen solubility, thereby contributing to prevent appearance of porosities. Balancing of Mn and other alloy elements of the steel of this invention, such as S and N, even in the presence of relatively high copper values, prevents problems related to hot-shortness and allows to obtain a corrosion resistance comparable to the more expensive AISI 304.

Copper, besides fostering formation of austenitic phase and contributing to its stability, improves the resistance of stainless steels towards generalized corrosion. As already seen in the foregoing, copper content should not be higher than 3% in order to prevent hot workability problems . Silicon is an important element, both for the fluidifying effect it exerts on the metal bath and for oxidation resistance. Due to its alphagenic character, it should be limited to 0.60% and, at higher contents, may create problems during pickling. However, for a good steel castability it should be present in contents higher than 0.15%.

Nitrogen is a strong stabilizer of the austenitic phase, moreover determining an improvement of pitting corrosion resistance. However, due to its reduced solubility in the liquid phase, nitrogen cannot be introduced in the alloy by conventional casting methods in contents sufficient to completely replace nickel. An important effect of nitrogen is the strengthening, induced by its interstitial solid solution, bringing about strength characteristics tendentially higher than the AISI 3XX class.

Molybdenum, in the percent range indicated, besides acting as alphagenic element and increasing nitrogen solubility in the alloy, is essential for the improvement of corrosion resistance and, in particular, of pitting corrosion resistance. However, a high percent of this element would not allow nickel to be decreased to desired levels. In this regard, it has to be pointed out that, in the context of the invention, molybdenum percents lower than the lower limit of the indicated range should be considered as impurities.

Boron, in the percent range indicated and suitably balanced with nitrogen, is effective at improving cold formability and mechanical strength (yield) . In this regard, it has to be pointed out that, in the context of the invention, boron percents lower than the lower limit of the indicated range should be considered as impurities .

Sulphur, in the composition range according to the invention and suitably balanced with manganese, also contributes to improve hot workability. Phosphor, in the composition range according to the invention, has no negative effect on mechanical properties and corrosion resistance.

Preferably, the composition of the steel according to the present invention is as follows: C 0.02-0.06; Cr 17.8-18.3; Ni 3.8-4.2; Mn 10.5-11.7; Cu 1.8-2.2; Si 0.15- 0.40; N 0.25-0.33; S < 0.01; P ≤ 0.03, and optionally Mo 0,2-1,0 and B 0.001-0.003, the remainder being substantially, unavoidable impurities apart, iron.

The present invention is not limited to the low- nickel austenitic stainless steel described hereto. It extends also to a process for preparing rolled sections of austenitic stainless steel having the composition according to the invention and the rolled sections thus obtainable. Therefore, subject-matter of the present invention is also a process for producing rolled sections of low- nickel austenitic stainless steel, characterized by subjecting a steel containing the following components expressed as percent by weight: C 0.02-0.10; Cr 17.6-19.0; Ni 3.5-4.5; Mn 10.5-12.5; Cu 1.5-3.0; Si 0.15-0.60; N 0.25-0.40; S < 0.01; P < 0.03, and optionally Mo 0.2-1.0 and B 0.001-0.003, the remainder being substantially, unavoidable impurities apart, iron, to the following operations: continuous casting in an ingot mold with a casting rate ranging from 0.5 to 5 m/min and a steel overheating at the casting ranging from 10 to 60 ⁰C; solidification of said steel cast in the form of slabs having a thickness ranging from 50 to 250 mm, with a cooling rate such as to complete solidification in a time ranging from 30 to 900 s;

- heat equalization treatment of said slabs at a temperature ranging from 1150 to 1400 °C;

- hot rolling of said slabs, with a start-of- rolling temperature ranging from 950 to 1250 °C and an end-of-rolling temperature ranging from 750 to 1150°C, so as to obtain said rolled sections.

Preferably, the above-described process is applied to a steel having the following composition expressed as percent by weight: C 0.02-0.06; Cr 17.8-18.3; Ni 3.8-4.2; Mn 10.5-11.7; Cu 1.8-2.2; Si 0.15-0.40; N 0.25-0.37; S < 0.01; P < 0.03, and optionally Mo 0.2-1.0 and B 0.001- 0.003, the remainder being substantially, unavoidable impurities apart, iron.

The above-described process, applied to steel having the chemical composition of the present invention, increases the content of nitrogen in solution thanks to the high solidification rate and high casting rate.

The greater amount of nitrogen in solution with respect to the traditional casting cycle allows to reduce Ni content and concomitantly increase the mechanical characteristics of the steel.

Further advantages of said cycle are represented by the reduction of interdendritic segregation, the reduction of the content of delta-ferrite and the more homogeneous in-matrix dispersion thereof. Moreover, these advantages contribute to give high isotropy, and therefore improved cold formability, to the rolled sections thus produced.

