Title of Invention	AUSTENETIC STEEL HAVING HIGH STRENGTH AND FORMABILITY AND A METHOD FOR ITS PRODUCTION
Abstract	Substantially austenitic steel having, high strength and good formabilitv for cold rolling comprising (in weight percent) - 0.05 to 1.0 %C - 11.0 to 14.9% Mn - 1.0 to 5.0% Al - 0 to 2.5% Ni the remainder being iron and unavoidable impurities, wherein the microstructure comprises at least 75% in volume of austenite, and wherein (Ni + Mn) is from 11.0 to 15.9%.

Full Text

AUSTENITIC STEEL HAV1NG HiGH STRENGTH AND FORMABILiTY, METHOD OF RODUC1NG SAiD STEEL AND USE THEREOF The invention relates to a substantialiy austenitic steel having high strength and iood formability for cold rolling. The invention also relates to a method of producing *. aid steel and the use thereof. Austenitic steels having a high strength, such as Hadfield steels, comprising rpnganese (11 to 14%) and carbon (1.1 to 1.4%)as tts rnain ailoying eiements, have ^een known for a long time. The original Hadfield steel, containing about 1.2% C and \2% Mn, was invented by Sir Robert Hadfield in 1882. This steel combines *high oughness and a reasonable ductility with high work-hardening capacity and, usually, jood resistance to wear. However, Hadfield steels do not have good formability due to arge amounts of brittie carbides. Due to the high work-hardening rate, the steels are jifficult to machine. GB 297420 discloses a cast Hadfield-type steel with additions of aluminium to improve the machinability. The addition of aluminium results in the rormation of particles which improve the machinability, particuiarty machinability by material detaching tools. A disadvantage of these types of steel is that they are difficult to cold roll. The high work-hardening rate and the presence of brittle carbides makes the steel work harden very quickly. US Patent 2,448,753 attempted to solve this problem by repeatedly heating, quenching, pickling and cold-rolling the hot rolled material until the desired cold rolled thickness is reached. However, this is a very costly process. US 5,431,753 discloses a process for manufacturing a cold rolled steel having a J 0 to 2.5% Ni the remainder being iron and unavoidable impurities, wherein the microstructure comprises at least 75% in voiume of austenite, and wherein (Ni+Mn) is from 11.0 to 15.9%. The carbon content of the steel according to the invention is much lower than the «Hadfield steels, which is known to be about 1.2%. The contribution of the alloying eiements is beiieved to be as foilows heretnafter. Carbon inhibits the formation of e-martensite by increastng the Stacking Fault Energy (SFE). Stacking fautts are precursors to e-martens'rte, so increasing the SFE decreases the tendency to form E-martensite. The lower carbon content results in a iower tendency to form embrittling phases and/or precipitates during cooling after rolling, and the lower carbon content in comparison to Hadfield steeis is also beneficial for the weldability of the steel. In addition carbon improves the stability of the austenite since carbon is an austenite stabilising eiement The main deformation mechanisms in the austenttic ^teel according to the invention are strain induced twinning and transformation induced plasticity. Manganese improves the strength of the steel by substitutional hardening and it is an austenite stabilising eiement Lowering the manganese content results in a reduction of the SFE of the alloy and hence in a promotion of strain induced twinning, The manganese ränge according to the'invention'provides a stable or rneta-stable austenite at room temperature. diffusivity lead to a reduced or no formation of brittle carbides, particuiany ^reivinj-carbides, and therefore to an improved formability and also an improved coid rollability. It was fourid that beiow 1% aluminium the suppression of £-martensite was insufficient, and at (evels exceeding 5% aluminium, tbe SFE becomes too high, thereby adversely affecting the twinning deformation mechanism. Since aluminium is also a ferrite stabiiising eiement, the influence on the austenite stability of the aluminium additiöns has to be compensated for by manganese and other austenite stabiiising etements. htenganese can, at least partly, be repiaced by elements which also promote austenite stabflity such as nickel. It is beiieved that Nicke! has a beneficiai effect on the elongation values and impact strength. Since the amount of alloying additiöns is kept as low as possible whilst maintaining favourabie coid rolling and mechanica! properües, the austenite is meta-stable and the microstructure of the steel may not be fully austenitic. The microstructure in the steel according to the present invention as a function of composition may comprise a mixture of ferrite and austenite with components of martensite. Upon deforming the steei according to the invention, a beneficiai combination of the deformation mecharusms of plasticity induced by twinning and piasticity induced by transformation under the influence of deformation provides excellent formability, whereas the iower strain hardening and work hardening rate as compared to conventional Hadfield steel in combination with a Iower susceptibility to dynamic strain ageing as a resuSt of the aluminium addition and the absence of coarse and/or brittle In