METHOD FOR MANUFACTURING A COATED ULTRA-HIGH STRENGTH STEEL SHEET AND THE RESULTING SHEET

Abstract

A process for producing a cold-rolled steel sheet having a tensile strength ≥ 1470 MPa and a total elongation te ≥ 19%, the process comprising the annealing steps at an annealing temperature at ≥ ac3 of an untreated steel sheet whose chemical composition contains, in % by weight: 0.34% ≤ c ≤ 0.40%, 1.50% ≤ mn ≤ 2.30%, 1.50% ≤ si ≤ 2.40%, 0% <cr ≤ 0,7%, 0% ≤ mo ≤ 0,3%, 0,01% ≤ al ≤ 0,07%, le reste étant fe et des impuretés inévitables, tremper la tôle d'acier recuite en la refroidissant à une température de trempe qt <point de transformation ms et entre 150 ° c et 250 ° c, et effectuer un traitement de partitionnement en réchauffant la tôle d'acier trempée à une température de partitionnement pt entre 350 ° c et 420 ° c et maintien de la tôle d'acier à cette température pendant un temps de partage pt compris entre 15 secondes et 250 secondes.

Description

[0001] The present invention concerns the manufacture of coated or non-coated high strength steel sheet having improved tensile strength and improved total elongation and the sheets obtained by this method.

[0002] The article of Guhui Gao et al., Enhanced ductility and toughness in an ultrahigh-strength Mn-Si-Cr-C steel: The great potential of ultrafine filmy retained austenite (ACTA MATERIALIA, vol. 76, 26 June 2014 (2014-06-26), pages 425-433, XP055175674, ISSN: 1359-6454, DOI: 10.1016 / j.actamat.2014.05.055) discloses an example of a method for producing a steel sheet.

[0003] EP 2 546 368 discloses another example of method for producing a steel sheet.

[0004] Further, the article of Guhui Gao et al. A carbide-free bainite / martensite / austenite triplex steel with enhanced mechanical properties treated by a novel quenching-partitioning-tempering process (MATERIALS SCIENCE AND ENGINEERING A, vol. 559, 1 January 2013 (2013-01-01), pages 165-169, XP055176061, ISSN: 0921-5093, DOI: 10.1016 / j.msea.2012.08.064) discloses another example of method for producing a steel sheet.

[0005] To manufacture various equipment such as parts of body structural members and body panels for automotive vehicles, it is now usual to use bare, electro-galvanized, galvanized or galvannealed sheets made of DP (dual phase) steels multi-phase, complex phase or martensitic steels.

[0006] For example, a high strength multi-phase may include a bainite-martensitic structure with / without some retained austenite and contains about 0.2% of C, about 2% of Mn, about 1.5% of Si which would result in yield strength of about 750 MPa, a tensile strength of about 980 MPa, a total elongation of about 10%. These sheets are produced on continuous annealing line by quenching from an annealing temperature higher than Ac3 transformation point, down to an overaging temperature above Ms Transformation point and maintaining the sheet at the temperature for a given time. Optionally, the sheet is galvanized or galvannealed.

[0007] To reduce the weight of the automotive parts in order to improve their fuel efficiency in view of the global environmental conservation it is desirable to have sheets having improved strength-ductility balance. But such sheets must also have a good formability.

[0008] In this respect, it was proposed to produce sheets made of steel using so called quenched and partitioned having improved mechanical properties and good formability. Coated or non-coated (bare) sheets having, a tensile strength TS of about 1470 MPa and a total elongation of at least 19%, are targeted. These properties are targeted at least when the sheet is not coated or galvanized.

[0009] When the sheet is galvannealed, a tensile strength TS of at least 1470 MPa and a total elongation of at least 15%, preferably at least 16% are targeted.

[0010] Therefore, the purpose of the present invention is to provide such sheet and a method to produce it.

[0011] For this purpose, the invention relates to a method for producing a cold rolled steel sheet according to claim 1.

[0012] Preferably, the annealing temperature AT is between 870°C and 930 °C.

