Method of manufacturing a steel part

Abstract

The invention relates to a method for manufacturing a steel part comprising the following sequential steps: providing a steel sheet having the following composition comprising in weight percent C: 0.05% to 0.25%, Mn: 3.5% to 8%, Si: 0.1% to 2%, Al: 0.01% to 3%, S < 0.010%, P < 0.020%, N < 0.008%, and optionally one or more of the following elements in weight percent: Cr: 0% to 0.5%, Mo: 0% to 0.25%, the remainder of the composition being iron and unavoidable impurities resulting from the melting, and the following microstructure comprising in surface fraction 10% to 50% of retained austenite, 50% or more of the sum of ferrite, bainite and tempered martensite, less than 5% of fresh martensite, less than 2% of carbides and strictly more than 0.4% and strictly less than 0.7% of austenite in carbon [C] A content; cutting the steel sheet into a predetermined shape to obtain a steel blank; heating the steel blank to a temperature T of (Md30 - 150°C) to (Md30 - 50°C) 加温 ; stamping or shearing and forming the heat-treated steel blank at the temperature T 加温 to obtain a steel part.

Description

[0001] The present invention relates to a method for manufacturing a steel part from a steel sheet having a high hole expansion ratio during hot working.

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

[0003] The strength of the cut edge of a TRIP steel is highly dependent on the stability of the retained austenite. Indeed, when the part is cut, unstable austenite can destabilize into martensite, thus becoming a potential site for initiation of damage. To limit this effect, the steelmaking industry is constantly developing new high strength steels and methods to obtain steel parts having improved yield and tensile strength, good ductility and formability, more particularly good stretch flangeability.

[0004] The publication WO2017131052 discloses a hot workable high strength steel sheet having excellent hot workability and residual ductility after hot working. The annealed steel sheet has an elongation at a temperature of 150°C greater than 27%. To achieve such properties, the carbon content in austenite must be controlled to less than 0.4% by weight, which is a particular limitation. Indeed, to ensure this low carbon level in the retained austenite, the cooling of the annealed steel sheet must be controlled and performed in two steps: one step to cool up to 500°C with an average cooling rate of 50°C / s and a holding step at this temperature such as galvanizing, and one cooling step to cool from Ms to room temperature with an average cooling rate of not less than 10°C / s. Moreover, no information is given about the stretch flangeability, which is a key feature for manufacturing steel parts.

[0005] It is therefore an object of the present invention to solve the above-mentioned problems and to provide a method easy to process on a conventional process route to obtain a steel part from a steel having a high hole expansion ratio greater than or equal to 25% during hot working.

[0006] The object of the present invention is achieved by providing a method according to claim 1. The method can further comprise the features of any one of claims 2 to 9.

[0007] Hereinafter, the term "warm cutting" refers to the part of the process where the steel blank is heated before being punched or sheared.

[0008] Hereinafter, the term "room temperature" refers to a temperature of 20°C.

[0009] The composition of the steel according to the present invention will now be described, the contents being expressed in weight percent.

[0010] In the following, Ae1 represents the equilibrium transformation temperature below which austenite is completely unstable, Ae3 represents the equilibrium transformation temperature above which austenite is completely stable, and Ms represents the martensite start temperature, i.e. the temperature at which austenite starts to transform into martensite upon cooling. These temperatures can be calculated according to the formula based on the weight percentage of the corresponding element:

[0011] Ae1 = 670 + 15 * %Si - 13 * %Mn + 18 * %Al

[0012] Ae3 = 890 - 20 * %C + 20 * %Si - 30 * %Mn + 130 * %Al

[0013] Ms = 560 - (30 * %Mn + 13 * %Si - 15 * %Al + 12 * %Mo) - 600 * (1 - exp(-0,96 * %C))

[0014] According to the invention, the carbon content is comprised between 0.05% and 0.25%. Above 0.25%, the amount of carbon in austenite is higher than the targeted value, eliminating the positive effect of hot cutting. Moreover, the weldability of the steel sheet can be decreased. If the carbon content is lower than 0.05%, the fraction of retained austenite is not stable enough to obtain a sufficient elongation at room temperature. In a preferred embodiment of the invention, the carbon content is comprised between 0.05% and 0.2%. More preferably, the carbon content is comprised between 0.1% and 0.2%.