an embodiment of the invention (Ni+Mn) is at most 14.9%. This embodiment aliows the steel to be produced in "a more economica! way, because the amount of expensive alioytng elements is reduced. In an" embodiment of the invention the microstructure, in particular after cold-roliing and anneaiing, comprises at least 80%, preferabiy at least 85%T more preferabiy at least 90% and even more preferabiy at least 95% in volume of austenite. The inventor found that a further improvement of the cofd roiling and meGhanical properties could be obtained if the steel was chosen such that the austenite content in the microstructure comprises at least 80%, preferabiy at least 85%, more preferabiy at least 90% and even more preferabiy at least 95% in volume of austenite. Due to the meta-stability of the austenite, and the occurrence of transformation induced plasticity, the amount of austenite tends to decrease during subsequent processing steps. In order to ensure good formabiSity and high strength, even during a later or its last processing step, it is desirable to have an austenite content which is as high as possible at any stage of the processing, but in particuiar after cold-rolling and anneaiing. It was found that the amount of austenite is favourably influenced by selecting the carbon content to be at least 0.10% or at least 0.15%, but preferabiy to be at least 0.30% and more preferabiy at least 0.50%. In an embodiment of the invention the aluminium content is at most 4.0 %. This embodiment limits the increase in stacking-fault energy by the addition of Aluminium, . whilst still maintaining favourabie properties. In an embodiment of the invention the manganese content is at least 11.5%, preferabiy at least 12.0%. This embodiment allows a more stable austenite to be formed. * In an embodiment of the invention the manganese content is at most 14.7%. This embodiment aHows a further reduction in costs of the steel according to the invention. In an embodiment, the steel according to the invention is provided in the form of a continuousiy cast slab with a typical thickness of between 100 and 350 mm, or in the form of a continuousiy cast thin siab with a typicaf thickness of between 50 and 100 mm. Preferabiy, the steei according to the invention is provided in the form of a continuousiy cast and/or hot rolled strip, preferabiy with a typical thickness between 0.5 and 20 mm, more preferabiy between. 0.7 and 10 mm. Even more preferabiy the strip thickness is at most 8 mm or even at most 6 mm. In an embodiment, the steei according to the invention is provided in the form of a hot roiied steel having a thickness between 0.5 and 20 mm, preferabiy between 0.7 and 10 mm, more preferabiy the strip thickness is at most 8 mm, or even more preferabiy between" 0.8 and 5 mm. It was found that this type of hot-rolled steei has excellent tensile strength and "formabHity which renders it particuiarly useful for äpplications where these properties are calied for, for instance in automotive and other transpori äpplications. According to a second aspect of the invention, there is provided a method of producing 'a suhstantiaüy austenitic steel strip, having an austenite content as described above, comprising the steps of: providing molten steel having a composition as described above; casting said steei into an ingot, or a continuously cast slab, or a continuously cast thin slab or a strip-cast strip; providing a hot-roüed strip by hot rolling the ingot, the continuously cast slab, the continuously cast thin slab or the strip-cast strip to the desired hot rolled thickness In view of the composition of the steel according to the invention, the molten steel will most likely be provided by an EAF-process. The motten steei is then subsequently cast in a mould so as to obtain a solidSfied steel in a form suitable for hot rolling. This form nhay be an ingot which after slabbing and reheating is suitable for hot rolling. It may also be a continuously cast thick ör thin slab having a typical thickness of between 50 and 300 mm. Also, the form suitable for hot rolling may be a continuously cast strip, such as obtained after strip casting using some form of strip-casting device, such as twin-rol! casting, belt-casting or drum casting. !n order to convert the cast microstructure into a wrought microstructure, hot deformation such as rolling of the'solidifled steel is required. This can be done in a conventional rolling mill. comprising a Single conventional rolling stand or a plurality of rolling Stands, in the iatter case usuaily in a tandem set-up.ln case the deformation of the cast steel has to be obtained using a low amount of thickness reduction, such as after strip casting, the method as disclosed in EP 1 449 596 Ai may be used to generate a substantial In an embodiment of the invention, the cold-roiled strip is annealed after cold roliing to the desired finai thickness in a continuous or batch annealing process. This annealing treatment results in a substantialiy recrystallised product. 