[0013] The composition of the steel is preferably such that 0.34 % ≤ C ≤ 0.37 %.

[0014] The invention also relates to a method for producing a cold-rolled steel sheet according to claim 4.

[0015] The annealing temperature AT is advantageously between 870°C and 930 °C.

[0016] The composition of the steel is preferably such that 0% ≤ Mo ≤ 0.005%.

[0017] The invention relates also to a coated steel sheet according to claim 7.

[0018] The invention also relates to a coated steel sheet according to claim 8.

[0019] The invention will now be described in details but without introducing limitations.

[0020] According to the disclosure, the sheet is obtained by heat treating a hot or preferably a cold rolled non-treated steel sheet made of steel which chemical composition contains, in weight %: 0.34 % to 0.40 % of carbon to ensure a satisfactory strength and improve the stability of the retained austenite. This is necessary to obtain a sufficient elongation. If carbon content is too high, the hot rolled sheet is too hard to cold roll and the weldability is insufficient. 1.50 % to 2.40 % of silicon in order to stabilize the austenite, to provide a solid solution strengthening and to delay the formation of carbides during partitioning with appropriate procedures to prevent the formation of silicon oxides at the surface of the sheet which is detrimental to coatability. 1.50% to 2.30% of manganese to have a sufficient hardenability in order to obtain a structure containing at least 60 % of martensite, a tensile strength of more than 1470 MPa and to avoid having segregation issues which are detrimental for the ductility. 0% to 0.3 % of molybdenum and 0% to 0.7 % of chromium to increase the hardenability and to stabilize the retained austenite in order to strongly reduce austenite decomposition during partitioning. The absolute zero value is excluded due to residual amounts. According to an embodiment of the disclosure, the composition comprises from 0% to 0.5% of chromium. In the steel sheet of the invention coated by galvanizing, the molybdenum content is from 0.07% to 0.20% and the chromium content is from 0.35% to 0.45%. As an alternative, the sheet being coated, the molybdenum content is lower than 0.005%, and the chromium content is from 0.46% to 0.7%. A molybdenum content lower than 0.005% corresponds to the presence of molybdenum only as an impurity or a residual. 0.01% to 0.08% of aluminum which is usually added to liquid steel for the purpose of deoxidation, preferably.

[0021] The remainder is iron and residual elements or unavoidable impurities resulting from the steelmaking. In this respect, Ni, Cu, V, Ti, B, S, P and N at least are considered as residual elements which are unavoidable impurities. Therefore, generally, their contents are less than 0.05% for Ni, 0.05 for Cu, 0.007% for V, 0.001% for B, 0.005% for S, 0.02% for P and 0.010% for N.

[0022] Addition of microalloy elements such as Nb from 0 to 0.05% could be utilized to obtain the desired microstructure and an optimal combination of product properties.

[0023] In particular, when the sheet is coated, Nb may be added in an amount up to 0.05%. According to an an embodiment of the disclosure, Nb is comprised between 0.03 and 0.05%. According to this embodiment, the sheet is coated, by galvanizing. A Nb content of 0.03 to 0.05% allows obtaining satisfactory tensile strength and elongation, in particular a tensile strength of at least 1470 MPa and an elongation of at least 16%, when the sheet is coated by galvanizing.

[0024] Thus, when the sheet is coated, the composition may comprise Nb in an amount between 0.03% and 0.05%, Cr in an amount between 0.46% and 0.7%, and no addition of Mo.

[0025] The non-treated steel sheet is a cold rolled sheet prepared according to the methods known by those who are skilled in the art.

[0026] After rolling the sheets are pickled or cleaned then heat treated and optionally hot dip coated.