[0015] The manganese content is comprised between 3.5% and 8% to obtain a sufficient elongation as well as the stability of the austenite. Above 8%, the risk of centerline segregation increases, which is detrimental to the ductility of the steel sheet and steel parts. Below 3.5%, the final microstructure contains an insufficient fraction of retained austenite, making it impossible to achieve the desired ductility. Preferably, the manganese content is comprised between 3.5% and 7%. More preferably, the manganese content is comprised between 3.5% and 5%.

[0016] According to the invention, the silicon content is comprised between 0.1% and 2% to stabilize a sufficient amount of retained austenite. Above 2%, silicon oxides are formed at the surface, which impairs the coatability of the steel. In a preferred embodiment of the invention, the silicon content is comprised between 0.3% and 1.5%.

[0017] According to the invention, the aluminum content is comprised between 0.01% and 3% because aluminum is a very effective element for deoxidizing the steel in liquid phase during the processing and increasing the annealing process window. The aluminum content can be added up to a maximum of 3% to avoid the appearance of inclusions and to avoid oxidation problems.

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

[0019] Chromium can optionally be added up to 0.5%. Above 0.5%, a saturation effect is noticed and adding chromium is both useless and expensive.

[0020] Molybdenum can optionally be added up to 0.25% to increase toughness. Above 0.25%, adding molybdenum is expensive and inefficient considering the required properties.

[0021] The remainder of the composition of the steel is iron and impurities resulting from the smelting. In this respect, P, S and N are at least considered as residual elements, which are unavoidable impurities. Their content is lower than or equal to 0.010% for S, lower than or equal to 0.020% for P and lower than or equal to 0.008% for N.

[0022] The microstructure of the steel sheet according to the application will now be described. The steel sheet has a microstructure consisting of, in surface fraction: 10% to 50% of retained austenite, 50% or more of the sum of ferrite, bainite and tempered martensite, less than 5% of fresh martensite, less than 2% of carbides, strictly more than 0.4% and strictly less than 0.7% of carbon in austenite [C] A in weight percent and carbon in austenite [C] A such that Md30 is between 200°C and 350°C, Md30 being defined as

[0023] Md30(°C) = 551 - 462*([C] A + %N) - 9.2*%Si - 8.1*%Mn - 13.7*%Cr - 29*(%Ni + %Cu) - 18.5*(%Mo)

[0024] The microstructure of the steel sheet comprises 10% to 50% of retained austenite to ensure high ductility of the steel at room temperature.

[0025] The carbon content in austenite is strictly higher than 0.4% to ensure the stability of the austenite, the elongation at room temperature greater than 10% and to ensure that the steel part can reach the target reaming rate. Above 0.7%, the austenite is too stable and the hot cutting of the steel blank has no effect on the reaming rate. This carbon content is measured with XRD diffraction before the hot cutting.

[0026] The microstructure of the steel sheet comprises 50% or more of the sum of ferrite, bainite and tempered martensite. The ferrite is formed during the soaking of the steel sheet.

[0027] In a preferred embodiment of the present application wherein the provided steel sheet is a cold-rolled steel sheet which undergoes a cooling and partitioning process, tempered martensite is formed during the partitioning of the cold-rolled steel sheet. In a preferred embodiment of the present application wherein the provided steel sheet is a hot-rolled steel sheet, tempered martensite is self-tempered martensite formed during cooling above the Ms of the hot-rolled steel sheet.

[0028] If the sum of the ferrite, bainite and tempered martensite fractions is lower than 50%, the elongation at room temperature cannot reach 10%.

[0029] The microstructure of the steel sheet contains less than 5% of fresh martensite. Above 5%, fresh martensite decreases the toughness of the steel sheet. Fresh martensite is formed during the cooling of the steel sheet to room temperature. Furthermore, the microstructure of the steel sheet of the present application contains less than 2% of carbides.

[0030] The weight percentages of nitrogen %N, silicon %Si, manganese %Mn, chromium %Cr, nickel %Ni, copper %Cu, molybdenum %Mo and carbon in austenite [C] A such that Md30 is between 200°C and 350°C. This Md30 temperature corresponds to the temperature at which 50% of the residual austenite is transformed into martensite after 30% of deformation.

[0031] The steel part according to the present application can be produced by any suitable manufacturing method and one manufacturing method can be defined by the person skilled in the art. However, it is preferred to use a method according to the present application which comprises the following steps:

[0032] A steel sheet having the aforementioned composition and microstructure is provided and cut into a predetermined shape to obtain a steel blank.

