Cold rolled and heat-treated steel sheet and method of manufacturing the same

Abstract

NOT VISIBLE DUE TO STATUS OF PATENT

Description

[0001] Cold rolled and heat-treated steel sheet and method of manufacturing the same

[0002]

[0001] The present invention relates to a cold rolled and heat-treated steel sheet having a good resistance to hydrogen delayed fracture 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. One of 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. During heat treatments of such a steel sheet, hydrogen atoms present in the atmosphere of the furnace, can easily penetrate the steel, and can be absorbed. Indeed, hydrogen atoms may progressively diffuse and be trapped by the defects such as dislocations and grain boundaries.

[0004]

[0003] The introduction and diffusion of hydrogen in the steel sheet is one of the mechanism responsible of the brittleness of the steel sheet, which could lead for example, to cracks formation along grain boundaries and / or dislocations gliding planes. Because of hydrogen, steel strip can suffer ductility lost, also called hydrogen embrittlement or can show delayed fracture phenomena after part forming.

[0005]

[0004] The publication WO2023233036 relates to a high strength, cold rolled steel flat product with reduced sensitivity to hydrogen embrittlement, specially thanks to the presence of vanadium and fine V- precipitated particularly effective for trapping hydrogen.

[0006]

[0005] The purpose of the invention is to provide a steel sheet having a good resistance to hydrogen embrittlement combined with a good resistance to hydrogen delayed fracture. Preferably, the steel sheet presents no crack when submit to the U-bending SEP1970 standard test.

[0007]

[0006] The object of the present invention is achieved by providing a cold rolled and heat- treated steel sheet according to claim 1 . The steel sheet can also comprise characteristics of anyone of claims 2. Another object is achieved by providing the method according to claim 3. Another object of the invention is achieved by providing a method comprising characteristics of claims 4 to 5.

[0007] The invention will now be described in detail and illustrated by examples without introducing limitations.

[0008]

[0008] The composition of the steel according to the invention will now be described, the content being expressed in weight percent.

[0009]

[0009] The carbon content is from 0.30% and 0.45 %. If the carbon content is above 0.45%, the weldability of the steel sheet is insufficient, and the risk of delayed fracture is increased. If the carbon content is lower than 0.30%, the ductility of the steel sheet may be reduced. Preferably, the carbon content is from 0.35% to 0.45%.

[0010]

[0010] The manganese content is from 1.0% and 3.0% for ensuring a satisfactory strength and achieving stabilization of at least part of the austenite, to obtain a sufficient elongation. Above 3.0% of addition, the risk of central segregation at austenitic grain are increased, consequently the risk of fracture is increased in the presence of hydrogen. Below 1.0 %, the final structure comprises an insufficient retained austenite fraction, so that the ductility of the steel may be reduced. Preferably, the manganese content is from 1.0% to 2.0%, more preferably, from 1.0% to 1.9%, even more preferably from 1.0% to 1.8% or 1.1% to 1.8%.

[0011]

[0011] According to the invention, the silicon content is from 0.8% to 2.5%. A silicon addition of at least 0.8% helps to stabilize a sufficient amount of retained austenite. Above 2.0%, silicon oxides form at the surface, which impairs the coatability of the steel. Preferably, the silicon content is from 1.0% to 2.5%, more preferably from 1.2% to 2.5%, even more preferably from 1.5% to 2.5%, or 1.7% to 2.5%.

[0012]

[0012] Chromium is added in an amount of 0.1 % to 1.0%. Below 0.1%, the hardenability of the steel sheet may be reduced. A maximum of 1.0% of chromium is allowed, above a saturation effect is noted, and adding chromium is both useless and expensive. Higher chromium causes surface cleaning issues during pickling process and as a result, affects coatability of the steel. Preferably, the chromium content is from 0.1% to 0.8%, more preferably from 0.1% to 0.5%.

[0013]

[0013] Molybdenum is added, in a content comprised from 0.05% to 0.7%. Molybdenum helps to stabilize a sufficient amount of retained austenite. Below 0.05%, this stabilization may be reduced. Above 0.7%, the addition of molybdenum is costly and ineffective in view of the properties which are required. Preferably, the molybdenum content is from 0.05% to 0.5%, more preferably from 0.07% to 0.5%, even more preferably from 0.1 % to 0.5%.

