Annealed and tempered steel sheet, and method for manufacturing the same

Abstract

The invention relates to an annealed and tempered steel sheet, made of a steel having composition comprising, by weight percent C: 0.05 - 0.2 %, Mn: 5 – 11 %, Mo: 0.05 - 0.50%, B: 0.0005 – 0.005%, S ≤ 0.010 %, P ≤ 0.020 %, N ≤ 0.012 %, and comprising optionally one or more of the following elements, in weight percentage: Si ≤ 1.20 %, Al ≤ 2.5 %, Ti ≤ 0.050 %, 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 5% to 30% of retained austenite, the rest being tempered martensite, and carbon [C] γ and manganese [Mn] γ contents in retained austenite, expressed in weight percent, satisfying 565-(31*[Mn] γ +6.3)-600*(1-EXP(-0.96*[C] γ))≤ 100.

Description

[0001] Annealed and tempered steel sheet, and method for manufacturing the same

[0002]

[0001] The present invention relates to a high strength and high ductility annealed and tempered steel sheet, and to a method to obtain such steel sheet.

[0003]

[0002] To manufacture various items such as parts of body structural members and body panels for automotive vehicles, it is known to use sheets made of DP (Dual Phase) steels or TRIP (Transformation Induced Plasticity) steels.

[0004]

[0003] 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.

[0005]

[0004] One of the developments made to improve mechanical properties is to increase content of manganese in steels. The presence of manganese helps to increase ductility of steels thanks to the stabilization of austenite. But these steels present weaknesses of brittleness. To overcome this problem, elements as boron are added. These boron-added chemistries are very tough at the hot-rolled stage but the hot band is too hard to be further processed. The most efficient way to soften the hot band is batch annealing, but it can lead to a loss of toughness.

[0006]

[0005] The publication WO / 2022 / 018568 relates to a cold rolled annealed and tempered steel sheet, having a combination of high mechanical properties with a yield strength above or equal to 1000 MPa, with a tensile strength TS above or equal to 1450 MPa, and a uniform elongation UE above or equal to 6.5%. Nevertheless, to obtain such properties, the cold rolled has to be batch annealed before cold rolling to promote manganese inhomogeneous repartition during the rolling, which helps to obtain the targeted final properties.

[0007]

[0006] The publication WO / 2021 / 124203 relates to a hot rolled steel sheet, subjected to a quenching and partitioning step, to obtain a yield strength YS higher than 950MPa, tensile strength TS higher than 1 180MPa, a uniform elongation UE higher than 10% and a hole expansion ratio HER higher than 25%. The soaking step between Ae1 and Ae3 leads to the formation of a high fraction of ferrite, needed to obtain the targeted mechanical properties, with in particular a TS lower than 1350MPa.

[0008]

[0007] The aim of the present invention is therefore to remedy the drawbacks of the prior art by providing a steel sheet having a combination of high mechanical properties with a tensile strength TS and a uniform elongation UE satisfying TS*UE >8100MPa%, UE being above or equal to 5.5%. Preferably TS is above or equal to 1350MPa. Preferably the yield strength YS of the annealed and tempered steel sheet is above or equal to 950 MPa.

[0009]

[0008] The object of the present invention is achieved by providing a steel sheet according to claim 1 . The steel sheet can also comprise any of the characteristics of claims 2 to 4, taken alone or in combination. Another object is achieved by providing the method according to claim 5. The method can also comprise any of the characteristics of claims 6 to 10, taken alone or in combination.

[0010] Other characteristics and advantages of the invention will be described in greater detail in the following description.

[0011]

[0009] 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, with reference to:

[0012] Figure 1 , which represents the results of dilatometry test, allowing to determine Vc.

[0013]

[0010] 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.

[0014]

[0011] The composition of the steel sheet according to the invention will now be described, the content being expressed in weight percent (wt. %).

[0015]

[0012] According to the invention, the carbon content is from 0.05% to 0.20 % to ensure a satisfactory strength and good weldability properties. Above 0.20% of carbon, weldability of the steel sheet may be reduced. The temperature of the soaking depends on carbon content: the higher the carbon content, the lower the soaking temperature to form austenite. If the carbon content is lower than 0.05%, the strength of the tempered martensite is not sufficient to get TS above 1350MPa. Preferably, the carbon content is from 0.08% to 0.20%, more preferably from 0.08% to 0.18%, even more preferably from 0.10% to 0.18%, or 0.11 % to 0.18%.