[0033] The steel blank is then heated to a temperature T of (Md30-150°C) to (Md30-50°C) 加温 to obtain a heat-treated steel blank, and at said T 加温 temperature, stamping or shearing is performed, then forming at said T 加温 temperature to obtain a steel part. Above (Md30-50°C), the austenite is too stable to obtain an improvement of the hole expansion ratio. Below (Md30-150°C), the austenite is destabilized into martensite and becomes a potential site of damage initiation, leading to a low hole expansion ratio.

[0034] In a preferred embodiment of the present application, the steel sheet provided for the manufacture of a steel part is produced by the following sequential steps:

[0035] A steel ingot having the aforementioned composition is hot-rolled to obtain a hot-rolled steel sheet. The hot-rolled steel sheet is then coiled to a temperature T of 200°C to 700°C 卷取After coiling, the sheet can be pickled to remove oxides. The hot-rolled steel sheet is then annealed to an annealing temperature T HBA to obtain a hot-rolled and annealed steel sheet. This annealing causes the steel to soften and the austenite to be stabilized after the final annealing due to the high carbon and manganese concentration in the carbides or in the austenite.

[0036] The hot-rolled and annealed steel sheet is then cold-rolled to obtain a cold-rolled steel sheet. The cold-rolling reduction is preferably between 20% and 80%. Below 20%, recrystallization during the subsequent heat treatment is not favorable, which can impair the ductility of the steel sheet. Above 80%, there is a risk of edge cracking during cold-rolling.

[0037] The cold-rolled steel sheet is then heated to a temperature T 均热 and held at the soaking temperature T 均热 for a soaking time t 均热 less than 500 seconds to maintain a fine residual austenite grain size and thus achieve high strength and ductility.

[0038] The heat-treated steel sheet is then cooled to room temperature to obtain a steel sheet having the above-described microstructure.

[0039] In another preferred embodiment of the application, the steel sheet provided for the manufacture of a steel part is produced by the following sequential steps:

[0040] The steel ingot having the above-described composition is hot-rolled to obtain a hot-rolled steel sheet. The hot-rolled steel sheet is then coiled to a temperature T 卷取 After coiling, the sheet can be pickled to remove oxides. The hot-rolled steel sheet is then annealed to an annealing temperature T HBA to obtain a hot-rolled and annealed steel sheet. This annealing causes the steel to soften and the austenite to be stabilized during the final annealing due to the high carbon and manganese concentration in the carbides or in the austenite.

[0041] The hot-rolled and annealed steel sheet is then cold-rolled to obtain a cold-rolled steel sheet. The cold-rolling reduction is preferably between 20% and 80%. Below 20%, recrystallization during the subsequent heat treatment is not favorable, which can impair the ductility of the steel sheet. Above 80%, there is a risk of edge cracking during cold-rolling.

[0042] The cold-rolled steel sheet is then heated to a temperature T 均热 and held at the soaking temperature T 均热 for a soaking time t 均热To maintain a fine residual austenite grain size, and thus achieve high ductility.

[0043] The heat-treated steel plate is then cooled to a temperature T of 20°C to (Ms-50°C). Q And the partitioning temperature T for heating to 150°C to 550°C P At the distribution temperature T P The allocation time t is maintained from 1 second to 1800 seconds. P The heat-treated steel sheet is then cooled to room temperature to obtain a steel sheet with the aforementioned microstructure.

[0044] In another preferred embodiment, the steel sheet provided for manufacturing the steel component is produced by the following sequential steps:

[0045] The steel ingot having the above composition is hot-rolled to obtain a hot-rolled steel sheet. Then, the hot-rolled steel sheet is coiled at a temperature T of 200°C to 700°C. 卷取 Then cool to room temperature.

[0046] According to the present invention, heating to T 加温 HER porosity of heat-treated steel T加温 and the porosity HER of the steel at 20°C 20℃ Make (HER) T加温 -HER 20℃ ) / HER 20℃ Greater than or equal to 50%.

[0047] Preferably, T is heated to 150°C 加温 HER porosity of heat-treated steel 150℃ and the porosity HER of the steel at 20°C 20℃ Make (HER) 150℃ -HER 20℃ ) / HER 20℃ Greater than or equal to 50%.

[0048] HER is measured according to ISO 16630.

[0049] According to the present invention, the elongation E1 of the steel at room temperature is greater than or equal to 10%. E1 is measured according to ISO standard ISO 6892-1.

