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: 1.5–3.5%, Si: 0.01-1.0%, Al: 0.4-2.5%, Si / 4+Al ≥0.4%, Cr: 0.01-1.5%, 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%, the remainder of the composition being iron and unavoidable impurities resulting from the smelting, and having a microstructure comprising, in area fraction, from 10% to 60% of pearlite Fp, less than 50% of bainite, the rest being ferrite, pearlite having a pearlite band index PBI = (Lfp-Fp) / (100-Fp) × 10 ≤ 7, Lfp being the average linear fraction of pearlite and the product PBI × D30 ≤ 50, D30 being the circular equivalent grain diameter measured for each of the 30% area fractions of the largest ferrite and bainite grains

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 “superplasticity” 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 toclaim 7. The method can also comprise any of the characteristics of claims 8 to 11. Another object is achieved by providing a steel 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] The composition of the steel sheet according to the invention will now be described, the content being expressed in weight percent (wt. %).

[0011]

[0010] 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 carbon content is from 0.05% to 0.30%, more preferably from 0.08% to 0.30%, even more preferably from 0.10% to 0.30%.

[0012]

[0011] The manganese content is from 1.5% to 3.5 %. Above 3.5% 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 1.5% to 3.0%, more preferably from 1.5% to 2.8%, even more preferably from 1.5 to 2.5%.

[0013]

[0012] Silicon content is added in a content of 0.01% to 1.0% to increase the strength. Above 1.0%, silicon oxides form at the surface, which impairs the coatability of the steel. Preferably, a minimum of 0.05% of silicon content is added. More preferably a minimum of 0.08% of silicon is added, or even 0.10%. The maximum amount of silicon added is preferably 0.8%, more preferably 0.7%.

[0014]

[0013] Aluminium content is added in an amount of 0.4% 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 minimum amount of aluminium added is 0.5%, more preferably 0.6%, even more preferably 0.7% or 0.8%. Preferably, the maximum amount of aluminium is 2.0%, or more preferably 1.8%.

[0014] The aluminium and silicon contents according to the invention satisfy Si / 4 + Al > 0.4%.

[0015]

[0015] According to the invention, the chromium content is from 0.01% to 1.5% to provide improved hardenability and post-forming strength. Preferably the minimum amount of chromium is 0.05%. More preferably the minimum amount of chromium is 0.08%, even more preferably 0.10%.

[0016]

[0016] Optionally some elements can be added to the composition of the steel according to the invention.

[0017]

[0017] Molybdenum 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%.

[0018]

[0018] Niobium can be added up to 0.05% in order to provide precipitation strengthening and to refine prior austenite grain size.

[0019]

[0019] 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.

[0020]

[0020] Titanium can be added up to 0.050 % to provide precipitation strengthening and to protect boron against the formation of BN.

[0021]

[0021] The remainder of the composition of the steel is iron and unavoidable impurities resulting from the smelting process and depending on the process route.

[0022]

[0022] 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.

[0023]

[0023] 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 such 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.

[0024]

[0024] S, P 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.

[0025]

[0025] The microstructure of the hot rolled steel sheet according to the invention will now be described. It contains, in area fraction:from 10% to 60% of pearlite Fp, the pearlite having a pearlite band index PBI calculated through the formula PBI = (LfP-Fp) / (100-Fp) x 10, LfPbeing the average linear fraction of pearlite measured according to standard ASTM E 1268-19, on lines parallel to the rolling direction.

[0026] less than 50% of bainite,

[0027] the rest being ferrite,

[0028] and the product PBI x D30 being below or equal to 50, D30 being the average circular equivalent grain diameter measured in the 30% area fractions of the ferrite and bainite grains having the largest circular equivalent grain diameter.

[0029]

[0026] Pearlite is formed during the cooling of the hot rolled steel sheet to the coiling temperature, with a cooling rate vcand during the coiling. Thanks to this cooling rate, the pearlite is formed in less dense bands, which is underlined by low values of LfP, LfPbeing the average linear fraction of pearlite measured according to standard ASTM E 1268-19. The pearlite band index PBI, defined as PBI = (LfP-Fp) / (100-Fp) x 10, Fpbeing the pearlite area fraction, is an index which quantifies the homogeneity of the microstructure. The smaller is the PBI, the less dense the pearlite bands, and the more homogeneous the microstructure. Above 60% of pearlite, the elongation of the steel sheet measured at high temperature may be reduced. With a carbon content according to the invention, it is difficult to obtain less than 10% of pearlite.

