MANUFACTURING PROCESS OF A STEEL PART

Abstract

steel parts manufacturing process. The present invention relates to a process for manufacturing steel parts, comprising the following successive steps: supplying a steel sheet with a 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 comprising one or more of the following elements, in weight percent: cr: 0 to 0.5%, mo: 0 to 0.25% and the remainder of the composition consisting of iron and unavoidable impurities resulting from casting, with a microstructure comprising, in surface fraction, 10% to 50% retained austenite, 50% or more of the sum of ferrite, bainite and tempered martensite, less than 5% fresh martensite, less of 2% carbides and carbon content [c]a in austenite of strictly more than 0.4% and strictly less than 0.7%, cutting the aforementioned steel sheet into a previously determined shape to obtain a steel mold,Heating the steel mold to a hot temperature between (md30–150 °C) and (md30–50 °C), drilling or shearing and forming the hot-treated steel mold at the aforementioned hot temperature to obtain a steel part.

Description

[001] The present invention relates to a method of manufacturing steel parts from steel sheets with a high hole expansion ratio during hot working. Background of the Invention

[002] To manufacture a range of items, such as structural body parts and body panels for automotive vehicles, sheets made of DP (Dual Phase) steels or TRIP (Transformation Induced Plasticity) steels are known to be used.

[003] The shear strength of TRIP steels is highly dependent on the stability of the residual austenite. In fact, unstable austenite can be destabilized into martensite when the part is sheared, thus becoming a potential site of damage initiation. To limit this effect, new high-strength steels and methods are continuously being developed by the steel industry to obtain steel parts with improved yield and tensile strength, good malleability and formability, and, more specifically, good draw flangability.

[004] Patent WO 2017 / 131052 describes a high-strength, hot-workable steel sheet with superior hot-workability and residual malleability after hot working. The elongation of this annealed steel sheet at a temperature of 150 °C is more than 27%. To achieve this property, the carbon content in austenite needs to be controlled below 0.4% by weight, which is a specific constraint. In fact, to ensure this low level of retained austenite carbon, the cooling of the annealed steel sheet needs to be controlled and carried out in two stages: cooling to 500 °C at an average cooling rate of 50 °C / s, with a holding stage at this temperature, for example, galvanizing, and a stage of Petition 870260023783, dated 03 / 13 / 2026, page 16 / 50 2 / 12 Cooling of Ms to ambient temperature at an average cooling rate of not less than 10 °C / s. Furthermore, no information is provided regarding stretch flangability, which is a fundamental characteristic for the manufacture of steel parts.

[005] The purpose of the present invention, therefore, is to solve the problem mentioned above and to provide a method that is easily processable in conventional processing ways to obtain a steel part from steel with a high hole expansion ratio, greater than or equal to 25% during hot working. Description of the Invention

[006] The object of the present invention is achieved by providing a method according to claim 1. The method may also comprise features according to any of claims 2 to 9.

[007] Hereafter, the term “hot cutting” refers to the part of the process in which the steel mold is heated before being pierced or sheared.

[008] Hereafter, the expression “room temperature” refers to a temperature of 20 °C.

[009] The composition of the steel according to the present invention will now be described and its content is expressed as a percentage by weight.

[0010] Ae1 indicates the equilibrium transformation temperature below which austenite is completely unstable, Ae3 indicates the equilibrium transformation temperature above which austenite is completely stable, and Ms indicates the martensite onset temperature, i.e., the temperature at which austenite begins to transform into martensite upon cooling. These temperatures can be calculated from a formula based on the weight percentage of the corresponding elements: Ae1 = 670 + 15*%Si - 13*%Mn + 18*%Al Ae3 = 890 - 20*\%C + 20*%Si - 30*%Mn + 130*%Al Petition 870260023783, dated 03 / 13 / 2026, p. 17 / 50 3 / 12 Ms = 560 - (30*%Mn + 13*%Si - 15*%Al + 12*%Mo) - 600 * (1exp(-0.96*%C)).