It was observed that the steels and the rolled sections according to the invention exhibit the following values of yield strength based on a 0.2 percent permanent set (RpO.2), ultimate tensile strength (Rm), ultimate elongation (A) and Erichsen Index (E.I.)-

Steels according to the invention: Rpo.2 (MPa) 400- 450; Rm (MPa) 690-800; A (%) 35-50; I.E. (mm) >10.5.

Rolled sections according to the invention: Rpo.₂ (MPa) 400-550; Rm (MPa) 730-850; A (%) 30-45; I.E. (mm) >10.3.

Therefore, in addition to reduced production costs, these rolled sections exhibit mechanical properties higher than those of conventional AISI 3XX steel products, and, production costs being substantially equal, analogous formability and corrosion resistance higher than that of conventional AISI 2XX steel products.

A general description of the present invention has been given hereto. With the aid of the following examples, hereinafter there will be given further details of specific embodiments of the invention, aimed at making better understood the objects, features, advantages and application modes thereof.

Example 1 In the following Table 1 there are reported the chemical compositions of steels according to the present invention and of conventional comparison steels.

In Table 2 there are reported mechanical strength, formability and corrosion resistance data for the steels of Table 1.

As it is evident from analysis of Tables 1 and 2, steels according to the invention exhibit mechanical strength higher than that of conventional steels taken into account, high corrosion resistance and good formability properties.

EXAMPLE 2

According to the present invention a steel was made, complying with the chemical composition denoted by A in Table 1. This steel was cast by means of continuous casting technology, making slabs having a 220mm-thickness .

The resulting steel has, as shown in Table 2, a yield strength as RpO.2 improved with respect to that of the conventional steels shown in Table 1 and denoted by F and G.

Members intended for the automotive field, in particular bumper cross members, were made with this steel.

This steel was cast by means of continuous casting technology, making slabs having a 220mm-thickness .

Continuous casting occurs in an ingot mold with a casting rate of Im/min and a steel overheating at the casting of 40°C.

Solidification of this steel, cast in the form of slabs, occurs with a cooling rate such as to complete solidification in 600 s.

Heat equalization treatment of the slabs occurs at a temperature of 128O⁰C.

Hot rolling of the slabs is performed with a start- of-rolling temperature of HOO⁰C and an end-of-rolling temperature of 950⁰C, so as to obtain said rolled sections .

In an attempt at providing a further comparative exemplification, a steel was made complying, in terms of composition, with the chemical composition denoted by A in Table 1, and this steel was cast by a thermo- mechanical treatment differing from the thermo-mechanical cycle proposed by the present invention.

The resulting steel has, as shown in Table 2, case A-I, a yield strength as RpO.2 lower than that of steel A obtained according to the thermo-mechanical treatment complying with what is subject-matter of the present invention.

EXAMPLE 3

According to the present invention a steel was made, complying with the chemical composition denoted by C in Table 1.

This steel was cast by means of a traditional casting technology, making slabs having a 220mm- thickness .

The resulting steel has, as shown in Table 2, an improved formability/ability to undergo drawing (Erichsen Index) with respect to steel A of Table 1.

With this steel there were made, by means of forming and hydroforming techniques, members intended for the automotive field, in particular suspension arms. This steel was cast by means of a continuous casting technology, making slabs having a 180mm~thickness .

Continuous casting occurs in an ingot mold with a casting rate of 0.8m/min and a steel overheating at the casting of 50⁰C. Solidification of this steel, cast in the form of slabs, occurs with a cooling rate such as to complete solidification in 750 s.

Heat equalization treatment of the slabs occurs at a temperature of 1310⁰C. Hot rolling of the slabs is performed with a start- of-rolling temperature of 116O⁰C and an end-of-rolling temperature of 98O⁰C, so as to obtain said rolled sections .

In an attempt at providing a further comparative exemplification, a steel was made complying, in terms of composition, with the chemical composition denoted by C in Table 1, and this steel was cast by a thermo- mechanical treatment differing from the thermo-mechanical cycle proposed by the present invention. The resulting steel has, as shown in Table 2, case C-I, a yield strength as RpO.2 lower than that of steel C obtained according to the thermo-mechanical treatment complying with what is subject matter of the present invention.

EXAMPLE 4

According to the present invention, a steel strip having chemical composition D, according to Table 1, is produced:

- by continuous casting in an ingot mold, with a casting rate of 3.5m/min and a steel overheating at the casting of 40⁰C; - with solidification of the steel, cast in the form of slabs having a 60 mm-thickness, with a cooling rate such as to complete solidification in a 180 s-time;

- with heat equalization treatment of said slabs at a temperature of 128O⁰C; - with hot rolling of said slab, with a start-of- rolling temperature of 114O⁰C and an end-of-rolling temperature of 95O⁰C, so as to obtain a rolled section according to the invention.

This steel has a nitrogen content higher than that of the steels having compositions A and C. Moreover, it possesses a mechanical strength and a corrosion resistance higher than those of the same steels having compositions A and C.

