Hot rolled superplastic steel sheet, and method for manufacturing the same

Abstract

The invention deals with a hot rolled superplastic steel sheet having a composition comprising, by weight percent C 0.03-0.30 %, Mn 2.0–6.0 %, Si: 0.01-1.5%, Al: 0.3–2.5%, Si / 4+Al ≥ 0.4%, S ≤0.010 %, P≤0.020 %, N≤0.012 %, and comprising optionally one or more of the following elements: Mo≤ 0.5 %, Nb≤ 0.05 %, Ti≤ 0.050 %, B≤ 0.005%, Cr ≤ 1.5 %, the remainder of the composition being iron and unavoidable impurities resulting from the smelting, and having a microstructure comprising, in area fraction, 10% or less of pearlite, 10% or more and 45% or less of M-A islands, the rest being bainite and tempered martensite, the bainite and tempered martensite laths having an average aspect ratio AR30 below or equal to 15, AR30 being the average aspect ratio measured in the 30% area fraction of the bainite and tempered martensite laths having the largest aspect ratio.

Description

[0001] Hot rolled superplastic steel sheet, and method for manufacturing the same.

[0002]

[0001] The present invention relates to a hot rolled superplastic steel sheet, and to a method to obtain such steel sheet. The steel sheet according to the invention is particularly well suited for the manufacture of part and for use in manufacturing of automobiles such as land motor vehicles.

[0003]

[0002] One of the major challenges in the automotive industry is to decrease the weight of vehicles in order to improve their fuel efficiency in view of the global environmental conservation, without neglecting the safety requirements. To meet these requirements, new high strength steels are continuously developed by the steelmaking industry, to have sheets with improved yield and tensile strengths, and good ductility and formability.

[0004]

[0003] From a viewpoint of improvement in the formability of automotive steel sheets, super-plasticity has attracted attention. As used herein, the term “super-plasticity” refers to a phenomenon which is caused by grain boundary sliding (other than plastic deformation, dislocation or slip) when materials with fine grain size are tensile-strained at temperatures above half of their melting point so as to exhibit high ductility at very low strain rate. Namely, at deformation temperatures at which materials exhibit super plasticity, the materials have low strength and very high ductility, and thus it is possible to form or process complex shapes even via a small amount of force.

[0005]

[0004] The publication LIS20180179611 relates to a superplastic medium manganese steel which exhibits superplasticity without containing expensive components, such as chromium, nickel or the like, and a method of producing the same. To obtain such super-plasticity the steel sheet needs to be cold rolled to obtain finer grain size and further annealed to high temperature.

[0006]

[0005] The aim of the present invention is to provide a hot rolled superplastic steel sheet having a total elongation TE above or equal to 175%, when measured after the steel has been heated to a temperature comprised from TA2O, temperature at which 20% in area fraction of austenite is formed, to TASO, temperature at which 80% in area fraction of austenite is formed, TA2O and TASO being determined by thermodynamical calculations using software as Thermo-calc®.

[0007]

[0006] The object of the present invention is achieved by providing a steel sheet according to claim 1. The steel sheet can also comprise the characteristic of any one of claims 2 and 6. Another object is achieved by providing the method according to claim 7. The method can also comprise any of the characteristics of claims 8 to 10. Theinvention also relates to the use of the steel sheet according to claim 11. Another object is achieved by providing the method according to claim 12.

[0008]

[0007] Other characteristics and advantages of the invention will be described in greater detail in the following description.

[0009]

[0008] The invention will be better understood by reading the following description, which is provided purely for purposes of explanation and is in no way intended to be restrictive.

[0010]

[0009] According to the invention, the carbon content is from 0.03% to 0.30 % to ensure a satisfactory strength and good weldability properties. Above 0.30% of carbon, weldability of the steel sheet may be reduced. If the carbon content is lower than 0.03%, the strength of the steel sheet can be reduced. Preferably, the minimum carbon content is from 0.05%. Preferably the maximum carbon content is 0.25%, or more preferably 0.20%.