[0011] According to the present invention, the carbon content is from 0.05% to 0.25%. Above 0.25% carbon, the amount of carbon in austenite is greater than the target value, eliminating the positive effects of hot cutting. Furthermore, the weldability of the steel plate may be reduced. If the carbon content is less than 0.05%, the retained austenite fraction will not be sufficiently stabilized to obtain sufficient elongation at room temperature. In a preferred embodiment of the present invention, the carbon content is from 0.05% to 0.2%. More preferably, the carbon content is from 0.1% to 0.2%.

[0012] The manganese content is 3.5% to 8% to obtain sufficient elongation with austenite stabilization. Above 8% addition, the risk of central segregation increases, to the detriment of the malleability of the steel sheet and steel part. Below 3.5%, the final structure will comprise an insufficient retained austenite fraction, so the desired malleability is not achieved. Preferably, the manganese content is 3.5% to 7%. More preferably, the manganese content is 3.5% to 5%.

[0013] According to the present invention, the silicon content is from 0.1% to 2% to stabilize a sufficient amount of retained austenite. Above 2%, silicon oxides form on the surface, which impairs the coating capacity of the steel. In a preferred embodiment of the present invention, the silicon content is from 0.3% to 1.5%.

[0014] According to the present invention, the aluminum content is from 0.01% to 3%, since aluminum is a very effective element for deoxidizing steel in the liquid phase during preparation and increasing the tempering process window. The aluminum content can be added up to a maximum of 3% to avoid the occurrence of inclusions and oxidation problems.

[0015] Optionally, some elements can be added to Petition 870260023783, dated 03 / 13 / 2026, page 18 / 50 4 / 12 steel composition according to the present invention.

[0016] Chromium can optionally be added up to 0.5%. Above 0.5%, the saturation effect is noticeable and adding chromium is useless and expensive.

[0017] Molybdenum may optionally be added up to 0.25% to increase hardness. Above 0.25%, the addition of molybdenum is expensive and ineffective, given the properties that are required.

[0018] The remainder of the steel composition consists of iron and impurities resulting from smelting. In this particular case, P, S, and N, at least, are considered residual elements, which are unavoidable impurities. Their content is less than or equal to 0.010% for S, less than or equal to 0.020% for P, and less than or equal to 0.008% for N.

[0019] The microstructure of the steel plate according to the present invention will now be described. The steel plate has a microstructure consisting, on a surface fraction, of 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, a carbon [C]a content in austenite of strictly more than 0.4% and strictly less than 0.7%, and with a weight percentage 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 comprised of 200 °C to 350 °C, where Md is defined as: Md30 (°C) = 551 - 462*([C]a + %N) - 9.2*%Si - 8.1*%Mn 13.7*%Cr - 29*(%Ni+%Cu) - 18.5*(%Mo).

[0020] The microstructure of the steel sheet comprises 10% to 50% retained austenite, to ensure high malleability of the steel at room temperature.

[0021] The carbon content in austenite is strictly more than 0.4% to ensure austenite stability, elongation of more than 10% at room temperature, and to ensure that the steel part can achieve the desired hole expansion ratio. Above 0.7%, the austenite is stabilized. Petition 870260023783, dated 03 / 13 / 2026, page 19 / 50 5 / 12 more and hot cutting of the steel mold has no effect on the hole expansion ratio. This carbon content is measured before hot cutting, using XRD diffraction.

[0022] The microstructure of the steel sheet comprises 50% or more of the sum of ferrite, bainite and tempered martensite. Ferrite is formed during the immersion of the steel sheet.

[0023] In the preferred embodiment of the present invention, wherein the supplied steel sheet is a cold-rolled steel sheet that undergoes a cooling and partitioning process, tempered martensite is formed during the partitioning of the cold-rolled steel sheet. In the preferred embodiment of the present invention, wherein the supplied steel sheet is a hot-rolled steel sheet, the tempered martensite is self-tempered martensite, which is formed during cooling above Ms of the hot-rolled steel sheet.

[0024] If the sum of the fraction of tempered ferrite, bainite and martensite is less than 50%, the elongation does not reach 10% at room temperature.

[0025] The microstructure of the steel plate comprises less than 5% fresh martensite. Above 5%, fresh martensite reduces the hardness of the steel plate. Fresh martensite is formed during cooling to room temperature of the steel plate. Furthermore, the microstructure of the steel plate according to the present invention comprises less than 2% carbides.