[0027] The heat treatment according to the present disclosure is made preferably on a continuous annealing when the sheet is not coated and on a hot dip coating line when the steel sheet is coated, comprises the following successive steps: annealing the cold rolled sheet at an annealing temperature AT equal or higher than the Ac3 transformation point of the steel, and preferably higher than Ac3 + 15°C, in order to obtain an annealed steel sheet having a structure completely austenitic, but less than 1000°C in order not to coarsen too much the austenitic grains. Generally, a temperature higher than 870°C is enough for the steel according to the invention and this temperature does not need to be higher to 930°C. Then the steel sheet is maintained at this temperature i.e. maintained between AT - 5°C and AT + 10°C, for a time sufficient to homogenize the temperature in the steel. Preferably, this time is of more than 30 seconds but does not need to be more than 300 seconds. To be heated to the annealing temperature, the cold rolled steel sheet is, for example, first heated to a temperature of about 600°C at a speed typically below 20°C / s then heated again to a temperature of about 800°C at a speed typically below 10°C / s and eventually heated to the annealing temperature at a heating speed below 5 °C / s. In this case, the sheet is maintained at the annealing temperature for a duration between 40 and 150 seconds. quenching of the annealed sheet by cooling down to a quenching temperature QT lower than the Ms transformation point between 150°C and 250°C at a cooling rate fast enough to avoid ferrite formation upon cooling and preferably of more than 35°C / second, in order to obtain a quenched sheet having a structure consisting of martensite and austenite, then the final structure contains at least 60 % of martensite and between 12 % and 15% of austenite. If the steel contains less than 0.05% of molybdenum and less than 0.05% of chromium, the quenching temperature is preferably between 190°C and 210°C. When the steel sheet has to be galvanized and when the chemical composition of the steel is such that 0.34 % ≤ C ≤ 0.37 %, 0.35 % ≤ Cr ≤ 0.45 % and 0.07 % ≤ Mo ≤ 0.20 %, then the quenching temperature is between 200°C and 230°C. When the composition of the steel is such that 0.46% ≤ Cr ≤ 0.7 % and 0% ≤ Mo ≤ 0.005 %, the quenching temperature is also between 200°C and 230°C. reheating the quenched sheet up to a partitioning temperature PT between 350 °C and 450 °C. The heating speed is preferably at least 30°C / s. maintaining the sheet at the partitioning temperature PT for a partitioning time Pt between 15 sec and 250 sec, for example between 15 sec and 150 sec. During the partitioning step, the carbon is partitioned, i.e. diffuses from the martensite into the austenite which is thus enriched. heating the sheet to a coating temperature, hot dip coating the sheet and cooling it down to the room temperature if a coating is desired. The hot dip coating is, for example, galvanizing, and the coating temperature is about 460°C as it is known in the art.

[0028] The heating to the coating temperature is made preferably at a heating speed of at least 30° / s and the coating takes between 2 and 10s.

[0029] The cooling speed to the room temperature is preferably between 3 and 20°C / s.

[0030] When the sheet is coated, the composition and the treatment parameters are preferably adjusted according to the two following embodiments.

[0031] According to the first aspect of the invention, when the sheet is coated, the steel contains between 0.35 % and 0.45 % of chromium and between 0.07 % and 0.20 % of molybdenum and the partitioning time Pt is preferably between 25 seconds and 55 seconds. In these conditions it is even possible to obtain coated steel sheet having a tensile strength higher than 1510 MPa and a total elongation of at least 20%.

[0032] According to the second aspect of the invention, when the sheet is coated, the steel comprises between 0.46 and 0.7% of Cr, less than 0.005% of Mo and between 0.03 and 0.05% of Nb. With this composition, the partitioning time is higher than or equal to 50s.

[0033] When the sheet is coated by galvanizing, the partitioning time may be as high as 230 s.

[0034] As examples and comparison, it was manufactured sheets made of steels whose compositions in weight and characteristic temperatures such as Ac3 and Ms are reported in table I.

[0035] The sheets were cold rolled, annealed, quenched, partitioned and cooled to the room temperature or, galvanized after partitioning before being cooled to the room temperature.

[0036] The mechanical properties were measured in the transverse direction relative to the direction of rolling. As it is well known in the art, the ductility level is slightly better in the direction of rolling than in the transverse direction for such high strength steel. Measured properties are Hole expansion ratio HER measured according to the standard ISO 16630:2009, the yield strength YS, the tensile stress TS, the uniform elongation UE and the total elongation TE.

