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

Abstract

The invention deals with a cold-rolled and heat-treated steel sheet, made of a steel having a composition comprising, by weight percent: C : 0.03 - 0.25 %, Mn : 3.5 – 7.0 %, Si : 0.1 – 2.0 %, Al: 0.5 – 3.0%, Mo : 0.01 – 0.5 %, S ≤ 0.010 %, P ≤ 0.020 %, N ≤ 0.02 %, and comprising optionally one or more of the following elements, in weight percentage: B≤ 0.004 %, Ti ≤ 0.04 %, Nb ≤ 0.1 %, Cr ≤ 0.80 %, the remainder of the composition being iron and unavoidable impurities resulting from the smelting, said steel sheet having a microstructure comprising, in surface fraction, - from 5% to 50% of retained austenite, - the rest being ferrite.

Description

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

[0002] The present invention relates to a steel sheet having a good resistance to hydrogen absorption and to a method to obtain such steel sheet.

[0003] 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] 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.

[0005] One development made to improve mechanical properties is to increase manganese content in steels. The presence of manganese helps to increase ductility of steels thanks to the stabilization of austenite. But these medium manganese steels present weaknesses of brittleness. Moreover, during heat treatments of 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 inside the defects such as dislocations.

[0006] 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.

[0007] Because of hydrogen, steel strip can suffer ductility lost, also called hydrogen embrittlement.

[0008] The publication describes JP2008081788 relates to an ultra-high-strength thin steel sheet for punching, with improved hydrogen embrittlement resistance, an excellent delayed fracture resistance and a tensile strength higher than 1 180MPA. To obtain this improved hydrogen embrittlement resistance, the microstructure must be finely controlled to obtain an average grain size of blocks composed of lath-shaped ferrite of 20 pm or less and the lath width of the lath-shaped ferrite of 2 pm or less, the lath shaped ferrite being more than 80%.

[0009] The purpose of the invention therefore is to provide a steel sheet having a good resistance to hydrogen absorption after being annealed, without the constraint of a finely controlled microstructure and easily processable on conventional process route. The object of the present invention is achieved by providing a steel sheet according to claim 1 . Another object is achieved by providing the method according to claim 2. Another object of the invention is achieved by providing a method comprising characteristics of claims 3 and 4.

[0010] The invention will now be described in detail and illustrated by examples without introducing limitations.

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

[0012] The carbon content is comprised from 0.03% and 0.25 %. If the carbon content is above 0.25%, the weldability of the steel sheet is insufficient. If the carbon content is lower than 0.03%, the austenite fraction is not stabilized enough to obtain targeted properties. In a preferred embodiment of the invention, the carbon content is from 0.05% to 0.25%, more preferably 0.10% to 0.20%.

[0013] The manganese content is comprised from 3.5% and 7.0 %. Above 7.0% of addition, the risk of central segregation increases to the detriment of the toughness. Below 3.5 %, the final structure comprises an insufficient retained austenite fraction to obtain the desired properties.. Preferably, the manganese content is 3.5% to 6.5%, more preferably from 3.7% to 6.5% or 3.7% to 6.0%

[0014] According to the invention, the silicon content is comprised from 0.1% to 2.0%. A silicon addition of at least 0.1 % helps to stabilize a sufficient amount of retained austenite. Above 2.0%, silicon is detrimental for toughness. Moreover, silicon oxides form at the surface, which impairs the coatability of the steel. Preferably, the silicon content is from 0.1% to 1 .5%, more preferably from 0.1 % to 1 .0%, even more preferably from 0.1% to 0.8%.

[0015] Aluminium is added in a content of 0.5% to 3.0%, 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 3.0% to avoid the occurrence of inclusions and to avoid oxidation problems.

[0016] Preferably, the aluminium content is added in a content of 0.8% to 3.0%, more preferably in a content of 1 .0% to 3.0%.

[0017] Molybdenum is added, in a content comprised from 0.01% to 0.5%. Molybdenum stabilizes the austenite. Moreover, molybdenum addition reduces hydrogen absorption during the annealing, and also at room temperature. Above 0.5%, 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%.

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

[0019] Boron content can be added up to 0.004%. The presence of boron can increase the toughness of the steel sheet. Moreover, boron improves weldability of the steel sheet. Above 0.004%, 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.

[0020] Titanium can be added optionally up to 0.04% to provide precipitation strengthening. Preferably a minimum of 0.01% of titanium is added in addition of boron to protect boron against the formation of BN.

[0021] Niobium can be added up to 0.1% to refine the austenite grains during hot-rolling and to provide precipitation strengthening. Preferably, the minimum amount of niobium added is 0.0010%.

[0022] A maximum of 0.80% of chromium can be added. Above, a saturation effect is noted, and adding chromium is both useless and expensive.

[0023] 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, Lead, Copper, and Tin up to 0.03% which are considered as unavoidable impurities.

[0024] P, S and N are also part of the unavoidable impurities whatever the process 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 0.02 % for N.