[0014]

[0014] Aluminium is added in a content of 0.01% to 0.6%, as it is a very effective element for deoxidizing the steel in the liquid phase during elaboration. Moreover, aluminium improves weldability of the steel sheet. The aluminium content is lower than 0.6% to avoid the occurrence of inclusions and to avoid oxidation problems. Preferably, the aluminium content is from 0.01% to 0.4%, more preferably from 0.01% to 0.2%, even more preferably from 0.01% to 0.1%.

[0015]

[0015] Niobium is added in an amount comprised from 0.001 % to 0.1% to refine the austenite grains during hot-rolling and to provide precipitation strengthening. Above 0.1% of addition, the risk of formation of NbC or Nb(C,N) carbides increases, which can increase the risk of delayed fracture. Preferably, the niobium content is from 0.005% to 0.05%, more preferably from 0.008% to 0.05%, even more preferably from 0.01 % to 0.05%.

[0016]

[0016] Nickel is an important element of the invention and is added in an amount comprised from 0.05% to 0.7%. Nickel is added in substitution of a part of manganese in order to improve hydrogen embrittlement and reduce the risk of delayed fracture. Above 0.7%, a saturation effect is noted, and adding nickel is both useless and expensive. Preferably, the nickel content is from 0.1 % to 0.7%, more preferably from 0.2% to 0.7%, even more preferably from 0.3% to 0.7%, or from 0.3% to 0.6%.

[0017]

[0017] Preferably, the sum of manganese and nickel contents is from 2.1 % to 3.1%, more preferably from 2.1% to 2.5%, even more preferably from 2.1% to 2.3%.

[0018]

[0018] Optionally some elements can be added to the composition of the steel according to the invention.

[0019]

[0019] Titanium can be added optionally up to 0.06% to provide precipitation strengthening. Preferably a minimum of 0.005% of titanium is added in addition of boron to protect boron against the formation of BN.

[0020]

[0020] Boron content can be added up to 0.005%. The presence of boron can increase the toughness of the steel sheet. Moreover, boron improves weldability of the steel sheet. Above 0.005%, the formation of borocarbides at the prior austenite grain boundaries is promoted, making the steel more brittle. Preferably a minimum of 0.0003% of boron is added.

[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 scraps, 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 scraps, 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 scraps such as Antimony, Arsenic, Lead, Copper, and Tin each up to 0.03% which are considered as unavoidable impurities.

[0024]

[0024] P, S and N are also part of the unavoidable impurities whatever the process route. Their content is below or equal to 0.020 % for P, below or equal to 0.010 % for S, and below or equal to 0.008 % for N.

[0025]

[0025] The 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:

[0026]

[0026] A semi-product able to be further hot rolled, is provided with the steel composition described above. Such semi-product can for example be a slab.

[0027]

[0027] The semi product is obtained by casting liquid steel, which can be produced by a steelmaking process with or without the use of scraps.

[0028]

[0028] The semi product is heated to a temperature from 1150°C to 1350°C, so to make it possible to ease hot rolling. The steel sheet is then hot rolled at a finish hot rolling temperature from 750°C to 950°C. The hot-rolled steel is then cooled and coiled at a temperature lower than 670°C, and optionally pickled to remove oxidation.

[0029]

[0029] After the coiling, the steel sheet is heated up to a temperature TAI from 500°C to 800°C and maintaining at said TAI temperature for a holding time tAi from 1000s to 30h, in order to facilitate the cold rolling step.

[0030]

[0030] After this annealing, the sheet is cooled and cold rolled to obtain a cold rolled steel sheet having a thickness that can be, for example, from 0.7 mm and 3 mm, or even better in the range of 0.8 mm to 2 mm. The cold-rolling reduction ratio is preferably from 20% and 80%.

[0031]

[0031] The cold rolled steel sheet is then heated to an annealing temperature TA2 above or equal to Ac3, preferably from Ac3 to Ac3+150°C, and maintained at said TA2 temperature for a holding time tA2 from 30s to 600s, in a furnace having an atmosphere, which can comprise for example 5%vol. of H2 the rest being N2, or in a bath-salt furnace, in order to obtain a fully austenitic microstructure.

[0032]

[0032] The steel sheet is quenched to a temperature TQ below Ms, before being reheated to a temperature TP which is from 250°C to 500°C, and maintaining at said TP temperature for a holding time tpfrom 30s to 2000s. The steel sheet is then cooled to room temperature.