[0026] The weight percentage of nitrogen %N, silicon %Si, manganese %Mn, chromium %Cr, nickel %Ni, copper %Cu, molybdenum %Mo and carbon in austenite [C]a is suitable for Md30 to be from 200 °C to 350 °C. This Md30 temperature corresponds to the temperature from which 50% of the retained austenite is transformed into martensite after 30% deformation.

[0027] The steel part according to the present invention can be produced by any suitable manufacturing method to be defined by those skilled in the art. However, the method according to [the invention] is preferred. Petition 870260023783, dated 03 / 13 / 2026, page 20 / 50 6 / 12 with the present invention comprising the following steps.

[0028] A steel sheet with the composition and microstructure mentioned above is supplied and cut into a predetermined shape to obtain a steel mold.

[0029] The steel mold is then heated to a Thot temperature of (Md30-150 °C) to (Md30-50 °C) to obtain a heat-treated steel mold, with punching or shearing at the aforementioned Thot temperature, before being molded at the aforementioned Thot temperature to obtain a steel part. Above (Md30-50 °C), the austenite is too stable to obtain an improved hole expansion ratio. Below (Md30-150 °C), austenite is destabilized into martensite and becomes a potential site for damage initiation, resulting in a low hole expansion ratio.

[0030] In a preferred embodiment of the present invention, the steel sheet supplied for manufacturing the steel part is provided by the following successive step.

[0031] A steel plate with the composition described above is hot-rolled to obtain a hot-rolled steel plate. The hot-rolled steel plate is then coiled at a coil temperature of 200 °C to 700 °C. After coiling, the plate may be pickled to remove oxidation. The hot-rolled steel plate is then annealed at a tempering temperature of 500 °C to 680 °C to obtain an annealed and hot-rolled steel plate. This tempering generates steel smoothness and austenite stability after final rolling, thanks to the concentration of carbon and manganese in carbides or austenite.

[0032] Annealed and hot-rolled steel sheet is then cold-rolled to obtain a cold-rolled steel sheet. The reduction ratio of cold rolling is preferably from 20% to 80%. Ratios below 20% do not favor recrystallization during subsequent heat treatment, which can impair the malleability of the steel sheet. Above 80%, there is a risk of edge cracking during cold rolling. Petition 870260023783, dated 03 / 13 / 2026, p. 21 / 50 7 / 12

[0033] The cold-rolled steel sheet is then heated to a Timersão temperature greater than or equal to 680 °C and below the Ti temperature, where Ti is the temperature above which more than 5% of martensite is formed after cooling, and held at the aforementioned Timersão immersion temperature for a Timersão immersion time of less than 500 s, in order to maintain the fine retained austenite grain size and, consequently, high strength and malleability.

[0034] The heat-treated steel sheet is then cooled to room temperature to obtain a steel sheet with the microstructure described above.

[0035] In another preferred embodiment of the present invention, the steel sheet supplied for manufacturing the steel part is produced by the following successive step.

[0036] A steel plate with the composition described above is hot-rolled to obtain a hot-rolled steel plate. The hot-rolled steel plate is then coiled at a coil temperature of 200 °C to 700 °C. After coiling, the plate may be pickled to remove oxidation. The hot-rolled steel plate is then annealed at a tempering temperature of 500 °C to 680 °C to obtain an annealed and hot-rolled steel plate. This tempering softens the steel and helps stabilize the austenite during the final tempering, thanks to the high concentration of carbon and manganese in carbides or austenite.

[0037] Annealed and hot-rolled steel sheet is then cold-rolled to obtain a cold-rolled steel sheet. The reduction ratio of cold rolling is preferably from 20% to 80%. Ratios below 20% do not favor recrystallization during subsequent heat treatment, which can impair the malleability of the steel sheet. Above 80%, there is a risk of edge cracking during cold rolling.

[0038] The cold-rolled steel sheet is then heated to a Timersão temperature greater than or equal to 780 °C and held at the aforementioned Timersão immersion temperature for a Timersão immersion time of less than 500 s to maintain the low Petition 870260023783, dated 03 / 13 / 2026, p. 22 / 50 8 / 12 particle size of retained austenite and, consequently, high malleability.