[0016]

[0013] The manganese content is from 5.0% to 1 1.0 %. Above 11.0% of addition, weldability of the steel sheet may be reduced, and the productivity of parts assembly can be reduced. Moreover, the risk of central segregation increases to the detriment of the mechanical properties. As the temperature of soaking depends on manganese content too, the minimum of manganese is defined to stabilize austenite, to obtain, after soaking, the targeted microstructure and strengths. Preferably, the manganese content is from 5.0% to 10.0%, more preferably from 5.0% to 9.0%.

[0017]

[0014] The molybdenum content is from 0.05% to 0.50% to decrease the manganese segregation during casting. Moreover, an addition of at least 0.05% of molybdenum stabilizes the retained austenite thus reducing austenite decomposition during tempering. Above 0.50%, the addition of molybdenum is costly and ineffective in view of the properties which are required. Preferably, the molybdenum content is from 0.10% to 0.50%, more preferably from 0.10% to 0.40%, even more preferably from 0.10% to 0.30%, or 0.15% to 0.30%.

[0018]

[0015] According to the invention, the boron content is from 0.0005% to 0.005% to improve the toughness of the hot rolled steel sheet and the spot weldability of the cold rolled steel sheet. Above 0.005%, the formation of boro-carbides at the prior austenite grain boundaries is promoted, making the steel more brittle. Preferably, the boron content is comprised from 0.001% to 0.005%, more preferably from 0.001% to 0.004%.

[0019]

[0016] Titanium is added in a content from 0.010% to 0.050 % to provide precipitation strengthening. Moreover, titanium is added in addition of boron to protect boron against the formation of BN.

[0020]

[0017] Optionally some elements can be added to the composition of the steel according to the invention.

[0021]

[0018] Aluminium content can be added up to 2.5% to decrease the manganese segregation during casting. Aluminium is a very effective element for deoxidizing the steel in the liquid phase during elaboration. 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 0.010%. Preferably, the maximum amount of aluminium is 2.0%, or more preferably 1 .5%, even more preferably 1 .0% or 0.5%.

[0022]

[0019] The maximum addition of silicon content is limited to 1.20% to improve the stabilization of austenite. Above 1.20%, silicon oxides form at the surface, which impairs the coatability of the steel. Preferably a minimum of 0.10% of silicon content is added. More preferably a minimum of 0.15% of silicon is added, or even 0.20%. The maximum amount of silicon added is preferably 1 .00%, more preferably 0.80%, even more preferably 0.60% or 0.5%.

[0023]

[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. In the case of a production route using a blast furnace, the level of unavoidable impurities is very low. In the case of a production route using an Electric Arc Furnace loaded with scraps, the steel sheet can further comprise residual elements coming from such scraps such as Antimony, Arsenic, Copper, Nickel, Chromium, Tin, and Lead, each up to 0.03% which are considered as unavoidable impurities.

[0024]

[0021] P, S and N are also part of the unavoidable impurities whatever the process route. Their content is less than or equal to 0.020 % for P, less than or equal to 0.010 % for S, and less than or equal to 0.012 % for N.

[0022] The microstructure of the annealed and tempered steel sheet according to the invention will now be described. It contains, in area fraction:

[0025] - from 5% to 30% of retained austenite,

[0026] - the rest being tempered martensite,

[0027] - and mean carbon [C]Yand manganese [Mn]Ycontents in retained austenite, expressed in weight percent, satisfying:

[0028] 565-(31 *[Mn]Y+13*0, 3+12*0, 2)-600*(1-exp(-0,96*[C]Y))< 100.

[0029]

[0023] The microstructure of the annealed and tempered steel sheet according to the invention contains from 5% to 30% of retained austenite. Below 5% of austenite, the uniform elongation UE cannot reach the minimum values of 5.5%. Such austenite is formed during the soaking of the steel sheet at a temperature TH above or equal to Ae3, and below or equal to (Ae3+200°C), Ae3 designating the equilibrium transformation temperature above which the microstructure is fully transformed into austenite. During this soaking, areas containing a manganese content higher than nominal value and areas containing manganese content lower than nominal value are formed, creating a heterogeneous distribution of manganese.

[0030]

[0024] The mean carbon [C]Yand manganese [Mn]Ycontents in retained austenite, expressed in weight percent, are such that they satisfy 565-(31 x[Mn]v+6.3)-600x(1 -exp(- 0.96x[C]Y)) below or equal to 100. This formula is used to evaluate the stability of austenite as a function of the carbon and manganese amount in austenite. When this value is above 100, the austenite is not stable enough, which can impact the ductility, with a low uniform elongation. Preferably, it is below or equal to 90, more preferably below or equal to 80, even more preferably below or equal to 70 or even 60. Preferably, the [Mn]Yis above or equal to 7.5%.