[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] 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. The semi product is obtained by casting liquid steel, which can be produced by a steelmaking process using for example the Basic Oxygen Furnace (BOF) route. In BOF route, hot metal or pig iron obtained for example in a blast furnace or a smelting furnace, is decarburized to be turned into liquid steel. Optionally, ferrous scraps comprising elements such as chromium, copper, nickel, tin, arsenic, antimony, or lead are loaded in the furnace together with this hot metal or pig iron. Direct Reduced Iron (DRI) may also be charged.

[0027] The liquid steel can also be produced in an Electric Arc Furnace (EAF) by melting ferrous scraps to directly produce liquid steel. DRI may also be charged together with ferrous scrap in the EAF.

[0028] The semi product is heated to a temperature from 1 150°C to 1300°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] After the coiling, the sheet is 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 comprised from 20% and 80%.

[0030] The cold rolled steel sheet is then heated to an annealing temperature TAfrom Ac1 to Ac3, in a furnace having an atmosphere, which can comprise for example 5%vol. of H2 the rest being N2, 100%voL of H2 or 100%voL N2. The steel sheet is maintained at said TAtemperature for a holding time tAfrom 10s to 180000s, so to obtain, upon annealing, a microstructure comprising ferrite and austenite.

[0031] The steel sheet is then cooled to room temperature (RT).

[0032] In a first preferred embodiment of the invention, the cooling to room temperature is performed by quenching the steel sheet from TAto room temperature.

[0033] In a second preferred embodiment of the invention, the cooling to room temperature is performed by quenching the steel sheet from TAto a temperature TQcomprised from 300°C to 500°C. The steel sheet is then maintained at said TQtemperature for a holding time to of 50s to 1000s, before being cooled to room temperature.

[0034] The steel sheet has a microstructure consisting of, in surface fraction, from 5% to 50% of retained austenite, the rest being ferrite.

[0035] Such austenite is formed during the heating of the steel sheet to TAtemperature. In the first preferred embodiment of the invention, during the quenching to room temperature, a part of austenite is transformed into ferrite. A last part of the austenite remains in the final microstructure. In the second preferred embodiment of the invention, during the quenching from TA to TQ and the maintaining to TQ, a part of austenite is transformed into ferrite. A last part of the austenite remains in the final microstructure.

[0036] Preferably, the microstructure of the steel sheet consists of, in surface fraction, from 10% to 40% of retained austenite, the rest being ferrite. More preferably, the microstructure of the steel sheet consists of, in surface fraction, from 15% to 40% of retained austenite, the rest being ferrite, even more preferably from 20% to 40% of retained austenite, the rest being ferrite. The steel sheet according to the invention has a good resistance to hydrogen absorption after being annealed. In particular, the steel sheet has a diffusible hydrogen content Hdiff below the diffusible hydrogen content of a steel sheet having a similar composition without molybdenum and manufactured by a similar process. The invention will be now illustrated by the following examples, which are by no way limitative.

[0037] Example 1

[0038] 2 grades, whose compositions are gathered in table 1 , were cast in semi-products and processed into steel sheets

[0039] Table 1 - Compositions

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

[0041] Ac1 and Ac3 have been determined through dilatometry tests and metallography analysis. Table 2 - Process parameters

[0042] Steel semi-products, as cast, were reheated at 1250°C, hot rolled with a finish rolling temperature FRT, coiled at a temperature TCOii, and cold rolled with a reduction rate of 45%. The cold rolled steel sheets are then heated in a furnace with an atmosphere comprising hydrogen, to a temperature TA, maintained at said temperature for a holding time tA, and quenched either to room temperature (RT) or to a TQtemperature of 400°C and maintained at said TQtemperature for a holding time of to, before being cooled to room temperature, through the specific conditions of Table 2.

[0043] Underline values: not corresponding to the invention

[0044] 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.

[0045] Table 3 - Microstructure of the cold rolled and heat-treated steel sheet

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

[0047] The determination of the volume fraction of retained austenite is performed thanks to X-ray diffraction. Table 4 - Properties of the cold rolled and heat-treated steel sheet

[0048] The diffusible hydrogen content Hdiff is measured thanks to TDA (Thermal Desorption Analysis) experiments at the end of the manufacturing process.

[0049] 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.

[0050] By comparing similar manufacturing processes (namely trial 1 with trial 2, trial 3 with trial 4 and trial 5 with trial 6), the steel sheets according to the invention, namely examples 2, 4 and 6, have a hydrogen content lower than the steel sheets made of steel A without molybdenum in their composition, corresponding respectively to examples 1 , 3 and 5. Preferably, the Hdiff content of the cold-rolled and heat-treated steel sheet annealed in an atmosphere comprising 100v% of H2 and measured after a quenching at room temperature is below or equal to 0.35 ppm.

[0051] Preferably, the Hdiff content of the cold-rolled and heat-treated steel sheet annealed in an atmosphere comprising 5v% of H2 and 95 v% of N2 and measured after a quenching at room temperature is below or equal to 0.10 ppm, more preferably below or equal to 0.08ppm.