[0039] The heat-treated steel sheet is then cooled to a temperature Tq of 20 °C at (Ms-50 °C), heated to a partition temperature Tp of 150 °C at 550 °C and held at the said partition temperature Tp for a partition time tp of 1 s 1800 s. The heat-treated steel sheet is then cooled to room temperature to obtain a steel sheet with the microstructure described above.

[0040] In another preferred embodiment, the steel sheet supplied for the manufacture of the steel part is produced by means of the following successive step.

[0041] A steel plate with the composition described above is hot-rolled to obtain a hot-rolled steel plate. The hot-rolled steel plate is then coiled at a coil temperature of 200 °C to 700 °C, before being cooled to ambient temperature.

[0042] According to the present invention, the hole expansion ratio of hot-treated steel HERThot heated to Thot and the hole expansion ratio of steel at 20 °C HER20°c are suitable so that (HERThot—HER20°c) / HER20°c is greater than or equal to 50%.

[0043] Preferably, the hole expansion ratio of the hot-treated steel HER150°C heated to Thot of 150°C and the hole expansion ratio of the steel at 20°C HER20°C are suitable so that (HER150°C HER20°C) / HER20°C is greater than or equal to 50%.

[0044] HER is measured according to ISO 16630.

[0045] According to the present invention, the steel has an elongation El at room temperature greater than or equal to 10%. El is measured in accordance with ISO 6892-1.

[0046] In a preferred embodiment of the present invention, the steel has a HER20°C greater than or equal to 10%. In another preferred embodiment of the present invention, the heat-treated steel has a HER150°C greater than or equal to 25%. Petition 870260023783, dated 03 / 13 / 2026, page 23 / 50 9 / 12 Examples

[0047] Three degrees, whose compositions are gathered in Table 1 shows that the materials were molded into semi-finished products and processed into steel sheets. Table 1 Compositions

[0048] The compositions tested are compiled in the following table, where the element contents are expressed as a percentage by weight: Steel C Mn Si Al Cr Mo SPN Ae1 (°C) Ae3 (°C) Ms (°C) A 0.11 4.78 0.46 1.58 0 0 0.001 0.014 0.003 653 955 374 B 0.18 3.8 1.2 0.3 0 0.2 0.0008 0.011 0.003 652 831 337 C 0.37 1.93 1.95 0.041 0.33 0.098 0.0019 0.01 0.0029 679 864 297 Steels A and B are in accordance with the present invention, and steel C is not in accordance with the present invention. Table 2 Process parameters for steel sheets

[0049] Semi-finished steel products were reheated to 1200 °C, hot-rolled and then coiled at 450 °C. Hot-rolled steel sheets are then heated to a temperature Thba of 500 °C to 680 °C and held at the mentioned temperature for the retention time Ihba. The hot-treated and hot-rolled steel sheet is then cold-rolled with a 50% reduction ratio, before being heated to the immersion temperature Timersão and held at the mentioned temperature for the retention time timersão. In tests 3 and 4, the hot-treated steel sheets are cooled under Ms-50 °C until they are heated to the partition temperature Tp and held at the mentioned temperature Tp for the retention time tp.

[0050] The steel plates are then cooled to ambient temperature. The following specific conditions were applied to obtain the heat-treated steel plates: Petition 870260023783, dated 03 / 13 / 2026, p. 24 / 50 10 / 12 Hot Band Tempering (HBA) Steel Tests Immersion Cooling Temperature (°C) Partition Thba (°C) tHBA (h) Timersion (°C) Timersion (s) Tp (°C) tP (s) 1 A 600 15 737 210 - - - 2 A 600 15 725 213 - - - 3 B 620 15 830 170 175 450 90 4 C 650 8 870 120 230 410 280 5 A 600 15 770 200 - - - Underlined values: parameters that do not allow obtaining the desired properties.