[0011]

[0010] The manganese content is from 2.0% to 6.0 %. Above 6.0% of addition, the hardenability of steel is too high to form the desired hot rolled microstructure. Moreover, the risk of central segregation increases to the detriment of the mechanical properties. Preferably, the manganese content is from 2.3% to 6.0%, more preferably from 2.5% to 6.0%, even more preferably from 2.8% to 6.0%, or from 3.0% to 6.0%.

[0012]

[0011] Silicon content is added in a content of 0.01% to 1.5% to increase the strength. Above 1.5%, silicon oxides form at the surface, which impairs the coatability of the steel. Preferably, a minimum of 0.1% of silicon content is added, more preferably a minimum of 0.2% of silicon is added, even more preferably a minimum of 0.3% of silicon is added.

[0013]

[0012] Aluminium content is added in an amount from 0.3% to 2.5% to decrease the manganese segregation during casting and to improve weldability. Aluminium is a very effective element for deoxidizing the steel in the liquid phase during elaboration. Moreover, aluminum enlarges the intercritical domain and allows to obtain the ferrite area fraction according to the invention with an improved robustness with regard to the heated temperature. Above 2.5% of addition, the weldability of the steel sheet may be reduced, so as castability. Preferably, the maximum amount of aluminium is 2.0%.

[0014]

[0013] The aluminium and silicon contents preferably satisfy Si / 4 + Al > 0.4%.

[0015]

[0014] Optionally some elements can be added to the composition of the steel according to the invention.

[0015] Molybdenum content can be added up to 0.5% to decrease the manganese segregation during casting. Above 0.5%, the addition of molybdenum is costly and ineffective in view of the properties which are required. Preferably, the maximum amount of molybdenum added is 0.4%.

[0016]

[0016] Niobium can be added up to 0.05% in order to provide precipitation strengthening and to refine prior austenite grain size. Preferably, the minimum content of niobium added is 0.001%.

[0017]

[0017] Boron can be added up to 0.005% to improve the spot weldability of the sheet. Above 0.005%, the formation of boro-carbides at the prior austenite grain boundaries is promoted, making the steel more brittle.

[0018]

[0018] Titanium can be added up to 0.050 % to provide precipitation strengthening and, in addition of boron, to protect boron against the formation of BN.

[0019]

[0019] Chromium content can be added up to 1.5% to provide improved hardenability and post-forming strength.

[0020]

[0020] The remainder of the composition of the steel is iron and unavoidable impurities resulting from the smelting process and depending on the process route.

[0021]

[0021] In the case of a production route without the use of scrap, as it is generally the case in the Blast Furnace-Basic Oxygen Furnace (BF-BOF) route, the level of unavoidable impurities is very low.

[0022]

[0022] In the case of a production route using scrap, as in an Electric Arc Furnace (EAF) or loaded in a converter in a BF BOF, the steel sheet can further comprise residual elements coming from scrap such as copper up to 0.4%, nickel up to 0.25%, tin up to 0.05%, arsenic up to 0.03%, antimony up to 0.03%, or lead up to 0.03% which are considered as unavoidable impurities.

[0023]

[0023] P, S and N are also part of the unavoidable impurities whatever the production route. Their content is below or equal to 0.010 % for S, below or equal to 0.020 % for P and below or equal to 0.012 % for N.

[0024]

[0024] The microstructure of the hot rolled steel sheet according to the invention will now be described. It contains, in area fraction:

[0025] - 10% or more and 45% or less of martensite-austenite islands (M-A islands), - 10% or less of pearlite,

[0026] - the rest being bainite and tempered martensite, the bainite and tempered martensite laths having an average aspect ratio AR30 below or equal to 15, AR30 being the average aspect ratio measured in the 30% area fraction of the bainite and tempered martensite laths having the largest aspect ratio.