[0031]

[0025] The rest of the microstructure of the annealed and tempered steel sheet according to the invention is tempered martensite. Martensite is formed during the cooling after the soaking of the steel sheet, by transformation of a part of austenite, that is less rich in carbon and manganese. This martensite is then tempered during the tempering of the steel sheet.

[0026] Preferably, the microstructure of the annealed and tempered steel sheet comprises from 5% to 20% of retained austenite, the rest being tempered martensite.

[0032]

[0027] The annealed and tempered 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:

[0028] A semi-product able to be further hot-rolled, is provided with the steel composition described above. The semi product is heated to a temperature Treheat from 1 100°C to 1300°C, so to make it possible to ease hot rolling, with a final hot rolling temperature FRT from 800°C to 1000°C. Preferably, the FRT is from 800°C to 950°C, more preferably from 800°C to 930°C, even more preferably from 830°C to 930°C.

[0033]

[0029] The hot-rolled steel sheet is then cooled and coiled at a temperature TCOii below or equal to 650°C, and preferably from 300°C to 500°C. At this stage, the manganese distribution in the microstructure of the hot rolled steel sheet is homogeneous.

[0034]

[0030] The hot rolled steel sheet is then cooled to a temperature Tcibelow or equal to 100°C. Preferably Tciis above or equal to 10°C. Preferably the temperature Tciis room temperature. The steel sheet can then be pickled.

[0035]

[0031] The hot rolled steel sheet can optionally be cold rolled. The reduction ratio can be comprised from 5% to 80%, depending on the targeted thickness. The aim of this cold rolling would be to reduce the thickness of the steel sheet, without the need of an intermediate batch annealing before the cold rolling step, or to improve the flatness of the hot rolled sheets. At this stage, the manganese distribution in the microstructure of the cold rolled steel sheet is homogeneous.

[0036]

[0032] Whether hot-rolled or cold-rolled, the microstructure of the sheet obtained after these rolling operations comprises from 0% to 30 % of bainite, the rest being martensite and martensite-austenite (MA) islands.

[0037]

[0033] The steel sheet is then heated from Tcito a temperature T 1 comprised from 425°C to 475°C, with a heating rate Vi, for which there is no limitation. For example, Vi can be from 0.5°C / s to 500°C / s, preferably from 0.5°C / s to 200°C / s, more preferably from 0.5°C / s to 100°C°C / s or even from 0.5°C / s to 50°C / s.

[0038] The steel sheet is then heated from T1 to T2, T2 being comprised from 625°C to 675°C, with a heating rate v2below or equal to the critical speed Vc. Vc is determined with dilatometry test and is defined as the heating rate allowing to form 7% of austenite at T2. Figure 1 represents the critical speed needed to obtain 7% of austenite for T2=650°C, which is 15°C / s. As shown, with a heating rate of 30°C / s, the austenite formed would be too low. This austenite is called reverse austenite. The reverse austenite formation during the heating in this temperature range is very slow, allowing a high level of Mn partitioning into austenite to form Mn-rich areas.

[0039] When the heating rate is above Vc, the austenite fraction formed is too low. Consequently, no significant fraction of stable retained austenite can be obtained after annealing and tempering. Preferably, Vc is above or equal to 0.1 °C / s.

[0034] The steel sheet is then heated from T2 to a temperature TH at a heating rate v3, which can be for example from 0.5°C / s to 1000°C / s, or 0.5°C / s to 500°C / s, preferably from 0.5°C / s to 200°C / s, more preferably from 0.5°C / s to 100°C°C / s or even from 0.5°C / s to 50°C / s. TH is comprised from Ae3 to Ae3+200°C, in order to form a fully austenite structure, Ae3 being the equilibrium transformation temperature Such annealing can be performed by continuous annealing. Preferably v3is from 0.5°C / s to 500°C / s, more preferably from 0.5°C / s to 200°C / s, even more preferably from 0.5°C / s to 100°C / s or 0.5 to 50°C / s.

[0040]

[0035] The steel sheet is maintained at said TH temperature for a holding time tn from 1 s to 1200s. Preferably, the holding time is from 1 s to 800s, more preferably from 1 s to 500s, even more preferably from 1 s to 300s, or from 1 s to 200s.

[0041]

[0036] The steel sheet is then cooled to a temperature TC2 below or equal to 100°C, preferably with a cooling rate above or equal to 5°C / s. Preferably the TC2 temperature is room temperature. The cooling can be done through a water quenching, or a gas quenching with a cooling rate above or equal to 5°C / s. A large part of the austenite present at the end of the soaking will be turned into fresh martensite.