[0051] The steel plates were analyzed and the corresponding microstructures are compiled in Table 3. Table 3 Microstructure of steel sheet

[0052] The microstructure of the steel plate was determined: Tests Retained Austenite (%) Ferrite + Tempered Martensite + Bainite (%) Fresh Martensite (%) Carbides (%) [C]a (% by weight) Md30 (°C) 1 25.5 72.5 2 0 0.43 308 2 23 76 0 1 0.47 290 3 14 85 0 1 0.60 227 4 17.9 80.6 1 0.50 0.89 99 5 11 60 29 0 0.29 373 Underlined values: do not conform to the present invention.

[0053] [C]a corresponds to the amount of carbon in austenite, in weight percent. It is measured using X-ray diffraction.

[0054] The surface fractions of phases in the microstructure are determined by the following method: a sample is cut from the steel sheet, polished and etched with a reagent known intrinsically to reveal the microstructure. The section is then examined using a scanning electron microscope, for example, with a Field Emission Source Scanning Electron Microscope (“FEG-SEM”) at a magnification of more than 5000x, in secondary electron mode.

[0055] The determination of the ferrite surface fraction is carried out Petition 870260023783, dated 03 / 13 / 2026, page 25 / 50 11 / 12 thanks to SEM observations after recording with Nital or Picral / Nital reagent.

[0056] The determination of the retained austenite volume fraction is carried out using X-ray diffraction.

[0057] The type of martensite can be determined and quantified using a Scanning Electron Microscope.

[0058] The percentage of carbides is determined by examining a section of the plate using a Field Emission Scanning Electron Microscope (“FEG-SEM”) and analyzing images with magnifications greater than 15000x.

[0059] The steel sheets were then cut to obtain a steel mold. The steel molds were analyzed at room temperature (20 °C) and the corresponding mechanical properties are compiled in Table 4.

[0060] The steel molds were then reheated to a Thot temperature of 150 °C before being drilled or sheared at the aforementioned Thot temperature.

[0061] The hot-treated steel molds were analyzed and the corresponding mechanical properties are compiled in Table 4. Table 4

[0062] Mechanical properties of steel molds: Tests (HERThot-HER20°C) / HER20°C at Thot = 150°C Properties at room temperature Properties at Thot = 150°C El (%) HER20°C (%) HER150°C (%) 1 77% 14.6 15.8 28.02 2 84% 19 22.9 42.1 3 58% 13.3 26.7 42.1 4 3% 18.4 22.1 22.8 5 nd 9.1 11 nd Underlined values: not consistent with the present invention; and nd: value not determined.

[0063] In tests 1 to 3, the compositions and manufacturing conditions Petition 870260023783, dated 03 / 13 / 2026, p. 26 / 50 12 / 12 correspond to the present invention. The desired properties are therefore obtained. The effect of hot cutting of the steel mold is particularly highlighted by the increase in the hole expansion ratio HERi5o°ca at 150 °C compared to the hole expansion ratio HER2o°c at room temperature.

[0064] In test 4, the carbon content of the steel sheet is too high, which generates a high carbon content in austenite. This indicates that the austenite is stabilized, which eliminates the effect of hot cutting on the hole expansion ratio.

[0065] In test 5, the steel is annealed at a higher temperature compared to tests 1 and 2. This results in a high quantity of low-carbon austenite forming within the steel, making it less stable than in tests 1 and 2. This austenite is then transformed into fresh martensite during cooling and hot cutting. This quantity of fresh martensite generates an elongation of the steel part at room temperature below 10%.