[0030] Preferably, the pearlite amount in area fraction is from 20% to 60%, more preferably from 20% to 50%.

[0031] Preferably, PBI is below or equal to 7. Preferably LfPis below or equal to 80%.

[0032]

[0027] The product PBI x D30 is below or equal to 50 to assure a global homogeneous microstructure, D30 being the average circular equivalent grain diameter measured in the 30% area fractions of the ferrite and bainite grains having the largest circular equivalent grain diameter, method known per se, and explained below. Preferably, PBI x D30 is below or equal to 40.

[0033]

[0028] Bainite can be formed during the cooling of the steel sheet down to the coiling temperature and to room temperature. Bainite can help to break down the pearlite bands and decrease the PBI. Preferably the bainite amount in area fraction is less than 40%.

[0034]

[0029] The rest of the microstructure is ferrite. Ferrite is formed during the cooling step. Ferrite is necessary to prevent the grain growth during a further hot deformation of the steel according to the invention.

[0030] The hot rolled superplastic 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:

[0035]

[0031] A semi-product able to be further hot rolled, is provided with the steel composition described above.

[0036]

[0032] The semi product is obtained by casting liquid steel, which can be produced by a steelmaking process with or without the use of scrap.

[0037]

[0033] The semi product is heated to a temperature from 1100°C to 1300°C, so to make it possible to ease hot rolling. The steel sheet is then hot rolled with a finish hot rolling temperature FRT comprised from 800°C to 950°C. During this hot rolling, the reduction ratio R from 1000°C to FRT is above or equal to 30%, to refine the prior austenite grain size. Preferably, R is below or equal to 70%, more preferably below or equal to 60%.

[0038]

[0034] The hot-rolled steel sheet is then cooled to a temperature TCOii below or equal to 680°C, with a cooling rate vccomprised from 20°C / sto 100°C / s, to avoid the formation of coarse ferrite or bainite grains and the formation of large pearlite bands in the microstructure. Preferably, the cooling rate vcis above or equal to 25°C / s, more preferably above or equal to 30°C / s, even more preferably above or equal to 50°C / s. Preferably TCOii is above or equal to 400°C to limit the formation of large pearlite bands. More preferably TCOii is above or equal to 450°C, even more preferably above or equal to 470°C, or above or equal to 500°C.

[0039]

[0035] The hot rolled steel sheet is then coiled at the temperature TCOii, and cooled to room temperature, to obtain a hot rolled superplastic steel sheet.

[0040]

[0036] 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.

[0041]

[0037] 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, comprised from TA2O, temperature at which 20% in area fraction of austenite is formedduring the said heating, and the temperature TASO at which 80% in area fraction of austenite is formed. Both temperatures can be defined by thermodynamical calculations using software as Thermo-calc®. The steel blank can be maintained at said Tstemperature for a holding time tsfrom 1s to 3600s, before being deformed with a strain rate below or equal to 0.01 per second.

[0042]

[0038] 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 Tswith a strain rate below or equal to 0.01s-1. 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 690°C.

[0043] Examples

[0044]

[0039] Two grades, whose compositions are gathered in table 1, were cast in semi- products and processed into steel sheets.

[0045] Table 1 - Compositions

[0046]

[0040] The tested compositions are gathered in the following table wherein the element contents are expressed in weight percent (wt.%).

[0047]

[0048]

[0041] TA2oand TASO of the steel sheet have been determined through thermodynamic calculations with software as Thermo-calc®.

[0049]

[0042] 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.

[0050] Table 2 - Process parameters

[0051] Steel semi-products, as cast, were reheated at 1250°C, hot rolled with a finish rolling temperature FRT, the reduction rate from 1000°C to FRT being R (%). The steel sheets were then cooled to the coiling temperature Tcoii with a cooling rate vc, before beingcoiled. The following specific conditions to obtain the hot rolled superplastic steel sheets were applied:

[0052]

[0053] The hot rolled superplastic steel sheets were then analyzed, and the corresponding microstructure elements were gathered in table 3.

[0054] Table 3 - Microstructure of the hot rolled superplastic steel sheets

[0055]

[0043] The area fractions of phases in the microstructure are determined through the following method: a specimen is cut from the hot rolled superplastic steel sheet, polished and etched with a reagent known per se, to reveal the microstructure. The section is afterwards examined through scanning electron microscope (SEM) at a magnification above or equal to 1000x, in secondary electron mode to determine the area fraction of bainite and ferrite after Nital or Picral / Nital reagent etching.

[0056]

[0044] The area fractions of pearlite are measured using SEM images.

