Steel part manufacturing method

Abstract

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

Description

[Technical Field]

[0001] The present invention relates to a method for producing steel parts from steel sheets that have a high hole expansion ratio during warm working. [Background technology]

[0002] It is known to use sheets made from DP (dual phase) steels or TRIP (transformation induced plasticity) steels to manufacture various articles such as automotive body structural members and body panel components.

[0003] The strength of the cut edge of TRIP steels depends heavily on the stability of the retained austenite. In fact, unstable austenite can destabilize to martensite when the part is cut, thus becoming a potential site for damage initiation. To limit this effect, the steelmaking industry is continually developing new high-strength steels and methods to obtain steel parts with improved yield and tensile strength, good ductility and formability, and more specifically, good stretch-flangeability.

[0004] WO2017131052 discloses a warm-workable high-strength steel sheet with excellent warm workability and residual ductility after warm working. The elongation of this annealed steel sheet at a temperature of 150°C is greater than 27%. To achieve such properties, the carbon content in the austenite must be controlled to less than 0.4% by weight, which is particularly restrictive. In fact, to ensure this low carbon level in the retained austenite, the cooling of the annealed steel sheet must be controlled and carried out in two steps: a single cooling to 500°C at an average cooling rate of 50°C / s, a holding step at this temperature (e.g., galvanizing), and a single cooling from Ms to room temperature at an average cooling rate of 10°C / s or more. Furthermore, no information is provided regarding stretch-flangeability, an important feature for manufacturing steel parts. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2017 / 131052 Summary of the Invention [Problem to be solved by the invention]

[0006] The object of the present invention is therefore to solve the above problems and to provide a method that can be easily processed on the route of conventional methods for obtaining steel parts from steels with a high hole expansion ratio of 25% or more during warm working. [Means for solving the problem]

[0007] The object of the present invention is achieved by providing a method as set forth in claim 1. The method may also include the features of any of claims 2-9. DETAILED DESCRIPTION OF THE INVENTION

[0008] Hereinafter, the term "warm cutting" refers to that part of the process where the steel blank is heated before being punched or sheared.

[0009] Hereinafter, the term "room temperature" refers to a temperature of 20°C.

[0010] The composition of the steel according to the invention will now be described, with the contents expressed in weight percent.

[0011] In the following, Ae1 refers to the equilibrium transformation temperature below which austenite becomes completely unstable, Ae3 refers to the equilibrium transformation temperature above which austenite becomes completely stable, and Ms refers to the martensite start temperature, i.e., the temperature at which austenite begins to transform to martensite on cooling. These temperatures can be calculated from formulas based on the weight percentages of the corresponding elements. Ae1=670+15 * %Si-13 * %Mn+18 * %Al Ae3=890-20 * √%C+20 * %Si-30* %Mn+130 * %Al Ms=560-(30 * %Mn+13 * %Si-15 * %Al+12 * %Mo)-600 * (1-exp(-0.96 * %C))

[0012] According to the present invention, the carbon content is 0.05% to 0.25%. At carbon levels exceeding 0.25%, the amount of carbon in austenite is higher than the target value, eliminating the beneficial effects of warm cutting. Furthermore, the weldability of the steel sheet may be reduced. At carbon levels below 0.05%, the residual austenite fraction is not stabilized to a degree that allows sufficient elongation at room temperature. In a preferred embodiment of the present invention, the carbon content is 0.05% to 0.2%. More preferably, the carbon content is 0.1% to 0.2%.

[0013] The manganese content is 3.5% to 8% to stabilize austenite and obtain sufficient elongation. At an addition of more than 8%, the risk of central segregation increases to the point of impairing the ductility of the steel sheet and steel parts. At an addition of less than 3.5%, the final structure contains an insufficient fraction of retained austenite, and as a result, the desired ductility is not achieved. Preferably, the manganese content is 3.5% to 7%. More preferably, the manganese content is 3.5% to 5%.

[0014] According to the present invention, the silicon content is 0.1% to 2% in order to stabilize a sufficient amount of retained austenite. If it exceeds 2%, silicon oxide will form on the surface, impairing the coatability of the steel. In a preferred embodiment of the present invention, the silicon content is 0.3% to 1.5%.