[0042]

[0037] After this cooling, the steel sheet is then submitted to a tempering step at a temperature Ttempcomprised from 150°C to 400°C, for a holding time ttempfrom 3s to 3600s, Ttempbeing below or equal to Ms, Ms being the martensite start temperature. Preferably, Ttempis below or equal to (Ms-10°C). The fresh martensite is transformed into tempered martensite at the end of this tempering step. Preferably Ttempis from 150°C to 350°C, more preferably from 150°C to 300°C, even more preferably from 150°C to 250°C. Preferably the holding time ttempis from 50s to 3600s, more preferably from 80s to 3600s, even more preferably from 100s to 3600s, or from 100s to 1800s.

[0043]

[0038] The steel sheet is then cooled to room temperature. The obtained annealed and tempered steel sheet can then be coated by any suitable process including electrodeposition or vacuum coating of zinc or zinc-based alloys or of aluminium or aluminium-based alloys.

[0044]

[0039] The annealed and tempered steel sheet according to the invention has a uniform elongation UE, expressed in % and a tensile strength TS, expressed in MPa, satisfying TS*UE above or equal to 8100MPa%.

[0045]

[0040] Preferably, the uniform elongation of the annealed and tempered steel sheet is above or equal to 5.5%.

[0046]

[0041] Preferably, the tensile strength TS of the annealed and tempered steel sheet is above or equal to 1350MPa.

[0042] Preferably, the yield strength YS of the annealed and tempered steel sheet is above or equal to 950MPa, more preferably above or equal to 1000MPa.

[0047]

[0043] The invention will be now illustrated by the following examples, which are by no way limitative.

[0048] Examples

[0049]

[0044] Three grades, whose compositions are gathered in table 1 , were cast in semiproducts and processed into steel sheets.

[0050] Table 1 - Compositions

[0051]

[0045] The tested compositions are gathered in the following table wherein the element contents are expressed in weight percent (wt.%).

[0052] Underlined values: out of the invention

[0053]

[0046] Ae3 and Ms temperatures of the steel sheet have been determined through thermodynamic calculations with software as Thermo-calc®.

[0054] Table 2 - Process parameters of the annealed and tempered steel sheets

[0055]

[0047] Steel semi-products, as cast, were reheated at 1200°C, hot rolled with a final rolling temperature FRT of 850°C, coiled at a TCOii temperature of 450°C and cooled to a temperature Tci, which is room temperature (RT). The steel sheets are then heated from Td to T 1 with a cooling rate Vi, then from T1 to T2 with a heating rate v2, and finally heated from T2 to a temperature TH with a heating rate v3. The steel sheets are maintained at said TH temperature for a holding time tn, before being water quenched to Tc2at a cooling speed of 5°C / s. The steel sheet is then heated to a temperature Ttempand maintained at said temperature for a holding time ttemp, before being cooled to room temperature. The following specific conditions to obtain the annealed and tempered steel sheets were applied:

[0056] Underlined values: out of the invention

[0057] 5

[0058] The steel sheets were analyzed, and the corresponding properties are gathered in table 3.

[0059] Table 3 Microstructure of the steel sheets

[0060] 10

[0048] The phase percentages of the microstructure of the hot rolled steel sheets before the soaking at TH and the microstructure of the annealed and tempered steel sheets were determined.

[0061]

[0049] The area fractions of phases in the microstructure are determined through the following method: a specimen is cut from the steel sheet, polished and etched with a reagent

[0062] 15 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 greater than 5000x, in secondary electron mode.

[0063]

[0050] The determination of the area fraction of bainite, martensite, and tempered 0 martensite is performed thanks to SEM observations after Nital or Picral / Nital reagent etching.

[0064]

[0051] The determination of the volume fraction of retained austenite is performed thanks to X-ray diffraction.

[0052] [C]Yand [Mn]Ycorresponds to the mean amount of carbon and manganese in austenite, in weight percent. The carbon content [C]Yis measured through X-rays diffraction and by using the known formula of Dyson and Holmes, J. Iron Steel Inst., 208 (1970), 469. The mean manganese [Mn]Yis obtained through Energy Dispersive X-ray Spectroscopy (EDS) on Transmission Electrons Microscopy (TEM), the method can be found in a publication such that Oyarzabal et al. Materials Characterization Volume 209 (2024), 1 13698.

[0065] Underlined value: not according to the invention nd: non determined value

[0066]

[0053] Mechanical properties of the obtained annealed and tempered steel sheets were determined and gathered in table 6.