[0025] Preferably, the area fraction of the sum of bainite and tempered martensite is 45% or more. Bainite and martensite are formed during the cooling of the hot rolled steel sheet to the coiling temperature and during coiling. The martensite is auto tempered during coiling. Thanks to the reduction ratio R from 1000°C to the finish rolling temperature FRT of the invention, a small prior austenite grain size is assured, which implies a small average aspect ratio of bainite and martensite laths in the hot rolled microstructure. The average aspect ratio AR of martensite and bainite laths is preferably below or equal to 8 to assure an easier break of laths into equiaxed grains during hot deformation in order to facilitate the rotation of grains, and to achieve superplastic forming. Preferably, AR is below or equal to 6.5. The 30% area fraction of the bainite and tempered martensite laths with the largest aspect ratio AR have an average aspect ratio AR30 below or equal to 15, to assure a more homogeneous microstructure, which is a key point to achieve superplastic forming.

[0027]

[0026] The M-A islands are produced through austenite partially transformed into martensite during coiling of the steel sheet. Above 45% in area fraction of M-A islands, the toughness of hot rolled sheet degrades. The microstructure comprises 10% or more of M-A islands in area fraction to assure a more homogeneous microstructure after hot deformation. Preferably, the microstructure comprises 15% or more of M-A islands.

[0028]

[0027] Pearlite can be formed up to 10% in area fraction during the cooling of the hot rolled steel sheet to the coiling temperature, and during the coiling. Preferably, the microstructure does not contain pearlite.

[0029]

[0028] The hot rolled steel sheet according to the invention can be produced by any appropriate manufacturing method and the man skilled in the art can define one. It is however preferred to use the method according to the invention comprising the following steps:

[0030]

[0029] A semi-product able to be further hot rolled, is provided with the steel composition described above.

[0031]

[0030] The semi product is obtained by casting liquid steel, which can be produced by a steelmaking process with or without the use of scrap.

[0032]

[0031] The semi product is heated to a temperature Treheat from 1100°C to 1300°C, so to make it possible to ease hot rolling. The semi-product is then hot rolled with a final hot rolling temperature FRT from 800°C to 950°C. During this hot rolling, the reduction ratio R from 1000°C to FRT is above or equal to 55%, to refine the prior austenite grain size, which implies a further small bainite and martensite laths aspect ratio. Preferably,R is above or equal to 60%, more preferably above or equal to 65%, or even more preferably above or equal to 70%.

[0033]

[0032] The hot-rolled steel sheet is then cooled and coiled to a temperature TCOii below or equal to 600°C. Preferably Tcoii is from 400°C to 600°C, more preferably from 400°C to 580°C, even more preferably from 400° to 550°C.

[0034]

[0033] The hot rolled steel sheet is then cooled to room temperature.

[0035]

[0034] The hot rolled superplastic steel sheet can be submitted to a metallic coating operation to improve its protection against corrosion. The coating process used can be any process adapted to the steel of the invention. The steel sheet can be hot dip coated, or can be also coated by electrochemical methods, for example electro-galvanizing, or through any vacuum coating process, like Jet Vapor Deposition. The metallic coating can be a zinc or a zinc-based alloy coating or an aluminium or aluminium based coating alloy.

[0036]

[0035] The hot rolled superplastic steel sheet according to the invention has a total elongation TE above or equal to 175%, when measured after being heated to Tsfrom TA2O, temperature at which 20% in area fraction of austenite is formed during the said heating, to the temperature TASO at which 80% in area fraction of austenite is formed , and maintained at said temperature for a holding time tsfrom 1s to 3600s, with a strain rate below or equal to 0.01s-1. Both temperatures TA2O and TASO can be defined by thermodynamical calculations using software as Thermo-calc®. Preferably, the hot rolled superplastic steel sheet according to the invention has a total elongation above or equal to 175%, when measured after being heated to a temperature Tsof at least 640°C with a strain rate below or equal to 0.01s-1. Preferably TE is above or equal to 200%.