'In an embodiment of the invention, the cold-roiled strip is gaivanised. The absence of Silicon as an alloying element, i.e. in the sense of a deliberate addition of Silicon for alloying purposes, is beneficial for the galvanisability of the austenitic steel. The adherence of the zinc layer to the Substrate is thereby greatiy improved. The steel according to the invention may be annealed at annealing temperatures between 550 to 1100°C, preferably between 650 to 1100°C either in a batch annealing process, in which case the maximum annealing temperature is preferably between 550 and 800°C, preferably between 650 and 800°C, more preferably at least at 700 and/or beiow 780°C, or in a continuous annealing process, in which case the maximum annealing temperature is at least 600°C, preferably wherein the maximum annealing temperature is between 700 and 1100°C, more preferably below 900°C. After the coid roliing step and/or the annealing step the strip may be subjected to a temper roliing process. According tö a third aspect an austenitic steel strip or sheet is provided as described above, produced according to a process as described above. These steels provide excelient strength' and'good formability in any process stage. Table 1: Steels according to the invention (in wt.%). Materia! 'C |Mn |Äi |Ni Hadfield 1.2 N2 - p 1 0.63 13.2 2Ü - 2 0.63 14.5 2.6 - 3 0.55 14.5 2k5 - 4 ä30 13.9 A5 - 5 0.90 14.5 "T5 ^ 6 Ö63~~T2 Z6 2Ü5 T-" rWÄ5~~\Ä2. 4\5 ~ 8 I0.05 14.5 \Ä5 ~ I J >___ I , I _ jo.66 14.1 Z2 - ^| US 0.52 14.9 fef - j _ |o.59 11.9 24 ^6 12 IÖ795 114.5 |2J5 - . ! I i I Roiled ingots of 30 mm thickness were reheated to a temperature of 1220°C (except for steel 12 where a reheating temperature of 1070 °C was used in view of the ductüity of the steel) and subsequently hot-roSied to a gauge of 3 mm using a 7-pass roiling schedule. A finishing temperature of 900°C was used. The coiüng temperatures ranged from 600°C to 680°C. Details of the finishing schedule are summarised in table 2 beiow. time of 4 hoürs was adequate to achieve substantiaily complete recrystailisation. in order to provide a reasonabie safety margin, a minimum annealing temperature of 715°C for 4 hours or 730°C for 4 hours is preferabie for batch-type annealing to provide complete recrystailisation. It should be noted that the annealing time and annealing temperature for batch anneaüng are exchangeable to a certain degree, reference is made to EP 0 876 514. Samples were removed from all plates and these were batch annealed (see table 4). The tensiie properties in the rolling direcöon for steel 1 and steels 9-12 are shown in tables 3 and 4. Dffferent levets-of cold neducöon appear to have little effect ön the driving force for recrystailisation. Fluctuations in coiling temperature between 600°C and 680° also appear to have little effect The tensiie tests were performed on a Standard tensiie specimen and a gauge fength of 80 mm was used, except for steel 12, where a gauge length of 50 mm was used. The tensiie tests were performed according to EN 10002-1 in the iongitudinal direction. AMENDED CLAIMS (clean version) 2. Steel according to damn 1, wherein the microstructure comprises at least 85%, preferably at least 90%,I more preferably at least 95% in volume of austenite. 3. Steel according to claim; 1 or 2 wherein the carbon content is between 0.30 and 0.75% 4. Steel according to any of the preceding claims, wherein the nickel content is at most 0.05%. - 5. Steel according to any of the preceding claims, wherein the aluminium content is at most 4.0%. 9. Steel according to any of the preceding claims wherein the steel is provided in the form of a cold-rolled strip,or! in the form of a cold-rolled and continuously annealed or batch- annealed strip, optionalfy coated with a coating system comprising one or more metallic and/or organic layer or layers. 10. Cold rolled steel according to claim 9, wherein the microstnjcture after rolling and annealing comprises at least 80%, preferably at least 85%, more preferably at least 90%? and even more preferably at least 95% in volume of austenite. i ! T 11. Method of producing austenitic steel strip, having an austenite content according to claim 1 or 2, comprising! the steps of providing molten steel having a composition according to any one of claims 1 to 7; casting said steef into an ingot, or a continuously cast slab, or a continuously cast thin slab or a strip-cast strip; providing a hot-rolled strip by hot roiling the ingot, the continuously cast slab, the continuously cast thin slab or the strip-cast strip to the desired hot rolled thickness. 12. Method according to claim 11, wherein the hot-rolled strip is cold-rolled to the desired final thickness, preferably wherein the cold-rolling reduction is between 10 to 90%, more preferably between 30 and 86, even more preferably between 45 and 80%. 17. Use of a steel according to claim 1 to 10 or a strip or sheet according to claim 15 for hydroforming applications.

Documents:

3371-CHENP-2007 CORRESPONDENCE OTHERS 18-11-2013.pdf

3371-CHENP-2007 AMENDED CLAIMS 15-07-2014.pdf

3371-CHENP-2007 AMENDED PAGES OF SPECIFICATION 15-07-2014.pdf

3371-CHENP-2007 EXAMINATION REPORT REPLY RECEIVED. 15-07-2014.pdf

3371-CHENP-2007 FORM-1 15-07-2014.pdf

3371-CHENP-2007 OTHERS 15-07-2014.pdf

3371-chenp-2007-abstract.pdf

3371-chenp-2007-claims.pdf

3371-chenp-2007-correspondnece-others.pdf

3371-chenp-2007-description(complete).pdf

3371-chenp-2007-form 1.pdf

3371-chenp-2007-form 3.pdf

3371-chenp-2007-form 5.pdf

3371-chenp-2007-pct.pdf

3760-2007-FORM 3.pdf

3760-2007-Petition 137 for Form 3.pdf