[0067] Table 6 - Mechanical properties of the annealed and tempered steel sheet

[0068]

[0054] The yield strength YS, the tensile strength TS, both expressed in MPa, and the uniform elongation UE, expressed in %, are measured according to ISO standard ISO 6892- 1 , published in October 2009.

[0069] Trials 1 and 2 are according to the invention. Trials 3, 5 and 6 were submitted to a heating from 450°C to 650°C at a cooling rate higher than the critical speed. The austenite fraction formed at this temperature range is thus too low. Moreover, in trials 4-6, the heating rate of this step heating does not allow to obtain a high amount of manganese in austenite, as seen by the low level of [Mn]Y. Both phenomena lead to a lower elongation of the sheet, and consequently a low compromise between strength and elongation TS * UE.

[0070] The steel sheet in trial 4 made of the same chemical composition as trials 1 and 2, are only hot rolled and coiled. It can be seen that without the further annealing step, the manganese content in austenite is lower than in trial 1 and 2, which leads to a low uniform elongation and yield strength.

Claims

CLAIMS1) Annealed and tempered steel sheet, made of a steel having a composition comprising, by weight percent:C: 0.05 - 0.20 %Mn: 5.0 - 11.0 %Mo: 0.05 - 0.50 %B: 0.0005 - 0.005%Ti 0.010 - 0.050 %S < 0.010 %P < 0.020 %N < 0.012 % and comprising optionally one or more of the following elements, in weight percentage:Si < 1.20 %Al < 2.5 % the remainder of the composition being iron and unavoidable impurities resulting from the smelting, said steel sheet having a microstructure comprising, in area fraction,- from 5% to 30% of retained austenite,- the rest being tempered martensite,- and a mean carbon [C]Yand mean manganese [Mn]Ycontents in retained austenite, expressed in weight percent, satisfying565-(31*[Mn]Y+6.3)-600*(1 -exp (-0.96*[C]Y)) < 100.2) An annealed and tempered steel sheet according to claim 1 wherein the steel sheet has a tensile strength TS, expressed in MPa, and a uniform elongation UE, expressed in %, satisfying TS*UE > 8100 MPa%.3) An annealed and tempered steel sheet according to any one of claims 1 or 2 wherein the steel sheet has a uniform elongation UE above or equal to 5.5%.4) An annealed and tempered steel sheet according to any one of claims 1 to 3, wherein the annealed steel sheet has a tensile strength TS above or equal to 1350MPa.5) A method for producing an annealed and tempered steel sheet, said method comprising the following successive steps:- casting a steel to obtain a semi product, said steel having a composition according to claim 1 ,- heating the semi product at a temperature Treheat from 1 100°C to 1300°C,- hot rolling the heated semi product at a finish hot rolling temperature FRT from 800°C to 1000°C- coiling the hot rolled steel sheet at a coiling temperature TCOii below or equal to 650°C,- cooling the hot rolled steel sheet to a temperature Tcibelow or equal to 100°C,- heating the hot rolled steel sheet from Tcito a temperature T 1 comprised from 425°C to 475°C at a heating rate Vi,- heating the hot rolled steel sheet from T1 to a temperature T2 comprised from 625°C to 675°C, at a heating rate v2below or equal to the critical speed Vc, Vc being the heating rate allowing to form 7% of austenite at T2,- heating the steel sheet from T2 to a soaking temperature TH comprised from Ae3 to (Ae3+200°C), at a heating rate v3, and maintained at said TH temperature for a holding time tn from 1 to 1200 s,- cooling the heated steel sheet from TH to a temperature Tc2below or equal to 100°C,- heated the steel sheet from Tc2to a temperature Ttempcomprised from 150°C to 400°C, and maintained at said Ttemptemperature for a holding time ttempcomprised from 3 to 3600s,- cooling the steel sheet to room temperature in order to obtain an annealed and tempered steel sheet.6) A method for producing an annealed and tempered steel sheet according to claim 5, wherein the hot rolled steel sheet is pickled after the coiling.7) A method for producing an annealed and tempered steel sheet according to any one of claims 5 and 6 wherein, the steel sheet is cold rolled, after the cooling step to a temperature Td.8) A method for producing an annealed and tempered steel sheet, according to any one of claims 5 to 8 wherein the critical speed Vc is above or equal to 0.1 °C / s.9) A method for producing an annealed steel sheet according to any one of claims 5 to 9, wherein the cooling step from TH to Tc2is made by water quenching.10) A method for producing an annealed steel sheet according to any one of claims 5 to 9, wherein the cooling step from TH to TC2 is made by gas cooling.