[0015] According to the present invention, aluminum is a very effective element for deoxidizing steel in the liquid phase during refining and increasing the window for the annealing process, so the aluminum content is 0.01% to 3%. The aluminum content can be added up to 3% to avoid the occurrence of inclusions and avoid oxidation problems.

[0016] Optionally, some elements can be added to the composition of the steel according to the invention.

[0017] Chromium can optionally be added up to 0.5%. Above 0.5%, a saturation effect is observed and the addition of chromium becomes unnecessary and expensive.

[0018] Molybdenum can optionally be added up to 0.25% to improve toughness. Above 0.25%, the addition of molybdenum becomes costly and ineffective given the properties required.

[0019] The remainder of the steel composition consists of iron and impurities resulting from smelting. In this respect, P, S and N are considered to be residual elements, at least unavoidable impurities. Their contents are not more than 0.010% for S, 0.020% for P and 0.008% for N.

[0020] Next, the microstructure of the steel sheet according to the present invention will be described. The steel sheet has, in terms of surface fraction, 10% to 50% retained austenite, 50% or more of the total of ferrite, bainite, and tempered martensite, less than 5% fresh martensite, less than 2% carbides, and carbon [C] in the austenite. A Contains strictly more than 0.4% and strictly less than 0.7% of the following: Nitrogen %N, Silicon %Si, Manganese %Mn, Chromium %Cr, Nickel %Ni, Copper %Cu, Molybdenum %Mo and Carbon [C] in austenite A The weight percent of is such that Md30 is between 200°C and 350°C, and Md30 has a microstructure defined as follows: Md30(℃)=551-462 * ([C] A +%N)-9.2 * %Si-8.1 * %Mn-13.7 * %Cr-29 * (%Ni+%Cu)-18.5 * (%Mo)

[0021] The microstructure of the steel sheet contains 10% to 50% retained austenite to ensure high ductility of the steel at room temperature.

[0022] A carbon content in the austenite strictly greater than 0.4% ensures austenite stability, greater than 10% elongation at room temperature, and ensures that the steel component can achieve the target hole expansion ratio. Above 0.7%, the austenite becomes too stable, and warm cutting of the steel blank does not affect the hole expansion ratio. This carbon content is measured by XRD diffraction before warm cutting.

[0023] The microstructure of the steel sheet contains a total of 50% or more ferrite, bainite, and tempered martensite. Ferrite is formed during the soaking of the steel sheet.

[0024] In a preferred embodiment of the present invention, the provided steel sheet is a cold-rolled steel sheet that has been subjected to a cooling and distributing process, and the tempered martensite is formed during distributing of the cold-rolled steel sheet. In a preferred embodiment of the present invention, the provided steel sheet is a hot-rolled steel sheet, and the tempered martensite is self-tempered martensite, which is formed during cooling of the hot-rolled steel sheet above Ms.

[0025] If the total fraction of ferrite, bainite and tempered martensite is less than 50%, the elongation at room temperature does not reach 10%.

[0026] The microstructure of the steel plate contains less than 5% fresh martensite. If the content exceeds 5%, the fresh martensite reduces the toughness of the steel plate. Fresh martensite forms during cooling of the steel plate to room temperature.

[0027] Additionally, the microstructure of the steel sheet of the present invention contains less than 2% carbides.

[0028] Nitrogen %N, silicon %Si, manganese %Mn, chromium %Cr, nickel %Ni, copper %Cu, molybdenum %Mo and carbon [C] in austenite AThe weight percentage of is such that Md30 is between 200°C and 350°C. This Md30 temperature corresponds to the temperature at which 50% of the retained austenite is transformed into martensite after a deformation of 30%.

[0029] The steel part according to the invention can be manufactured by any suitable manufacturing method, which can be defined by a person skilled in the art. However, it is preferable to use a method according to the invention, which comprises the following steps:

[0030] A steel sheet having the above composition and microstructure is provided and cut into a predetermined shape to obtain a steel blank.

[0031] The steel blank is then heated to a temperature T warm to obtain a heat-treated steel blank, warm After punching or shearing at temperature, the T warm Steel parts are formed at temperatures above (Md30-50°C), where the austenite is too stable to achieve an improved hole expansion ratio. Below (Md30-150°C), the austenite becomes unstable in the martensite, becoming a potential site for damage initiation and resulting in a low hole expansion ratio.

