WO2013064202A1

WO2013064202A1 - Method of manufacturing a duplex steel sheet having enhanced formability

Info

Publication number: WO2013064202A1
Application number: PCT/EP2012/003902
Authority: WO
Inventors: Christiaan Theodorus Wilhelmus Lahaye; Radhakanta RANA
Original assignee: Tata Steel Nederland Technology B.V.
Priority date: 2011-11-03
Filing date: 2012-09-19
Publication date: 2013-05-10

Abstract

The invention relates to a method of manufacturing a duplex steel sheet having a microstructure comprising 10 - 50 vol.% ferrite in an austenite matrix, and having a steel composition, in wt.%, Mn: 5-30% Al: 3-12% C: 0.3-1.0% inevitable impurities: <0.1% the remainder being iron provided that %AI/%Mn ≤%C, and the process comprising the steps of: a) casting an ingot from the composition; b) heating the cast ingot to a temperature above 1050 °C; c) hot rolling the heated cast ingot to obtain a hot rolled strip having a reduced thickness; d) quenching the hot rolled strip from the final hot rolling temperature (FRT) at a rate of at least 75 °C/second to a temperature below 200 °C; e) pickling the quenched strip, f) cold rolling the pickled strip to a sheet having a further reduced thickness; g) annealing the sheet having a further reduced thickness at a temperature in the range of 700-1200°C for a period of time of 0.5-8 minutes; h) quenching the annealed sheet from the annealing temperature at a rate of at least 75 °C/second to a temperature below 200 °C.

Description

METHOD OF MANUFACTURING A DUPLEX STEEL SHEET HAVING ENHANCED FORMABILITY

The present invention relates to a method of manufacturing a duplex steel sheet having a particular high manganese steel composition, and to a duplex steel sheet having such a composition.

Duplex stainless steels commonly comprising high amounts of costly alloying elements like Cr, Ni, Mo, V, are typically used in corrosion resistant, high toughness applications, e.g. ships, pipelines, bridges etc.

In automotive applications, high strength steel sheets having high mechanical properties, such as high strength and high formability (elongation) and high resistance to delayed fracture are required. Furthermore, weight saving is desired in view of reduction of fuel consumption.

WO 2009/084792 A1 has disclosed inter alia a method of manufacturing a high manganese steel, comprising the steps of heating a steel slab having a specific composition based on C, Mn, Si and Al to a temperature of 1200 °C or less, finish hot rolling the steel slab into a steel sheet at a temperature of 950 °C or less, cooling the finish-hot-rolled steel sheet with water and coiling the cooled steel sheet at a temperature of 550 °C or less; pickling the hot-rolled steel sheet and cold rolling the pickled steel sheet at a rolling reduction of 40% or more; and continuously annealing the cold-rolled steel sheet at a temperature of 700 °C to 830 °C. According to the application as published this known steel sheet has high strength and elongation as well as excellent delayed fracture resistance for use in automotive applications.

A similar process for producing TWIP steel sheet is disclosed in US2009/0010793A1 , comprising the subsequent steps of belt casting a particular steel composition and cooling into a pre-strip, optionally heat treating the pre-strip, hot rolling thereof to a hot strip with a completely recrystallized structure, and the optional additional steps of winding into a coil, pickling, cold rolling and annealing.

The present invention aims at providing a steel having a relatively low density with good mechanical properties regarding formability (bendability, hole expandability, stretchability, elongation), which sheet is manufactured from relatively cheap and light alloying elements. In a first aspect of the invention a method of manufacturing a duplex steel sheet having a microstructure comprising 10 - 50 vol.% ferrite in an austenite matrix, and having a steel composition, in wt.%,

Mn: 5-30%

Al: 3-12%

C: 0.3-1.0%

inevitable impurities: <0.1%

the remainder being iron provided that %AI/%Mn≤%C, and the process comprising the steps of:

a) casting an ingot from the composition;

b) heating the cast ingot to a temperature above 1050 °C;

c) hot rolling the heated cast ingot to obtain a hot rolled strip having a reduced thickness;

d) quenching the hot rolled strip from the final hot rolling temperature (FRT) at a rate of at least 75 °C/second to a temperature below 200 °C;

e) pickling the quenched strip,

f) cold rolling the pickled strip to a sheet having a further reduced thickness;

g) annealing the sheet having a further reduced thickness at a temperature in the range of 700-1200°C for a period of time of 0.5-8 minutes;

h) quenching the annealed sheet from the annealing temperature at a rate of at least 75 °C/second to a temperature below 200 °C.

Instead of using relatively costly alloying elements like Cr, Ni, Mo, V etcetera the present invention uses a steel composition wherein Mn and Al as relatively cheap and light elements in addition to C are the main alloying elements allowing to achieve a sheet product having a relatively low density.