[0037] The conditions of treatment and the mechanical properties are reported in Table II for the non coated sheets (not according to the invention) and in Table III for the coated sheets.

[0038] In these tables, AT is the annealing temperature, QT the quenching temperature, PT the partitioning temperature. In Table II, GI is the temperature of galvanizing. Table IRef steelC %Mn %Si %Cr %Mo %Al %Ac3 °CMs °CS1800.292.022.440.004Residual (<0.003)0.059920290S1810.392.031.950.003Residual (< 0.003)0.058860240S800.361.991.950.410.0880.045850250S810.381.981.930.340.141.047860270 Table II ExampleSteelATQTPTPtHEYSTSUETE°C°C°Csec%MPaMPa%%1S18092024040010-982149711.415.92S180920240400100171073135413.919.93S180920240400500-1082130913.218.44S18190020040010-1095158312.513.85S181900200400100211238149313.019.46S181900200400500-1207141713.117.77S8090022040010-92515186.66.88S8090022040030-92914388.98.99S8090022040050-897146213.518.510S80900220400100-948144715.719.611S8190024040010-86716238.19.312S8190024040030-878158411.411.813S8190024040050-833152010.812.214S81900240400100-840149515.917.3 Table III Exam pleSteelATQTPTGlPtHEYSTSUETE°C°C°C°Csec%MPaMPa%%15S180920240400460100241127131013.720.716S18190020040046010-933.4134814.018.017S18190020040046030-1170142513.820.118S181900180400460100-135315078.014.119S181900200400460100191202139913.020.220S181900220400460100-936128014.318.021S18190020042046010-906134611.210.622S18190020042046030-841129814.719.323S181900200420460100-900132214.519.124S18190020036046010-910135714.519.025S18190020036046030-992135614.018.926S8090022040046010-756157610.511.127S8090022040046030-836154318.320.328S8090022040046050-906153418.621.629S80900220400460100-94113948.18.5830S8190024040046010-92516589.49.3931S8190024040046030-929160315.120.532S8190024040046050-897155416.121.133S81900240400460100-948154218.121.4

[0039] The examples 1 to 14 show that it is only with the steel S181, which contains neither chromium nor molybdenum, and steel S80, which contains both chromium and molybdenum, that it is possible to reach the desired properties i.e. TS ≥ 1470 MPa and TE ≥ 19%. In alloy S181, the desired properties are achieved for a quenching temperature QT of 200°C and a partitioning time of 100 seconds. In this case, the yield strength is higher than 1150 MPa. In alloy S80, which contains chromium and molybdenum, the desired properties are achieved for a quenching temperature QT of 220°C and a partitioning time between 30 to 100 seconds (examples 7 to 10). In this case, the tensile strength is higher than 1520 MPa and the total elongation is more than 20%. Moreover, it is worth mentioning that all the examples containing Cr and Mo (7 to 14) have yield strengths significantly lower than the examples 1 to 6, concerning a steel without Cr and Mo.

[0040] The examples 15 to 33 show that only the examples corresponding to steels containing Cr and Mo are able to reach the desired properties when the sheets are galvanized (examples 27 and 28). For the steel S80, the quenching temperature has to be of 220°C and a partitioning of 10 seconds is too short while a partitioning time of 100 seconds is too long. When the steel does not contain Cr and does not contain Mo, the tensile strength always remains lower than 1470 MPa.

[0041] Other sheets made of an alloy having the composition shown in Table IV were cold rolled, annealed, quenched, partitioned, galvanized or galvannealed and cooled to the room temperature. Table IVC %Mn %Si %Cr %Mo %Al %NbAc3 °CMs °C0.381.981.930.510.0030.0480.039825290

[0042] The mechanical properties of the sheets were measured in the transverse direction relative to the direction of rolling. As it is well known in the art, the ductility level is slightly better in the direction of rolling than in the transverse direction for such high strength steel. Measured properties are the Hole Expansion Ratio HER measured according to the standard ISO 16630:2009, the yield strength YS, the tensile strength TS, the uniform elongation UE and the total elongation TE.