[0032] In a preferred embodiment of the present invention, the steel sheet provided for manufacturing the steel part is manufactured by the following sequence of steps:

[0033] The steel slab having the above composition is hot-rolled to obtain a hot-rolled steel sheet. Then, the hot-rolled steel sheet is heated to a temperature T coil After winding, the sheet can be pickled to remove oxidation.

[0034] Next, the hot-rolled steel sheet is annealed at an annealing temperature T HBA The hot rolled annealed steel sheet is then annealed to a temperature of 1000°C to obtain a hot rolled annealed steel sheet, which, due to the carbon and manganese concentrations in the carbides or austenite, results in softening of the steel and stability of the austenite after the final annealing.

[0035] Next, the hot-rolled annealed steel sheet is cold-rolled to obtain a cold-rolled steel sheet. The cold-rolling reduction is preferably between 20% and 80%. If it is less than 20%, recrystallization during the subsequent heat treatment is unfavorable, and the ductility of the steel sheet may be impaired. If it exceeds 80%, there is a risk of edge cracking during cold rolling.

[0036] Next, the cold-rolled steel sheet is heated to a temperature T soak where T1 is the temperature above which more than 5% martensite forms after cooling, and a soaking time t of less than 500 seconds is used to maintain a fine retained austenite grain size and, consequently, high strength and ductility. soak During this time, the soaking temperature T soak is maintained.

[0037] The heat-treated steel sheet is then cooled to room temperature to obtain a steel sheet having the above-described microstructure.

[0038] In another preferred embodiment of the present invention, the steel sheet provided for manufacturing the steel part is manufactured by the following sequence of steps:

[0039] The steel slab having the above composition is hot-rolled to obtain a hot-rolled steel sheet. Then, the hot-rolled steel sheet is heated to a temperature T coil After winding, the sheet can be pickled to remove oxidation.

[0040] Next, the hot-rolled steel sheet is annealed at an annealing temperature T HBA to obtain a hot rolled annealed steel sheet. This annealing results in steel softening and helps to stabilize the austenite during the final annealing due to the high carbon and manganese concentrations in the carbides or austenite.

[0041] Next, the hot-rolled annealed steel sheet is cold-rolled to obtain a cold-rolled steel sheet. The cold-rolling reduction is preferably between 20% and 80%. If it is less than 20%, recrystallization during the subsequent heat treatment is unfavorable, and the ductility of the steel sheet may be impaired. If it exceeds 80%, there is a risk of edge cracking during cold rolling.

[0042] Next, the cold-rolled steel sheet is heated to a temperature T soak and a soaking time of less than 500 seconds t to maintain a fine retained austenite grain size and therefore high ductility. soak During this time, the soaking temperature T soak is maintained.

[0043] Next, the heat-treated steel sheet is heated to a temperature T Q The mixture is cooled to a dispensing temperature T of 150°C to 550°C. P The heating time is t P During this time, the distribution temperature T P The heat-treated steel sheet is then cooled to room temperature to obtain a steel sheet having the above-described microstructure.

[0044] In another preferred embodiment, the steel sheet provided for manufacturing the steel part is manufactured by the following successive steps:

[0045] The steel slab having the above composition is hot-rolled to obtain a hot-rolled steel sheet. Then, the hot-rolled steel sheet is heated to a temperature T of 200°C to 700°C before being cooled to room temperature. coil It is wound up with.

[0046] According to the present invention, T warm Hole expansion ratio HER of heat-treated steel heated to Twarm , and the hole expansion ratio HER of steel at 20°C 20℃ (HER Twarm -HER 20℃ ) / HER 20℃ is more than 50%.

[0047] Preferably, T of 150 °C warmHole expansion ratio HER of heat-treated steel heated to 150℃ , and the hole expansion ratio HER of steel at 20°C 20℃ (HER 150℃ -HER 20℃ ) / HER 20℃ is more than 50%.

[0048] HER is measured according to ISO 16630.

[0049] According to the invention, the steel has an elongation El of greater than or equal to 10% at room temperature, El being measured in accordance with ISO standard ISO 6892-1.