[0052] Steel semi-products, as cast, were reheated at 1250°C, hot rolled with a finish rolling temperature FRT, coiled at a temperature TCOii, and cold rolled with a reduction rate of 45%. The cold rolled steel sheets are then heated in a furnace having an atmosphere of 100%voL of N2, to a temperature TA, maintained at said temperature for a holding time tA, and quenched either to room temperature (RT) or to a TQtemperature of 400°C, and maintained for a holding time of to, before being cooled to room temperature (RT), through the specific conditions of Table 5.

[0053] Table 5 - Process parameters

[0054] The steel sheets of examples 9 -12 have a microstructure consisting of 23% of retained austenite, the rest being ferrite.

[0055] In order to quantify the hydrogen intake at room temperature, after an annealing in an atmosphere of 100v% of N2, a cathodic hydrogen charging method is used. The cold rolled and heat-treated steel sheet is electrogalvanized with a zinc coating. Three sections of this coated steel sheet are cut and immersed in a solution comprising H2O + NaCI 3% or in a solution comprising H2O + NaCI 3% + 0.03% NH4SCN, in which a current density of 1 mA / cm2is applied for two hours.

[0056] These sections are then kept for 48h at room temperature.

[0057] The diffusible hydrogen contents are then measured by TDA after these 48h for each of the three sections, and the average Hdiff are gathered in Table 6 for the cathodic hydrogen charging method using H2O + NaCI 3% and in Table 7 for the method using H2O + NaCI 3% + 0.03% NH4SCN.

[0058] The reference tensile strength TS_ref is the average tensile strength measured of the specimen without being charged by the cathodic hydrogen charging method.

[0059] TS_H is the average tensile strength measured after the cathodic hydrogen charging method.

[0060] TS_ref and TS_H are measured according to standard ISO 6892-1 :2009. The Hydrogen embrittlement HE index is calculated as the ratio from (TS_ref - TS_H) and TS_ref.

[0061] Table 6 - Cathodic charging method with H2Q +NaCI 3% solution and mechanical properties

[0062] Trials with steel having molybdenum (10 and 12) have a combination of low hydrogen embrittlement HE index and low diffusible hydrogen content Hdiff, in comparison to trials 9 and 11 without molybdenum in which diffusible hydrogen is higher and associated to a higher HE index, as calculated after the cathodic charging method with NaCI 3%. Preferably, the HE index of the cold rolled and heat treated steel sheet annealed in an atmosphere of 100v% of N2 and measured after the cathodic charging with NaCI 3% is below or equal to 5%, more preferably below or equal to 4%.

[0063] Preferably, the diffusible hydrogen Hdiff measured after the cathodic charging with NaCI 3% of the steel sheet is below or equal to 0.13ppm.

[0064] Table 7 - Cathodic charging method with H2O + NaCI 3% + 0.03% NH4SCN solution and mechanical properties The contribution of molybdenum in the hydrogen embrittlement resistance is even more visible when using a solution of H2O + NaCI 3% + 0.03% NH4SCN: specimens with molybdenum (10 and 12) have a combination of low hydrogen embrittlement index HE index and low diffusible hydrogen content Hdiff, in comparison to specimens without molybdenum (9 and 1 1 ) in which diffusible hydrogen is higher, associated to a higher HE index.

[0065] Preferably, the HE index of the cold rolled and heat treated steel sheet annealed in an atmosphere of 100v% of N2 and calculated after the cathodic charging method with H2O + NaCI 3% + 0.03% NH4SCN of the steel sheet is below or equal to 10%, more preferably below or equal to 8%.

[0066] Preferably, the diffusible hydrogen of the steel sheet Hdiff measured after the cathodic charging method with NaCI 3% + 0.03% NH4SCN is below or equal to 0.50ppm, more preferably below or equal to 0.40ppm.

Claims

CLAIMS1 . Cold-rolled and heat-treated steel sheet, made of a steel having a composition comprising, by weight percent:C : 0.03 - 0.25 %Mn : 3.5 - 7.0 %Si : 0.1 - 2.0 %Al: 0.5 - 3.0%Mo : 0.01 - 0.5 %S < 0.010 %P < 0.020 %N < 0.02 % and comprising optionally one or more of the following elements, in weight percentage:B< 0.004 %Ti < 0.04 %Nb < 0.1 %Cr < 0.80 % the remainder of the composition being iron and unavoidable impurities resulting from the smelting, said steel sheet having a microstructure comprising, in surface fraction,- from 5% to 50% of retained austenite,- the rest being ferrite.

2. 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 claim 1 ,- heating the semi-product at a temperature Treheat comprised from 1150°C to 1300°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,- cold rolling the steel sheet,- heating the steel sheet up to a temperature TAfrom Ac1 to Ac3, and maintaining at said TA temperature for a holding time tAfrom 10s to 180000s, - cooling to room temperature.

3. A method for manufacturing a cold rolled and heat-treated steel sheet according to claim 2, wherein the cooling to room temperature is performed by quenching from TA to room temperature.

4. A method for manufacturing a cold rolled and heat-treated steel sheet according to claim 3, wherein the cooling to room temperature is performed by quenching from TA to a temperature TQcomprised from 300°C to 500°C and maintained at said temperature TQfor a holding time to of 50s to 1000s, before being cooled to room temperature.

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