The process according to the invention comprises at least the following steps. An ingot may be obtained by a traditional casting process. Next the cast ingot is heated to a substantial high temperature of more than 1050 °C, preferably more than 1200 °C to avoid segregation of Mn and Al. The heating time is not particularly limited. Usually the higher the heating temperature, the smaller the heating time may be. E.g. for an experimental cast ingot having a thickness of 100 mm a heating time of about 1 hour at a heating temperature of 1250 °C has proven to be satisfactory. Then the hot cast ingot is subjected to hot rolling. There are no particular limitations to be set to the hot rolling conditions. The final thickness of the hot rolled strip is for example in the range of less than 10 millimetres, such as 2-9 mm, typically 3-6 mm. The finish rolling temperature is advantageously 900 °C or more, such as 950 °C, in view of reducing rolling load. Subsequently the thus produced hot rolled product is quenched. Quenching is essential to avoid precipitation of κ precipitates (FeMnAI carbides) as much as possible under the prevailing conditions, which precipitates would seriously hamper subsequent processing steps and final mechanical properties. Water quenching is preferred, because it offers the fast cooling rate - at least within the relevant temperature range - where formation of these κ precipitates occurs. Roughly this temperature range of the formation of κ precipitates is from about 450 to about 200 °C. Therefore quenching is carried out from the finish hot rolling temperature to a temperature below 200 °C. The quenching rate is higher than 75°C/second, such as about 100 °C/second and higher. Typically these kind of rates require forced application of water, such as jetting, among others depending on the thickness of the hot-rolled strip. Quenching is different from coiling as disclosed in e.g. WO 2009/084792 A1 and US2009/0010793A1 , as coiling slows down the cooling process. Pickling e.g. using HCI, is a necessary step in order to remove the hard oxide scale at the surface brought about by the hot rolling step. The thus pickled strip is then subjected to cold rolling in order to reduce its thickness further to a desired final sheet thickness, followed by annealing in the temperature range of 700 to 1200 °C for a period of time ranging from 0.5 to 8 minutes. Quenching after annealing, through at least the relevant temperature range down to 200 °C at a rate of at least 75° C/second in order to avoid the formation of κ precipitates as explained above, is performed on the cold rolled sheet. Cold rolling is preferably carefully controlled. E.g. advantageously reduction per pass is less than 20%, more preferably less than 10%, such as equal to or less than 0.25 mm/pass for a 3 mm hot rolled sheet. For automotive applications the final thickness of the finished sheet is usually in the range of 0.4-2.5 mm, typically.0.5-2.0 mm. Annealing of the thus cold rolled sheet is performed, typically at a temperature above 700°C up to 1200 °C advantageously above 800 °C preferably in the range of 820-930 °C, e.g. 830°C or 900 °C, for a sufficient period of time, such as 0.5 to 8 minutes, usually in the order of 1-5 minutes. The higher the annealing temperature, the higher the amount of ferrite present in the final product. Thus proper selection of the annealing conditions offers a tool of manipulating the microstructure and thus the mechanical properties of the finished sheet. Regarding the composition the following explanation is provided:

C is in the range of 0.3-1.0 wt.%, preferably 0.4-0.6%. If C content is less than 0.3 wt.%, then strength is too low. If C content is higher than 1.0 wt.%, then the product is too brittle and less formable, and it is difficult to avoid the formation of precipitates. Therefore the preferred range is 0.4-0.6 wt.% C in view of strength.

Mn is in the range of 5-30 wt.%, preferably 20-26%. Mn together with C is a stabilizing element of the austenite matrix in the steel according to the invention.

Al is in the range of 3-12 wt.%, preferably 8-10%. Al acts as a ferrite stabilizing element.

Mn and Al are considered in combination. If Mn is less than 5 wt.% and/or Al is higher than 12. wt.%, then the amount of austenite is too low. If Mn is higher than 30 wt.% and/or Al is less than 3 wt.%, then the amount of ferrite is too low. The preferred ranges of Al and Mn relate to obtaining the preferred ratio of austenite and ferrite in view of strength. If Al is low, then density cannot be reduced. Optionally some Al may be substituted by a small amount of Si at the costs of complexity of the manufacturing process and density.

The relatively light weight elements Al and Mn to a lesser extent allow preparing a stable microstructured steel composition having a reduced density compared to traditional steel compositions of similar strength. The provision %AI/%Mn-%C ≤ 0 reflects the compositional balance in view of the austenite and ferrite microstructure and therefore balanced mechanical properties, in particular strength and elongation (formability).