[0050] In a preferred embodiment of the invention, the steel HER 20℃ Greater than or equal to 10%. In another preferred embodiment of the invention, the HER of heat-treated steel... 150℃ Greater than or equal to 25%. Example

[0051] The compositions thereof are summarized in Table 1 in 3 grades cast as semi-products and processed into steel sheets.

[0052] Table 1 - Composition

[0053] The compositions tested are summarized in the following table, in which the elemental contents are expressed in weight percent.

[0054]

[0055] Steels A and B are according to the application, steel C is outside the application

[0056] Table 2 - Process parameters for the steel sheet

[0057] The cast steel semi-products are reheated at 1200°C, hot rolled and then coiled at 450°C. The hot rolled steel sheets are then heated to a temperature T HBA and held at said temperature for a time t HBA . The hot rolled and heat treated steel sheets are then cold rolled at a reduction of 50% and then heated to an annealing temperature T 均热 and held at said temperature for a time t 均热 . In tests 3 and 4, the heat treated steel sheets are quenched below Ms-50°C and then heated to a partitioning temperature T P and held at said T P temperature for a time t P .

[0058] The steel sheets are then cooled to room temperature. The following specific conditions are applied to obtain heat treated steel sheets:

[0059]

[0060] Underlined values: parameters for which the targeted properties cannot be obtained

[0061] The steel sheets are analyzed and the corresponding microstructures are summarized in Table 3.

[0062] Table 3 - Microstructure of the steel sheet

[0063] The microstructure of the steel sheets is determined:

[0064]

[0065] Underlined values: outside the application

[0066] [C] A Corresponds to the amount of carbon in the austenite in weight percent. It is measured by X-ray diffraction.

[0067] The surface fraction of the phases in the microstructure was determined by cutting the specimen from a steel plate, polishing it, and etching it with reagents known per se to expose the microstructure. The cross-section was then examined in secondary electron mode at a magnification greater than 5000x using a scanning electron microscope, such as a scanning electron microscope with a field emission gun (“FEG-SEM”), in secondary electron mode.

[0068] The surface fraction of ferrite was determined by SEM observation after etching with Nital or Picral / Nital reagents.

[0069] The volume fraction of retained austenite was determined by X-ray diffraction.

[0070] The type of martensite can be determined and quantified using scanning electron microscopy.

[0071] The percentage of carbides was determined by examining cross-sections of the plate with a scanning electron microscope (“FEG-SEM”) equipped with a field emission gun and by image analysis at magnifications greater than 15,000.

[0072] The steel plate was then cut to obtain steel billets. The mechanical properties of the steel billets were analyzed at room temperature (20°C), and are summarized in Table 4.

[0073] Then the steel billet is reheated to a temperature T of 150°C. 加温 Then in the T 加温 Stamping or shearing at temperature.

[0074] The mechanical properties of the heat-treated steel billets are analyzed and summarized in Table 4.

[0075] Table 4 - Mechanical properties of the steel billet

[0076]

[0077] Underlined values: outside of this invention

[0078] nd: Undetermined value

[0079] In experiments 1 through 3, the composition and manufacturing conditions corresponded to those of this invention. Therefore, the desired properties were obtained. This was in contrast to the porosity HER at room temperature. 20℃ Compared to HER, the porosity at 150℃ 150℃ The addition of this feature particularly highlights the effect of hot cutting of steel billets.

[0080] In test 4, the carbon content of the steel sheet was too high, resulting in a high carbon content in the austenite. This means that the austenite is stabilized, eliminating the effect of the hot cutting on the hole expansion ratio.

[0081] In test 5, the steel was annealed at a higher temperature compared to tests 1 and 2. As a result, a large amount of austenite with a low carbon content was formed inside and was therefore less stable than in tests 1 and 2. As a result, this austenite transformed into fresh martensite during cooling and hot cutting. This amount of fresh martensite resulted in an elongation of less than 10% at room temperature for the steel part.

