High-strength and highly flexible steel parts

Abstract

The chemical composition includes, by weight %, 0.18≦C≦0.27, 0.18≦Si≦0.30, 1.0≦Mn≦1.5, 0.14≦Cr≦0.25, 0.02≦Al≦0.06, 0.02≦Ti≦0.06, 0.0020≦B≦0.0040, 0≦S≦0.008, and 0.008≦N≦0.020, with the remainder being iron and unavoidable impurities resulting from the refining process. The microstructure contains more than 95% martensite by surface fraction, and the surface fraction of TiN particles in the skin portion is 200×10⁻⁶. -6 Inclusions / mm 2 The steel part wherein the average equivalent diameter of the TiN particles in the skin portion is 2.0 microns or less.

Description

[Technical Field]

[0001] The present invention relates to high-strength steel parts and methods for manufacturing the same. [Background technology]

[0002] High-strength steel components can be used as structural elements in automobile vehicles for intrusion prevention or energy absorption functions.

[0003] For such applications, it is desirable to manufacture steel components that combine high mechanical strength with high impact resistance. Furthermore, one of the major challenges in the automotive industry is reducing vehicle weight to improve fuel efficiency without neglecting safety requirements.

[0004] This weight reduction can be achieved, in particular, by using steel components that primarily have a martensite microstructure.

[0005] It is difficult to manufacture very high-strength steel that also has good resistance to crack formation under bending. In fact, very high-strength steel tends to crack prematurely when subjected to bending loads. This is detrimental to the impact safety of parts made from such high-strength steel, because even though the material can withstand very high loads thanks to its high tensile strength, once cracks begin to appear in a part, these cracks propagate rapidly under continuous load, causing the part to fail prematurely. [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] The objective of the present invention is to address the above-mentioned problems and provide a steel component that combines high strength and high flexibility. [Means for solving the problem]

[0007] An object of the present invention is achieved by providing a steel part as described in claim 1, having optionally the features of claims 2 to 6 individually or in any possible combination. Another object of the present invention is achieved by applying the method for manufacturing such a steel part as described in claim 7.

[0008] Here, the present invention will be described and illustrated in detail by examples, without introducing any limitations, with reference to Figure 1, which is a schematic cross-sectional view of a steel part according to the present invention.

[0009] A steel blank refers to a flat sheet of steel cut into any shape suitable for its use. The blank has a top and bottom surface, also called the upper and bottom sides or upper and bottom surfaces. The distance between these surfaces is specified as the thickness of the blank. The thickness can be measured, for example, using a micrometer, with its spindle and anvil positioned on the top and bottom surfaces. Similarly, the thickness can also be measured for molded parts.

[0010] Steel parts refer to parts formed from steel blanks.

[0011] The average thickness of a part or a portion of a part refers to the overall average thickness of the material that makes up the part after it has been formed from a flat sheet into a three-dimensional part.

[0012] When referring to the thickness of a steel component, it refers to the local thickness measured, for example, using the spindle and angle described above, or, for example, using a cross-sectional micrograph. [Brief explanation of the drawing]

[0013] [Figure 1] Referring to Figure 1, the steel component 1 comprises a bulk portion 3 and top and bottom skin layers 2. The total thickness of the steel component 1 is t0, and the thickness ts of the skin layer 2 is ts = t0 × 10%. In other words, the skin layer 2, or simply the skin, occupies the outermost 10% of the thickness on both sides of the bulk. Conversely, the bulk portion 3, or simply the bulk, occupies the central 80% of the thickness of the steel component. [Modes for carrying out the invention]

[0014] Hot stamping is a forming technique that involves heating a blank to a temperature at which the steel microstructure transforms at least partially into austenite, forming the blank at a high temperature by stamping it, and rapidly cooling the formed part to obtain a microstructure with very high strength. Hot stamping makes it possible to obtain very high-strength parts with complex shapes and presents many technical advantages. It should be understood that the heat treatment undergone by the part includes not only the thermal cycle of the hot stamping process itself described above, but also, in some cases, other subsequent heat treatment cycles performed after the part has been painted to cure the paint, such as a paint curing step. The mechanical properties of the hot-stamped parts below were measured, for example, after a complete thermal cycle including an optional paint curing step.

[0015] Yield strength and ultimate tensile strength are measured according to ISO standard ISO 6892-1, published in October 2009. Tensile test specimens are cut from the flat areas of the hot-stamped parts. Smaller tensile test samples are taken, if necessary, to correspond to the entire available flat area on the part.

[0016] The bending angle is measured according to the VDA-238 bending standard. For the same material, the bending angle depends on the thickness. For simplicity, the bending angle values ​​in this invention refer to those for a thickness of 1.5 mm. If the thickness is different from 1.5 mm, the bending angle values ​​must be normalized to 1.5 mm by the following calculation, where α1.5 is the bending angle normalized to 1.5 mm, t is the thickness, and αt is the bending angle for thickness t.

[0017] α1.5 = (αt × √t) / √1.5

[0018] In the present invention, the bending angle was measured in the transverse direction, i.e., the direction transverse to the rolling direction in which the steel sheet moved during the hot rolling step. The bending angle was measured using a laser measuring device. The reported value is the value reached when the maximum bending force Fmax, represented by N, was reached. When performing a bending test on a hot-stamped part, the sample is cut out from the flat area of the part. If necessary, small-sized samples are taken to correspond to the entire available flat area on the part. If the rolling direction of the hot-stamped part is unknown, it can be determined, for example, using the following protocol.