[0037]

[0036] The hot rolled superplastic steel sheet can then be cut to a predetermined shape, to obtain a steel blank. The steel blank can be heated to a temperature Ts,from TA2oto TASO . The steel blank can be maintained at said Tstemperature for a holding time tsfrom 1s to 3600s, and deformed with a strain rate below or equal to 0.01 per second.

[0038]

[0037] The steel blank obtained by the above method has a TE above or equal to 175%.

[0039]

[0038] Three grades, whose compositions are gathered in table 1, were cast in semiproducts and processed into steel sheets.Table 1 - Compositions

[0040]

[0039] The tested compositions are gathered in the following table wherein the element contents are expressed in weight percent (wt.%). The remainder of the composition is iron and unavoidable impurities resulting from the smelting.

[0041]

[0042]

[0040] TA2O and TASO of the steel sheet have been determined through thermodynamic calculations with software as Thermo-calc®.

[0043]

[0041] TA2ois the temperature at which 20% of austenite in area fraction is formed. TASO is the temperature at which 80% of austenite in area fraction is formed.

[0044] Table 2 - Process parameters

[0045] Steel semi-products, as cast, were reheated at 1250°C and hot rolled with a finish rolling temperature FRT, the reduction rate from 1000°C to FRT being R (%). The steel sheets were then cooled and coiled at the coiling temperature Tcoii, before being cooled to room temperature. The following specific conditions to obtain the hot rolled steel sheets were applied:

[0046]

[0047] Underlined values: out of the invention

[0048] The hot rolled steel sheets were then analyzed, and the corresponding microstructure elements were gathered in table 3.Table 3 - Microstructure of the hot rolled superplastic steel sheets

[0049]

[0042] The area fractions of phases in the microstructure are determined through the following method: a specimen is cut from the hot rolled steel sheet, polished and etched with a reagent known per se, to reveal the microstructure. The section is afterwards examined through scanning electron microscope, for example with a Scanning Electron Microscope with a Field Emission Gun (“FEG-SEM”) at a magnification above or equal to 1000x, in secondary electron mode.

[0050]

[0043] The determination of the area fraction of bainite and tempered martensite are performed thanks to SEM observations after Nital or Picral / Nital reagent etching.

[0051]

[0044] The area fractions of M-A islands are measured using image analysis on SEM photos with magnifications of 2000x and 5000x.

[0052]

[0045] The aspect ratio of bainite and tempered martensite lath is the ratio of the longest intercept grain dimension maximum Feret diameter (Fmax) to the longest intercept grain dimension measured at 90° of said Fmax (Fmax90°) : Aspect Ratio = (Fmax) / ( Fmax90°). The average aspect ratio AR is the average of these values. On the images obtained through scanning electron microscope, the 30% area fraction of the bainite and tempered martensite laths having the largest aspect ratio are selected and the mean aspect ratio AR30 is calculated.

[0053]

[0054] Underlined values: out of the invention

[0055]

[0046] In trials 2-4 the mean aspect ratio AR30 is higher than 15, which implies a more heterogeneous microstructure, which impair the superplastic forming.

[0056]

[0047] To demonstrate the superplasticity of the hot rolled steel sheet of the present invention, the hot rolled steel sheets are cut to a predetermined shape, to obtain a steel blank. The steel blanks obtained are then heated to a temperature Ts. The steel blanksare maintained at said temperature for a holding time tsand deformed at a strain rate of 0.01 s-1, with following parameters given in Table 4.

[0057] Table 4 - Hot deformation process

[0058]

[0059]

[0048] The total elongation TE (%) measured according to ISO standard ISO 6892-1, published in October 2009, is measured after the deformation and is gathered in the following table 5.

[0060] Table 5 - Mechanical properties of the parts

[0061]

[0062] Underlined values: out of the invention

[0063]

[0049] Samples a and b are according to the invention. Thanks to the microstructure obtained before the deformation, the steel sheet presents a high elongation.