Claims

1. MANUFACTURING PROCESS OF A STEEL PART, characterized by comprising the following successive steps: - supply of a steel sheet with a composition comprising, in 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% which optionally comprises one or more of the following elements, in weight percentage: Cr: 0 to 0.5% Mo: 0 to 0.25% and the remainder of the composition is composed of iron and unavoidable impurities resulting from casting, with a 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% fresh martensite; - less than 2% carbides; - carbon content ([C]a) in austenite of more than 0.4% by weight and less than 0.7% by weight;where the weight percentage of nitrogen (%N), silicon (%Si), manganese (%Mn), chromium (%Cr), nickel (%Ni), copper (%Cu), molybdenum (%Mo) and carbon in austenite ([C]a) are adequate for Md30 Petition 870260023783, dated 03 / 13 / 2026, page. 28 / 50 2 / 4 is from 200 °C to 350 °C, where Md30 is defined as: Md30 (°C) = 551 - 462*([C]a + %N) - 9.2*%Si - 8.1*%Mn 13.7*%Cr - 29*(%Ni+%Cu) - 18.5*(%Mo), where Md30 is the temperature at which 50% of the retained austenite is transformed into martensite after 30% deformation of the steel sheet; - cutting the steel sheet into a previously determined shape to obtain a steel mold; - heating the steel mold to a temperature Thot from (Md30-150 °C) to (Md30-50 °C) to obtain a heat-treated steel mold; - drilling or shearing the heat-treated steel mold at temperature Thot; e - forming a mold from heat-treated steel at the hot temperature T to obtain a steel part.

2. A MANUFACTURING PROCESS FOR A STEEL PART, according to claim 1, characterized in that the steel sheet is provided by the following successive steps: - hot rolling of a steel plate with the same composition as the steel sheet to obtain a hot-rolled steel sheet; - coiling of the hot-rolled steel sheet at a coiling temperature Tcobina from 200 °C to 700 °C; - tempering of the hot-rolled steel sheet at a tempering temperature Thba from 500 to 680 °C to obtain a tempered and hot-rolled steel sheet; - cold rolling of the tempered and hot-rolled steel sheet to obtain a cold-rolled steel sheet; - heating of cold-rolled steel sheet to a temperature greater than or equal to 680 °C and below a temperature Ti, where Ti is the temperature above which more than 5% of martensite is formed after Petition 870260023783, dated 03 / 13 / 2026, page.29 / 50 3 / 4 Cooling and holding of cold-rolled steel sheet at immersion temperature. Timer for an immersion time of less than 500 seconds to obtain a hot-treated steel sheet; and - cooling of the hot-treated steel sheet to ambient temperature.

3. A MANUFACTURING PROCESS FOR A STEEL PART, according to claim 1, characterized in that the steel sheet is provided by the following successive steps: - hot rolling of a steel plate with the same composition as the steel sheet to obtain a hot-rolled steel sheet; - coiling of the hot-rolled steel sheet at a coiling temperature Tcoil from 200 °C to 700 °C; - tempering of the hot-rolled steel sheet at a tempering temperature Thba from 500 to 680 °C to obtain a tempered and hot-rolled steel sheet; - cold rolling of the tempered and hot-rolled steel sheet to obtain a cold-rolled steel sheet; - Heating the cold-rolled steel sheet to a temperature greater than or equal to 780 °C and maintaining the cold-rolled steel sheet at the immersion temperature for an immersion time of less than 500 seconds to obtain a heat-treated steel sheet;- Cooling the heat-treated steel plate to a Tq temperature of 20 °C at (Ms-50 °C) and heating the heat-treated steel plate to a partition temperature Tp of 150 °C to 550 °C and maintaining the steel plate at the partition temperature Tp for a partition time tp of 1 second to 1800 seconds; and - Cooling the heat-treated steel plate to ambient temperature. Petition 870260023783, dated 03 / 13 / 2026, p. 30 / 50 4 / 4; 4. A process for manufacturing a steel part, according to claim 1, characterized by the hot temperature (Tt) being from 50 °C to 250 °C.

5. A PROCESS FOR MANUFACTURING A STEEL PART, according to any one of claims 1 to 4, characterized in that the hole expansion ratio of the steel sheet at temperature Thot (HERThot) and the hole expansion ratio of the steel sheet at 20 °C (HER20°C) are suitable such that: (HERThot-HER20°C) / HER20°C > 50%.

6. A process for manufacturing a steel part, according to any one of claims 1 to 5, characterized by the elongation El of the steel sheet at 20 °C being greater than or equal to 10%.

7. A process for manufacturing a steel part, according to any one of claims 1 to 6, characterized by the expansion of the hole in the steel sheet at 20 °C, HER20°C being greater than or equal to 10%.

8. A process for manufacturing a steel part, according to any one of claims 1 to 7, characterized by the expansion of the hole in the steel sheet at 150 °C, HER150°C being greater than or equal to 25%.