[0033] The steel sheet can then be heated to a temperature comprised from 400 and 550°C, to ensure that the sheet undergoes the same temperature variation as in a zinc bath. The steel sheet can optionally be heated to a temperature comprised from 400 and 550°C and coated with a zinc-based coating, in a bath at a temperature comprised from 400°C to 550°C.

[0033]

[0034] The cold rolled and heat-treated steel sheet is finally cooled to room temperature.

[0034]

[0035] In one embodiment of the invention, after being heated and held at TA2 temperature, the steel sheet is slow cooled from TA2 to a temperature Tsc which is from 600°C to Ac3- 10°C before being quenched.

[0035] Preferably Tsc is from 600°C to 800°C. Preferably the slow cooling is done with a cooling rate vsc from 0.5°C / s to 1°C / s.

[0036]

[0036] The steel sheet has a microstructure consisting of, in surface fraction, from 5% to 30% of retained austenite, the rest being partitioned martensite.

[0037]

[0037] Such austenite is formed during the heating of the steel sheet to TA2 temperature. During the quenching to TQ, a part of austenite is transformed into martensite. A last part of the austenite remains in the final microstructure. When present in surface fraction of at least 5%, retained austenite contributes to increase ductility and improves the resistance to delayed fracture. Preferably, the retained austenite is from 10% to 30%.

[0038]

[0038] Partitioned martensite comes from the martensite formed during the quenching, which is then partitioned during the partitioning step at the TP temperature.

[0039]

[0039] The steel sheet according to the invention has a good resistance to hydrogen embrittlement and delayed fracture, in particular, by satisfying the discriminant U-bending SEP1970 standard test.

[0040]

[0040] The invention will be now illustrated by the following examples, which are by no way limitative.

[0041] Example

[0042]

[0041] Nine grades, whose compositions are gathered in table 1, were cast in semiproducts and processed into steel sheets. The element contents are expressed in weight percent (%wt.)

[0043] Table 1 - Compositions

[0044] Steels A-D are according to the invention.

[0045] Steels E-l are references.

[0046] Ac3 and Ms are measured through Andrews Formulas:

[0047] Ac3=910-203*^%C-15.2*%Ni+44.7*%Si+104*%V+31.5*%Mo+13.1*%W-30*%Mn-1*%Cr-

[0048] 5 20*%Cu+700*%P+400*%AI+120* %As+400*%Ti

[0049] Ms=539-423*%C-30.4*%Mn-17.7*%Ni-12.1*%Cr-11*%Si-7.5*%Mo

[0050]

[0042] Steel semi-products, as cast, were reheated at a temperature TH, hot rolled with a 10 finish rolling temperature FRT and coiled at a temperature Tcoii. The steel sheets are annealed to a temperature TAI and maintained at said TAI temperature for a holding time tAi. The steel sheets are then cooled and cold rolled with a reduction rate R. The steel sheets are heated to a temperature TA2 and maintained at said temperature for a holding time tA2. The steel sheets are guenched from TA2to a TQ temperature, before being

[0051] 15 reheated to a partitioning temperature TP and maintained at said TP temperature for a holding time of tp.. Finally, the steel sheets are heated to 460°C, for a duration of 22s, to coat them with zinc, before being cooled to room temperature.

[0052]

[0043] The trials 3,4, 8 and 9 are slow cooled at a cooling rate vsc from TA2 to a temperature Tsc, before being guenched to TQ.

[0053]

[0044] The parameters are gathered in the Table 2. Table 2 - Process parameters

[0054] Underline values: not corresponding to the invention

[0055]

[0045] The cold rolled and heat-treated steel sheets were then analyzed, and the corresponding microstructure elements and properties were respectively gathered in tables 3 and 4. Table 3 - Microstructure of the cold rolled and heat-treated steel sheet

[0056]

[0046] The phase percentages of the microstructures of the cold rolled and heat-treated steel sheet were determined, in surface fraction:

[0057]

[0047] The surface fractions of phases in the microstructure are determined through the following method: a specimen is cut from the cold rolled and heat-treated steel sheet, and polished. The determination of the surface fraction of retained austenite is performed thanks to X-ray diffraction. The rest of the microstructure is partitioned martensite, which can be distinguished on a section polished and etched with a reagent known per se, for example Nital reagent, observed by Scanning Electron Microscopy (SEM) or on a section polished, analysed by Electron Backscatter Diffraction (EBSD).