[0064]

[0050] For samples c-f, because of the lath microstructure with a too high aspect ratio and the too heterogeneous aspect ratio, the elongation of the steel sheet is reduced.

Claims

CLAIMS1. Hot rolled superplastic steel sheet, made of a steel having a composition comprising, by weight percent:C: 0.03 - 0.30 %,Mn: 2.0 -6.0 %,Si: 0.01 - 1.5%,Al: 0.3 -2.5%,S < 0.010 %,P < 0.020 %,N < 0.012 %,and comprising optionally one or more of the following elements, in weight percentage:Mo < 0.5 %,Nb < 0.05 %,Ti < 0.050 %,B < 0.005%,Cr < 1.5 %,the remainder of the composition being iron and unavoidable impurities resulting from the smelting, said steel sheet having a microstructure comprising, in area fraction,- 10% or more and 45% or less of martensite-austenite islands (M-A islands), - 10% or less of pearlite,- the rest being bainite and tempered martensite, the bainite and tempered martensite laths having an average aspect ratio AR30 below or equal to 15, AR30 being the average aspect ratio measured in the 30% area fraction of the bainite and tempered martensite laths having the largest aspect ratio.

2. A hot rolled superplastic steel sheet according to claim 1, wherein the steel has a manganese content comprised from 2.5% to 6%.

3. A hot rolled superplastic steel sheet according to any one of claims 1 and 2, wherein the steel has a silicon content comprised from 0.1% to 1.5%.

4. A hot rolled superplastic steel sheet according to any one of claims 1 and 3, wherein the bainite and tempered martensite laths have an aspect ratio AR below or equal to 8.

5. A hot rolled superplastic steel sheet according to any one of claims 1 to 4, wherein the steel has a total elongation above or equal to 175%, when measured after the steel has been heated to a temperature Tscomprised from TA2O at which 20% in area fraction of austenite is formed and TASO at which 80% in area fraction of austenite is formed, TA2O and TASO being determined by thermodynamical calculations, at a strain rate below or equal to 0.01s-1.

6. A hot rolled superplastic steel sheet according to claim 5, wherein Ts is of at least 640°C.

7. A method for manufacturing a hot rolled superplastic steel sheet, comprising the following successive steps:casting a steel to obtain a semi-product, said semi product having a composition according to claims 1 to 3,reheating the semi-product at a temperature comprised between 1100°C and 1300°C,hot rolling the semi-product with a finish hot rolling temperature FRT between 800°C and 950°C, the reduction rate R from 1000°C to FRT being above or equal to 55%, to obtain a hot rolled steel sheet,cooling the hot rolled steel sheet to a coiling temperature TCOii below or equal to 600°C,coiling the hot rolled steel sheet at the said coiling temperature Tcoii, cooling the coiled steel sheet to room temperature, to obtain a hot rolled superplastic steel sheet.

8. A method according to claim 7, wherein the reduction rate R is above or equal to 60%.

9. A method according to any one of claims 7 and 8, wherein the coiling temperature Tcoii is comprised from 400°C to 580°C.

10. A method according to any one of claims 7 and 9, comprising further a final coating step.

11. Use of a steel sheet according to anyone of claims 1 to 6 or obtainable according to the method of anyone of claims 7 to 10 for manufacturing a structural or safety part of a vehicle.

12. A method for manufacturing a superplastic steel part, comprising the following successive steps:providing a hot rolled superplastic steel sheet according to any one of claims 1 to 6, or produced by the method according to any one of claims 7 to 10, cutting the hot rolled superplastic steel sheet to a predetermined shape, to obtain a steel blank,- heating the steel blank to a temperature Ts, from TA2O and TASO and maintaining at said temperature for a holding time tsfrom 1s to 3600s,- deforming the steel blank with a strain rate below or equal to 0.01 per second to obtain a superplastic steel part.

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