Regarding the inevitable impurities such as S, N, O, Cr, Ni, Cu the upper limit is 0.1 wt.%. Preferably the impurities content is less than 0.05 wt.%. In particular, the S content is most preferably less than 0.005 wt.% to avoid the formation of sulfide inclusions which affect formability of the steel composition. N is preferably less than 0.004 wt.%. N and O may form precipitates of nitrides and oxides with Al respectively, which are unwanted in view of formability. The same applies to Cu since it forms Cu- rich intermetallic precipitates. Cr and Ni are unwanted elements in view of their high cost.

Thus in a preferred embodiment the composition comprises in wt.%,

Mn: 20-26%

Al: 8-10%

C: 0.4-0.6%

inevitable impurities: <0.05%. The duplex steel articles produced according to the invention comprise a stable microstructure, even after deformation. The microstructure comprises 10-50 vol.% ferrite in an austenite matrix, preferably 20-50 vol.% ferrite. If the ferrite content is too high, then strength leaves something to be desired. If ferrite is too low, formability is unsatisfactory. As mentioned above, the phase fractions of the duplex steel according to the invention can be manipulated by optimising the final annealing temperature after cold rolling. E.g. annealing at 830°C for 1 min of Fe-0.47C-26.0Mn-9.7AI (wt. %) resulted in about 22 vol.% ferrite, while annealing at 900°C for 1 min. yielded about 28 vol.%.

The invention also relates to a vehicle having a body part, made from a duplex steel sheet obtained from the manufacturing method according to the first aspect of the invention that has been deformed to shape said body part. The invention is illustrated by the Example below.

On laboratory scale an ingot having a final composition Fe-0.47C-26.0Mn-9.7AI(wt.%) was manufactured through vacuum induction melting. Impurities were below 0.05%. The ingot was heated at 1250°C for 1 h and then hot rolled from 100 mm to 3 mm thickness with a finish rolling temperature of 950°C and subsequently directly water quenched to ambient temperature. The hot rolled strips were pickled and cold rolled to sheet of 1.5 mm thickness. Then the cold rolled sheets were annealed for 1 min or 3 min at 830°C and 900°C respectively. The resulting sheets were examined and tested. The microstructure of the resulting sheets was characterised by optical microscopy with FeCI3 as etchant. Phase content in the microstructures was measured using electron back scattered diffraction (EBSD) and XRD. Mechanical properties were determined by a quasistatic tensile test using 80 mm gauge length following NEN 10002 standard. The formability of the resulting sheets was determined using Erichsen cupping, hole expansion and bending tests. Erichsen cupping tests of 80 mm diameter sheets were conducted using a 50 mm diameter hemispherical punch following ISO20582. For hole expansion tests, a conical shaped expanding tool was used to expand a hole of 10 mm diameter in a 90 mm X 90 mm sheet and the tests were carried out using the conditions of ISO TS 16630:2003(E). The so-called "guided bending tests" were performed in both longitudinal and transverse directions using 40 mm x 25 mm sample dimensions. In case of above formability tests, for each condition three samples (in stead of 10 as set in some test requirements) were tested and the average value is reported.

The results are summarized in the following Table 1 , wherein a sample is identified by an indication of the annealing temperature and time after cold rolling. Rp is yield strength, Rm is tensile strength, Ag is uniform elongation, A80 is total elongation.

Table 1.

*L=parallel to rolling direction; P=perpendicular to rolling direction.

Claims

Method of manufacturing a duplex steel sheet having a microstructure comprising 10 - 50 vol.% ferrite in an austenite matrix, and having a steel composition, in wt.%,

Mn: 5-30%

Al: 3-12%

C: 0.3-1.0%

inevitable impurities: <0.1%

the remainder being iron provided that %AI/%Mn ≤%C, and the process comprising the steps of:

a) casting an ingot from the composition;

b) heating the cast ingot to a temperature above 1050 °C;

d) quenching the hot rolled strip from the final hot rolling temperature (FRT) at a rate of at least 75 °C/second to a temperature below 200 °C; e) pickling the quenched strip,

Method according to claim 1 , wherein the steel composition comprises, in wt.%,

Mn: 20-26%

Al: 8-10%

C: 0.4-0.6%

inevitable impurities: <0.05%.

Method according to claim 1 or claim 2, wherein the microstructure of the steel sheet comprises at least 20 vol.% ferrite.

4. Method according to any one of the preceding claims, wherein in step b) a cast ingot is heated to a temperature of at least 1200 °C. Method according to any one of the preceding claims, wherein step g) is performed at a temperature in the range of 820-930°C for a period of time of 1- 5 minutes

Method according to any one of the preceding claims, wherein step f) comprises multiple passes at a thickness reduction of not more than 20% per pass.

Method according to any one of the preceding claims, wherein the final hot rolling temperature is 900°C or more.

Method according to any one of the preceding claims, wherein after cold rolling the final thickness is in the range of 0.4-2.5 mm.

Vehicle having a body part, made from a duplex steel sheet manufactured according to any one of claims 1-8.