Members intended for the automotive field, in particular reinforcement bars, were made with this steel.

Claims

1. A low-nickel austenitic stainless steel, characterized by containing the following components expressed as percent by weight: C 0.02-0.10; Cr 17.6- 19.0; Ni 3.5-4.5; Mn 10.5-12.5; Cu 1.5-3.0; Si 0.15-0.60; N 0.25-0.40; S < 0.01; P < 0.03, and optionally Mo 0.2- 1.0 and B 0.001-0.003, the remainder being, unavoidable impurities apart, iron.

2. The low-nickel austenitic stainless steel, according to claim 1, wherein Ni percent range is 3.8-

4.5.

3. The low-nickel austenitic stainless steel according to claim 1, containing the following components expressed as percent by weight: C 0.02-0.06; Cr 17.8- 18.3; Ni 3.8-4.2; Mn 10.5-11.7; Cu 1.8-2.2; Si 0.15-0.40; N 0.25-0.33; S < 0.01; P < 0.03, and optionally Mo 0.2- 1.0 and B 0.001-0.003, the remainder being substantially, unavoidable impurities apart, iron.

4. A process for producing rolled sections of low- nickel austenitic stainless steel, characterized by subjecting a steel, containing the following components expressed as percent by weight:

C 0.02-0.10; Cr 17.6-19.0; Ni 3.5-4.5; Mn 10.5-12.5;

Cu 1.5-3,0; Si 0.15-0.60; N 0.25-0.40; S < 0.01; P < 0.03, and optionally Mo 0.2-1.0 and B 0.001-0.003, the remainder being substantially, unavoidable impurities apart, iron, to the following operations: continuous casting in an ingot mold with a casting rate ranging from 0.5 to 5 m/min and a steel overheating at the casting ranging from 10 to 60⁰C;

- solidification of said steel cast in the form of slabs having a thickness ranging from 50 to 250 mm, with a cooling rate such as to complete solidification in a time ranging from 30 to 900 s;

- heat equalization treatment of said slabs at a temperature ranging from 1150 to 1400 °C; hot rolling of said slabs, with a start-of- rolling temperature ranging from 950 to 125O⁰C and an end-of-rolling temperature ranging from 750 to 115O⁰C, so as to obtain said rolled sections. 5. A process for producing rolled sections of low- nickel austenitic stainless steel, characterized by subjecting a steel containing the following components expressed as percent by weight:

C 0.02-0.10; Cr 17.6-19.0; Ni 3.5-4.5; Mn 10.5-12.5; Cu 1.

5-3.0; Si 0.15-0.60; N 0.25-0.40; S < 0.01; P <

0.03, and optionally Mo 0.2-1.0 and B 0.001-0.003, the remainder being substantially iron and the unavoidable impurities, to the following operations:

- continuous casting in an ingot mold with a casting rate ranging from 3 to 5 m/min and a steel overheating at the casting ranging from 20 to 50⁰C;

- solidification of said steel cast in the form of slabs having a thickness ranging from 50 to 120 mm, with a cooling rate such as to complete solidification in a time ranging from 30 to 300 s;

- heat equalization treatment of said slabs at a temperature ranging from 1200 to 1350⁰C;

- hot rolling of said slabs, with a start-of- rolling temperature ranging from 1000 to 1200⁰C and an end-of-rolling temperature ranging from 850 to HOO⁰C,

- obtainment of the hot-rolled sections.

6. The process for producing rolled sections of low-nickel austenitic stainless steel, according to claim 5, wherein nickel concentration range is 3.8-4.5%.

7. The process for producing rolled sections of low-nickel austenitic stainless steel, according to claim 5 or 6, wherein a steel containing the following components expressed as percent by weight: C 0.02-0.06; Cr 17.8-18.3; Ni 3.8-4.2; Mn 10.5-11.7; Cu 1.8-2.2; Si 0.15-0.40; N 0.25-0.37; S < 0.01; P < 0.03, and optionally Mo 0.2-1.0 and B 0.001-0.003, the remainder being substantially, unavoidable impurities apart, iron, is subjected to the following operations: continuous casting of said steel in an ingot mold with a casting rate ranging from 3 to 5 m/min and a steel overheating at the casting ranging from 20 to βO°C; - solidification of said steel cast in the form of slabs having a thickness ranging from 50 to 90 mm, with a cooling rate such as to complete solidification in a time ranging from 30 to 300 s; heat equalization treatment of said slabs at a temperature ranging from 1150 to 1300⁰C; hot rolling of said slabs, with a start-of- rolling temperature ranging from 950 to 1200⁰C and an end-of-rolling temperature ranging from 850 to 1050⁰C; obtainment of the hot-rolled sections.

8. Rolled sections of low-nickel austenitic stainless steel, characterized by being obtainable with the process according to claims 4 to 7.