[0050] In a preferred embodiment of the present invention, the steel has a HER of 10% or more. 20℃ In another preferred embodiment of the present invention, the heat treated steel has an HER of 25% or more. 150℃ It has. [Example]

[0051] Three grades, whose compositions are summarized in Table 1, were cast into semi-finished products and processed into steel sheets.

[0052] The compositions tested are summarized in the table below, with elemental contents expressed as weight percent.

[0053] [Table 1]

[0054] The cast steel semi-finished product was reheated at 1200 ° C, hot-rolled, and then coiled at 450 ° C. The hot-rolled steel sheet was then heated to a temperature T HBA Heat to 1000kJ / min and hold for t HBA The hot-rolled heat-treated steel sheet is then cold-rolled at a reduction ratio of 50%, and then maintained at the soaking temperature T soak Heat to 1000kJ / min and hold for t soakIn trials 3 and 4, the heat-treated steel sheet was quenched to less than Ms-50°C, and then the distribution temperature T P Heat to 1000kJ / min and hold for t P During the T P Maintain temperature.

[0055] The steel sheet is then cooled to room temperature. The following specific conditions were applied to obtain the heat treated steel sheet:

[0056] [Table 2]

[0057] The steel sheets were analyzed and the corresponding microstructures are summarized in Table 3.

[0058] The microstructure of the steel plate was determined.

[0059] [Table 3]

[0060] [C] A corresponds to the amount of carbon in austenite in weight percent, which is measured by X-ray diffraction.

[0061] The surface fraction of the phases in the microstructure is determined by the following method: a sample is cut from the steel sheet, polished and etched with reagents known per se to expose the microstructure, and the section is then examined in a scanning electron microscope, for example a field emission gun electron microscope ("FEG-SEM"), in secondary electron mode at a magnification of more than 5000 times.

[0062] The surface fraction of ferrite is determined by SEM observation after etching with Nital or Picral / Nital reagents.

[0063] The volume fraction of retained austenite is determined by X-ray diffraction.

[0064] The determination of the martensite type can be performed and quantified by scanning electron microscopy.

[0065] The percentage of carbides is determined by cross-sections of the plates examined with a scanning electron microscope equipped with a field emission gun (FEG-SEM) and image analysis at magnifications above 15,000x.

[0066] The steel plate was then cut to obtain steel blanks, which were analyzed at room temperature (20°C) and the corresponding mechanical properties are summarized in Table 4.

[0067] The steel blank is then heated to a temperature T warm After reheating to T warm Punched or sheared at temperature.

[0068] The heat-treated steel blanks were analyzed and the corresponding mechanical properties are summarized in Table 4.

[0069]

[0070] [Table 4]

[0071] trial 2 In the range of 100 to 300°C, the composition and manufacturing conditions correspond to the present invention. Therefore, the desired properties are obtained. The effect of warm cutting of the steel blank is particularly evident in the HER 20℃ The hole expansion ratio HER at 150°C compared to the hole expansion ratio at room temperature 150℃ This is emphasized by the increase in

[0072] In trial 4, the carbon content of the steel plate is too high, resulting in a high carbon content in the austenite, which means that the austenite is stabilized and eliminates the effect of warm cutting on the hole expansion ratio.

[0073] In test 5, steel was tested. Line 2Therefore, a large amount of austenite is formed at a low carbon content in the interior, and therefore the test Line 2 This austenite is less stable than in annealing. This austenite therefore transforms into fresh martensite during cooling and warm cutting. This amount of fresh martensite results in an elongation of the steel part of less than 10% at room temperature.