Claims

1. A method for manufacturing steel components, comprising the following sequential steps: - Provide a steel plate having the following composition and the following microstructure: the composition contains, by weight percentage C: 0.05% to 0.25% Mn: 3.5% to 8% Si: 0.1% to 2% Al: 0.01% to 3% S≤0.010% P≤0.020% N≤0.008% The remaining portion of the composition consists of iron and unavoidable impurities produced during smelting, and the microstructure contains, on a surface fraction basis... - 10% to 50% retained austenite - 50% or more of the sum of ferrite, bainite and tempered martensite - Less than 5% fresh martensite - Less than 2% carbides - Strictly more than 0.4 wt% and strictly less than 0.7 wt% of carbon [C] in austenite. A content, Weight percentages of nitrogen (N), silicon (Si), manganese (Mn), chromium (Cr), nickel (Ni), copper (Cu), and molybdenum (Mo) and carbon [C] in austenite. A This allows Md30 to be maintained at temperatures between 200°C and 350°C. Md30 is defined as... - The steel plate is cut into a predetermined shape to obtain a steel billet. - Heat the steel billet to a temperature T between (Md30-150℃) and (Md30-50℃). 加温 To obtain heat-treated steel billets, - In the T 加温 The heat-treated steel billet is then stamped or sheared at a specific temperature. - In the T 加温 The heat-treated steel billet is shaped at a certain temperature to obtain a steel component.

2. The method for manufacturing steel components according to claim 1, wherein the composition further comprises, by weight percentage, one or more of the following elements: Cr: 0% to 0.5% Mo: 0% to 0.25%.

3. The method for manufacturing a steel component according to claim 1 or 2, wherein the steel plate is provided by the following sequential steps: - Hot rolling of a steel ingot having the composition according to claim 1 to obtain a hot-rolled steel sheet. - Winding temperature T between 200°C and 700°C 卷取 The hot-rolled steel plate is then coiled. - The hot-rolled steel sheet is annealed to an annealing temperature T of 500°C to 680°C. HBA To obtain hot-rolled and annealed steel sheets, - The hot-rolled and annealed steel sheet is cold-rolled to obtain a cold-rolled steel sheet. - The cold-rolled steel sheet is heated to a temperature T above or equal to 680°C but below temperature T1. 均热 The cold-rolled steel sheet was then subjected to a homogenization temperature T. 均热 The heat spread time t is kept shorter than 500 seconds. 均热 To obtain heat-treated steel sheets, T1 is a temperature above which more than 5% martensite forms after cooling. - Cool the heat-treated steel plate to room temperature.

4. The method for manufacturing a steel component according to claim 1 or 2, wherein the steel plate is provided by the following sequential steps: - Hot rolling of a steel ingot having the composition according to claim 1 to obtain a hot-rolled steel sheet. - Winding temperature T between 200°C and 700°C 卷取 The hot-rolled steel plate is then coiled. - The hot-rolled steel sheet is annealed to an annealing temperature T of 500°C to 680°C. HBA To obtain hot-rolled and annealed steel sheets, - The hot-rolled and annealed steel sheet is cold-rolled to obtain a cold-rolled steel sheet. - Heating the cold-rolled steel sheet to a temperature T above or equal to 780°C. 均热 The cold-rolled steel sheet was then subjected to a homogenization temperature T. 均热 The heat spread time t is kept shorter than 500 seconds. 均热 To obtain heat-treated steel sheets, - Cool the heat-treated steel plate to a temperature T of 20°C to (Ms-50°C). Q And the distribution temperature T for heating the heat-treated steel plate to 150°C to 550°C. P The steel plate is then placed at the distribution temperature T P The allocation time t is maintained from 1 second to 1800 seconds. P , - Cool the heat-treated steel plate to room temperature.

5. The method for manufacturing a steel component according to claim 1 or 2, wherein the steel plate is provided by the following sequential steps: - Hot rolling of a steel ingot having the composition according to claim 1 to obtain a hot-rolled steel sheet. - Winding temperature T between 200°C and 700°C 卷取 The hot-rolled steel plate is then coiled. - Cool the hot-rolled steel sheet to room temperature.

6. The method for manufacturing steel components according to claim 1 or 2, wherein the T 加温 The temperature ranges from 50°C to 250°C.

7. The method for manufacturing steel components according to claim 1 or 2, wherein the heat-treated steel is subjected to a T... 加温 HER of the expanded porosity T加温 and the porosity HER of the steel at 20°C 20℃ Make (HER T加温 -HER 20℃ ) / HER 20℃ ≥ 50%。 8. The method for manufacturing steel components according to claim 1 or 2, wherein the steel has an elongation E1 at 20°C greater than or equal to 10%.

9. The method for manufacturing steel components according to claim 1 or 2, wherein the HER of the steel is... 20℃ Greater than or equal to 10%.

10. The method for manufacturing steel components according to claim 1 or 2, wherein the heat-treated steel has a HER 150℃ Greater than or equal to 25%.

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