[0058]

[0048] A section of the cold-rolled and heat-treated steel sheets are cut and bent according to U-bending SEP1970 standard test to evaluate hydrogen delayed fracture resistance of the steel. The tests are validated when no crack is formed in the sample after 720 hours.

[0059]

[0049] The diffusible hydrogen content Hdiff is measured thanks to TDA (Thermal Desorption Analysis) experiments at the end of the manufacturing process.

[0060]

[0050] The TDA set-up consists of a heating room in which a flat sample of 10 mm width and 50 mm length is heated in an infra-red furnace at a constant heating rate of 1200°C / h under a constant flow of pure nitrogen up to 900°C. Released hydrogen carried by nitrogen is detected by a quadrupole mass spectrometer.

[0061]

[0051] The Hdiff values and results to U-bending test are gathered in Table 4.

[0062] Table 4 - Properties of the cold rolled and heat-treated steel sheet

[0063]

[0052] The cold rolled and heat-treated steel sheet according to the invention, namely trials 1-4 have successfully validated the U-bending test, with at least 720 hours in which no cracks appear in the sample, even if the hydrogen diffusible content is high. The tests are stopped after 720h.

[0064]

[0053] On the contrary, trials 5 to 9 without nickel, are very sensitive to hydrogen, with the apparition of cracks in the samples, even with low hydrogen content as in trials 5 and 6.

[0065]

[0054] A substitution of manganese by nickel in trials 1 to 4, in comparison to trials 5-9, improves the resistance to delayed fracture and helps to validate the U-bending SEP1970 standard test.

Claims

CLAIMS1 . Cold-rolled and heat-treated steel sheet, made of a steel having a composition comprising, by weight percent:C : 0.30 - 0.45 %Mn : 1.0 - 3.0 %Si : 0.8 - 2.5 %Cr : 0.1 - 1.0 %Mo : 0.05 - 0.7 %Al : 0.01 - 0.6 %Nb 0.001 - 0.1 %Ni : 0.05-0.7%P < 0.020 %S < 0.010 %N < 0.008 % and comprising optionally one or more of the following elements, in weight percentage:Ti < 0.06 %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 surface fraction,- 5% to 30% of retained austenite,- the rest being partitioned martensite.

2. A cold rolled and heat-treated steel sheet according to claim 1 , wherein the manganese content is from 1.0% to 1.9%.

3. A method for manufacturing a cold rolled and heat-treated steel sheet, comprising the following successive steps: casting a steel to obtain a semi-product, said semi product having a composition according to any one of claims 1 and 2, heating the semi-product to a temperature THcomprised from 1150°C to 1350°C, hot rolling the semi-product with a finish hot rolling temperature from 750°C to 950°C to obtain a hot-rolled steel sheet, cooling the hot rolled steel sheet,coiling the hot rolled steel sheet at a coiling temperature Tcoii below 670°C , optionally pickling the steel sheet, heating the steel sheet up to a temperature TAI from 500°C to 800°C, and maintaining at said TAI temperature for a holding time tAi from 1000s to 30h to obtain a hot-rolled and annealed steel sheet, cooling the steel sheet, cold rolling the steel sheet, heating the steel sheet up to a temperature TA2 above or equal to Ac3 and maintaining at said TA2 temperature for a holding time tA2from 30s to 600s, quenching the steel sheet down to a temperature TQ below Ms, reheating the quenched steel sheet to a temperature TP from 250°C to 500°C, and maintaining at said TP temperature for a holding time tpfrom 30s to 2000s, cooling the cold rolled and heat-treated steel sheet.

4. A method for manufacturing a cold rolled and heat-treated steel sheet according to claim 3, wherein, after the cooling of said cold rolled and heat-treated steel sheet, the steel sheet is heated to a temperature which is from 400°C to 550°C and coated with a zinc-based coating.

5. A method for manufacturing a cold rolled and heat-treated steel sheet according to any one of claim 3 or 4, wherein, after being heated and held at TA2 temperature, the steel sheet is cooled from TA2 to a temperature Tsc which is from 600°C to Ac3-10°C, at a cooling rate vsc from 0.5 to 1°C / s before the quenching step to TQ.