Claims

1. 1. A method for manufacturing a steel part, comprising the following successive steps: - in weight percent: C: 0.05-0.25% Mn: 3.5 to 8% Si: 0.1 to 2% Al: 0.01~3% S≦0.010% P≦0.020% N≦0.008% and optionally comprising, in weight percent, the following elements: Cr: 0 to 0.5% Mo: 0-0.25% The remainder of the composition is iron and unavoidable impurities resulting from smelting, and in the 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 than 2% carbides - carbon in austenite [C] strictly greater than 0.4% by weight and strictly less than 0.7% by weight Ａ having a microstructure containing Nitrogen %N, silicon %Si, manganese %Mn, chromium %Cr, nickel %Ni, copper %Cu, molybdenum %Mo, and carbon in austenite [C] Ａ is such that Md30 is between 200°C and 350°C, Md30 being defined as follows: wherein Md30 is the temperature at which 50% of the retained austenite is transformed into martensite after 30% deformation of the steel plate; Md300(c)=551-462 ＊ (b) Ａ 008)-9.2 ＊ 031-8.1 ＊ 7.1-13.7 ＊ 300-29 ＊ (08)+135)-18.5 ＊ (b) - cutting the steel plate into a predetermined shape to obtain a steel blank; - The steel blank is heated to a temperature T ｗａｒｍ to obtain a heat treated steel blank. - the T ｗａｒｍ stamping or shearing the heat treated steel blank at temperature; - Heat-treated steel blanks ｗａｒｍ Process of forming steel parts at high temperatures A method for manufacturing a steel part, comprising:

2. The steel sheet is subjected to the following consecutive steps: - hot rolling a steel slab having the composition according to claim 1 to obtain a hot-rolled steel sheet, - The hot-rolled steel sheet is wound at a coiling temperature T of 200 ° C to 700 ° C. ｃｏｉｌ The winding process is - The hot-rolled steel sheet is annealed at an annealing temperature T of 500 to 680 ° C. ＨＢＡ to obtain a hot-rolled annealed steel sheet; - cold rolling the hot-rolled annealed steel sheet to obtain a cold-rolled steel sheet; - The cold-rolled steel sheet is heated to a temperature of 680 ° C or higher and a temperature T １ Less than temperature T ｓｏａｋ where T １ is the temperature above which more than 5% martensite is formed after cooling, and the cold-rolled steel sheet is heated to the soaking temperature T ｓｏａｋ and a soaking time t of less than 500 seconds ｓｏａｋ a step of obtaining a heat-treated steel sheet by maintaining the temperature for a period of time; - cooling the heat-treated steel sheet to room temperature. The method for manufacturing a steel part according to claim 1 provided by

3. The steel sheet is subjected to the following consecutive steps: - hot rolling a steel slab having the composition according to claim 1 to obtain a hot-rolled steel sheet, a coiling temperature T between 200°C and 700°C; ｃｏｉｌ a step of coiling the hot-rolled steel sheet; - The hot-rolled steel sheet is annealed at an annealing temperature T of 500 to 680 ° C. ＨＢＡ to obtain a hot-rolled annealed steel sheet; - cold rolling the hot-rolled annealed steel sheet to obtain a cold-rolled steel sheet; - The cold-rolled steel sheet is heated to a temperature T of 780 ° C or higher. ｓｏａｋ Heat to the soaking temperature T ｓｏａｋ and a soaking time t of less than 500 seconds ｓｏａｋ maintaining the cold-rolled steel sheet during this time to obtain a heat-treated steel sheet; -20 ° C and (Ms-50 ° C) Ｑ The heat-treated steel sheet is cooled to a distribution temperature T Ｐ The heat-treated steel sheet is heated to the distribution temperature T Ｐ The distribution time t is 1 second to 1800 seconds. Ｐ The process of maintaining the steel plate between - cooling the heat-treated steel sheet to room temperature. The method for manufacturing a steel part according to claim 1 provided by

4. Said T ｗａｒｍ The method for producing a steel part according to claim 1, wherein the temperature is between 50°C and 250°C.

5. The temperature T ｗａｒｍ Hole expansion rate HER Ｔｗａｒｍ and the hole expansion ratio HER of the steel plate at 20 ° C. ２０℃ The relationship is as follows: (HER Ｔｗａｒｍ -HER ２０℃ ) / HER ２０℃ ≧50% The method for manufacturing a steel part according to any one of claims 1 to 4, wherein

6. A method for manufacturing a steel part described in any one of claims 1 to 5, wherein the elongation El of the steel plate at 20°C is 10% or more.

7. The hole expansion ratio HER of the steel plate at 20°C ２０℃ The method for producing a steel part according to any one of claims 1 to 6, wherein the content of Cr is 10% or more.

8. The spread ratio HER of the steel plate at 150°C １５０℃ The method for producing a steel part according to any one of claims 1 to 7, wherein the content of Cr is 25% or more.

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