[0043] The conditions of treatment and the mechanical properties of the galvanized sheets are reported in Table V.

[0044] In this table, Gl is the temperature of galvanizing. Table VExampleATQTPTPtGlHEYSTSUETE°C°C°Csec°C%MPaMPa%%3490020540030460-103216241415.73590020540050460-1102160616.119.836900205400150460-1139159415.320.937900205400230460-1179160615.219.2

[0045] Examples 35 to 37 show that with a steel comprising higher amounts of chromium and niobium and a lower amount of molybdenum, the desired properties, i.e. TS ≥ 1470 MPa and TE ≥ 19%, can be reached with a partitioning time of more than 30s, in particular of at least 50s.

[0046] The conditions of treatment and the mechanical properties of the galvannealed sheets are reported in Table VI.

[0047] In this table, TGA is the alloying temperature and t GA is the holding time at this alloying temperature TGA. ExampleATQTPTPtTGAt GA HEYSTSUETE°C°C°Csec°Csec%MPaMPa%%389002054003050020-85015899.812.8399002054005050020-858156312.1124090020540010050020-881153413.415.7419002054005050010-1062154814.716.54290020540010050010-990156114.316.54390020540015050010-998158112.714.3449002054005048010-1035160314.417.9

[0048] Examples 38-44 show that a partitioning time Pt between 40 seconds and 120 seconds, in particular between 50 and 100 seconds, allow obtaining a galvannealed steel sheet having a tensile strength higher than 1510 MPa and a total elongation of at least 16%.

[0049] In particular, example 44 show that an alloying temperature of 480°C and a holding time at the alloying temperature of 10s even allow obtaining a tensile strength of more than 1510 MPa and a total elongation of more than 16%, even more than 17%.

Claims

1. A method for producing a cold rolled steel sheet having a tensile strength TS of at least 1470 MPa and a total elongation TE of at least 19%, the mechanical properties being measured in the transverse direction relative to the direction of rolling, the method comprising the successive steps of: - annealing at an annealing temperature AT a cold rolled steel sheet made of steel whose chemical composition contains in weight %: 0.34 % ≤ C ≤ 0.40 % 1.50 % ≤ Mn ≤ 2.30 % 1.50 ≤ Si ≤ 2.40% 0.35 % ≤ Cr ≤ 0.45% 0.07 % ≤ Mo ≤ 0.20 % 0.01 % ≤ Al ≤ 0.08 %, and optionally 0% ≤ Nb ≤ 0.05%, the remainder being Fe and unavoidable impurities, Ni, Cu, V, Ti, B, S, P and N being unavoidable impurities with contents of less than 0.05% for Ni, 0.05% for Cu, 0.007% for V, 0.001 % for B, 0.005% for S, 0.02% for P and 0.010% for N, the annealing temperature AT being higher than the Ac3 transformation point of the steel, - quenching the annealed steel sheet by cooling it down to a quenching temperature QT lower than the Ms transformation point of the steel and between 200 °C and 230 °C, during quenching, the annealed steel sheet being cooled down to said quenching temperature at a cooling rate fast enough to avoid ferrite formation upon cooling, in order to obtain a quenched steel sheet having a structure consisting of martensite and austenite, and, - making a partitioning treatment by reheating the quenched steel sheet at a partitioning temperature PT between 350°C and 450°C and maintaining the steel sheet at this temperature during a partitioning time Pt between 25 seconds and 55 seconds, - after partitioning, the steel sheet is galvanized then cooled to the room temperature in order to obtain a coated steel sheet.

2. The method according to claim 1, wherein the annealing temperature AT is between 870°C and 930 °C.

3. The method according to any one of claims 1 or 2, wherein the composition of the steel is such that 0.34 % ≤ C ≤ 0.37%.