[0057]

[0045] In the following table, LfPis the average linear fraction of pearlite on lines parallel to the rolling direction and is evaluated according to ASTM E1268-19 using SEM images with a magnification of x1000 and x2000. Five lines parallel to the rolling direction are chosen to cross the five densest pearlite bands in each image obtained by SEM, and the linear fraction of pearlite is evaluated in each line. LfPis the average values of these linear fraction values in these five SEM images.

[0058]

[0046] The pearlite band index PBI is an index which quantifies the homogeneity of the microstructure. The smaller is the PBI, the more homogeneous the microstructure. PBI is defined through the following equation PBI= (LfP-Fp) / (100-Fp) x 10, Fpbeing the pearlite fraction.

[0059]

[0047] The microstructure can still contain large grains which can deteriorate the superplasticity of the hot rolled steel sheets. On the images obtained through scanning electron microscope, the circular equivalent grain diameter is measured in the bainiteand ferrite grains, the circular equivalent grain diameter being determined by a method known per se, by drawing a circle around the grain with a surface area equivalent to that of the grain, and by calculating its diameter. The 30% area fraction of the largest ferrite and bainite grains are selected, and the average circular equivalent grain diameter D30 is measured from these grains. To assure a global homogeneous microstructure, the hot rolled superplastic steel sheet has a PBI x D30 value below or equal to 50.

[0060]

[0061] Underlined values: not according to the invention

[0062]

[0048] In trials 2 and 4 the hot rolled superplastic steel sheets are cooled to the coiling temperature with a too slow cooling rate, which leads to the presence of dense pearlite band structures, as shown with the high LfPand PBI values. Moreover, a significant area fraction of large ferrite and / or bainite grains is present, as shown with the high D30 values. This leads to a high global index PBI x D30, indicating a non-homogeneous microstructure, which impairs the superplasticity of the steel sheet.

[0063]

[0049] 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 blanks are 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.

[0064] Table 4 - Hot deformation process

[0065]

[0066]

[0050] 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.

[0067] Table 5 - Mechanical properties of the parts

[0068]

[0069]

[0051] Trials 5, 6, 9 and 10 are according to the invention. Thanks to the microstructure obtained before the hot deformation, the steel presents a high elongation TE.

[0070]

[0052] In trials 7, 8, 11 and 12 because of the combination of the large ferrite and bainite grains and a very heterogeneous banded microstructure before the hot deformation, the elongation of the steel 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: 1.5 - 3.5 %,Si: 0.01 - 1.0%,Al: 0.4 -2.5%,Si / 4+AI > 0.4%Cr: 0.01 - 1.5 %,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%,the remainder of the composition being iron and unavoidable impurities resulting from the smelting, said steel sheet having a microstructure comprising, in area fraction,- from 10% to 60% of pearlite Fp,- less than 50% of bainite,- the rest being ferrite,- the pearlite having a pearlite band index calculated through the formula PBI = (Lfp- Fp) / (100-Fp) x 10, LfPbeing the average linear fraction of pearlite measured according to standard ASTM E 1268-19, such that the product PBI x D30 is below or equal to 50, D30 being the average circular equivalent grain diameter measured in the 30% area fractions of the ferrite and bainite grains having the largest circular equivalent grain diameter.

2. A hot rolled superplastic steel sheet according to claim 1, wherein the steel has a manganese content comprised from 1.5% to 2.8%.

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 0.01% to 0.7%.

4. A hot rolled superplastic steel sheet according to any one of claims 1 to 3, wherein PBI is below or equal to 7.

5. A hot rolled superplastic steel sheet according to any one of claims 1 to 4, wherein the steel has a total elongation TE above or equal to 175%, when measured after the steel has been heated to a temperature Tscomprised from TA2oat 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 Tsis of at least 690°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 4,reheating the semi-product at a temperature comprised from 1100°C to 1300°C, hot rolling the semi-product with a finish hot rolling temperature FRT comprised from 800°C to 950°C, the reduction rate from 1000°C to FRT being above or equal to 30%, to obtain a hot rolled steel sheet,cooling the hot rolled steel sheet to a coiling temperature TCOii below or equal to 680°Cwith a cooling rate vccomprised from 20°C / sto 100°C / s,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 coiling temperature TCOii is comprised from 400°C to 680°C.

9. A method according to any one of claims 7 and 8, wherein the cooling rate vcis above or equal to 25°C / s.

10. A method according to any one of claims 7 to 9, wherein the cooling rate vcis above or equal to 30°C / s.

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

12. 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 11 for manufacturing a structural or safety part of a vehicle.

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