4. A method for producing a cold rolled steel sheet having a tensile strength TS of at least 1470 MPa and a total elongation TE of at least 19%, the mechanical properties being measured in the transverse direction relative to the direction of rolling, the method comprising the successive steps of: - annealing at an annealing temperature AT a cold rolled steel sheet made of steel whose chemical composition contains in weight %: 0.34 % ≤ C ≤ 0.40 % 1.50 % ≤ Mn ≤ 2.30 % 1.50 ≤ Si ≤ 2.40% 0.46 % ≤ Cr ≤ 0.7% 0 % ≤ Mo ≤ 0.3% 0.01 % ≤ Al ≤ 0.08 %, and 0.03% ≤ Nb ≤ 0.05%, the remainder being Fe and unavoidable impurities, Ni, Cu, V, Ti, B, S, P and N being unavoidable impurities with contents of less than 0.05% for Ni, 0.05% for Cu, 0.007% for V, 0.001% for B, 0.005% for S, 0.02% for P and 0.010% for N, the annealing temperature AT being higher than the Ac3 transformation point of the steel, - quenching the annealed steel sheet by cooling it down to a quenching temperature QT lower than the Ms transformation point of the steel and between 200 °C and 230 °C, during quenching, the annealed steel sheet being cooled down to said quenching temperature at a cooling rate fast enough to avoid ferrite formation upon cooling, in order to obtain a quenched steel sheet having a structure consisting of martensite and austenite, and, - making a partitioning treatment by reheating the quenched steel sheet at a partitioning temperature PT between 350°C and 450°C and maintaining the steel sheet at this temperature during a partitioning time Pt between 50 seconds and 250 seconds, - coating the steel sheet then cooling to the room temperature in order to obtain a coated steel sheet, the coating step being a galvanizing step.

5. The method according to claim 4, wherein the annealing temperature AT is between 870°C and 930 °C.

6. The method according to any one of claims 4 or 5, wherein the composition of the steel is such that 0% ≤ Mo ≤ 0.005%.

7. A coated steel sheet made of steel whose chemical composition comprises in weight %: 0.34 % ≤ C ≤ 0.40 % 1.50 % ≤ Mn ≤ 2.30 % 1.50 ≤ Si ≤ 2.40 % 0.35 % ≤ Cr ≤ 0.45 % 0.07 % ≤ Mo ≤ 0.20 % 0.01 % ≤ Al ≤ 0.08 % and optionally 0% ≤ Nb ≤ 0.05%, the remainder being Fe and unavoidable impurities, Ni, Cu, V, Ti, B, S, P and N being unavoidable impurities with contents of less than 0.05% for Ni, 0.05% for Cu, 0.007% for V, 0.001 % for B, 0.005% for S, 0.02% for P and 0.010% for N, the structure comprising at least 60 % of martensite and between 12 % and 15 % of residual austenite, the steel sheet being galvanized, the tensile strength being at least of 1510 MPa and the total elongation being at least 20% the mechanical properties being measured in the transverse direction relative to the direction of rolling.

8. A coated steel sheet made of steel whose chemical composition comprises in weight %: 0.34 % ≤ C ≤ 0.40 % 1.50 % ≤ Mn ≤ 2.30 % 1.50 ≤ Si ≤ 2.40 % 0.46 % ≤ Cr ≤ 0.7 % 0% ≤ Mo ≤ 0.005 % 0.01 % ≤ Al ≤ 0.08 % and 0.03% ≤ Nb ≤ 0.05%, the remainder being Fe and unavoidable impurities, Ni, Cu, V, Ti, B, S, P and N being unavoidable impurities with contents of less than 0.05% for Ni, 0.05% for Cu, 0.007% for V, 0.001% for B, 0.005% for S, 0.02% for P and 0.010% for N, the structure comprising at least 60 % of martensite and between 12 % and 15 % of residual austenite, the tensile strength being at least of 1470 MPa and the total elongation being at least 19%, the mechanical properties being measured in the transverse direction relative to the direction of rolling, at least one face of the sheet being galvanized.