[0019] - Electron backscatter diffraction (EBSD) analysis is performed across a fragment of the sample with a scanning electron microscope (SEM).

[0020] - The rolling direction is determined according to the intensity of the orientation density function (ODF) representing the main fiber at φ2 = 45°, where φ2 is the Euler angle defined in "H.-J. Bunge: Texture Analysis in Materials Science - Mathematical Methods. 1st English Edition by Butterworth Co(Publ.) 1982" (see Figures 2.2 and 2.3 of the said publication for the definition of φ2).

[0021] The bending angle of the part represents the ability of the part to withstand deformation without cracking.

[0022] Here, an example of an experimental protocol for determining the average prior austenite grain size (PAGS) will be described. This protocol is given by way of example and is in no way limiting:

[0023] - The test piece is cut from a press-hardened steel part and polished using a solution containing fine oxides in suspension to produce a completely scratch-free and deformation-free surface known as a mirror-polished surface.

[0024] Next, EBSD maps (electron backscatter diffraction) are acquired using, for example, a JEOL IT800 Field-Emission-Gun Secondary-Electron-Microscope (FEG-SEM) equipped with a Symmetry2-Oxford CMOS EBSD camera. Each EBSD map represents a 0.25 mm × 0.20 mm zone. Two EBSD maps are performed for each measurement, so that they represent both the area statistics and the microstructure characteristics of the area being considered.

[0025] - The prior austenite grains can be reconstructed by crystallographic calculations based on both the measured crystallographic orientation of the martensite grains (measured EBSD map) and the orientation relationship linking the martensite and austenite.

[0026] - The calculations are performed using, for example, Merengue2(R) software. The calculated austenite map is then post-processed using, for example, AZtecCrystal software (Oxford Instruments(R)). Grain boundaries are defined as a minimum misorientation of 8°. The mean prior austenite grain size (PAGS) is the equivalent diameter (d) of each particle in the map considered. eq Based on this, the calculation is as follows:

[0027]

number

[0028] The following is an example of a methodology used to measure the surface fraction of inclusions (in this invention, only TiN particles are considered) in steel components. It should be understood that this is only one possible methodology, and other protocols can also be implemented.

[0029] Inclusions present in steel components are characterized using a scanning electron microscope (SEM) equipped with a field emission electron gun (FEG). The Tescan Mira 3 SEM can be used with a 14kV acceleration power setting. Furthermore, the inclusions are analyzed using energy-dispersive X-ray spectroscopy (EDS). 120mm 2 The Bruker EDS probe can be used.

[0030] The sample is divided into three regions (upper skin, lower skin, and bulk (as described above)). Each region is divided into a field of view. In each field of view, inclusions are detected. Each inclusion is zoomed in to capture its morphological features, and EDS analysis is performed. A dual gray level threshold is set to capture particles (on a scale of 0 to 255, where 0 is black and 255 is white):

[0031] Classic dark particles such as oxides, which have a gray level of less than -150.

[0032] - Bright particles such as NbC particles, where the gray level exceeds 220

[0033] Using information from the EDS probe, shape, and brightness level, each particle is then classified into one of the following categories: TiN, NbC, TiNbCN, alumina, composite oxides, oxysulfides, and MnS.

[0034] The next step is to calculate the following properties for the entire set of inclusions and for each particle family:

[0035] - Average diameter in microns

[0036] - Density by number of inclusions / mm 2

[0037] - The surface fraction of an inclusion, defined as the sum of the surface areas occupied by a given family of inclusions across all analytical fields divided by the total surface area of ​​all analytical regions. The surface fraction of an inclusion is defined (for a particle of type called "X" here) by the following formula:

[0038]

number

[0039] The surface fraction of inclusions is mm 2 It is expressed as the number of inclusions per unit area. This combines single parameter information regarding the particle density level and their average size.

[0040] Next, the composition of the steel component according to the present invention will be described.

[0041] The chemical composition is indicated with respect to the lower and upper limits of the composition range, where said limits are included within the possible composition range according to the present invention. Where a preferred range for a given element is disclosed, the present invention also discloses all possible combinations of these preferred ranges for each individual element.

[0042] According to this invention, the carbon content is in the range of 0.18% to 0.27% to ensure satisfactory strength. If the carbon content exceeds 0.27%, the weldability and flexibility of the steel plate may decrease. If the carbon content is less than 0.18%, the tensile strength will not reach the target value.

[0043] The silicon content is in the range of 0.18% to 0.30%. Silicon is an element that is involved in the hardening of solid solutions and limits carbide formation. If it exceeds 0.30%, silicon oxide forms on the surface, impairing the coating properties of the steel. If it is less than 0.18%, the target mechanical properties will not be met.

[0044] The manganese content is in the range of 1.0% to 1.5%. Above 1.5%, the risk of MnS formation increases, impairing flexibility. Below 1.0%, the hardenability of the steel sheet during the hot stamping process decreases, preventing it from achieving the target mechanical properties.

[0045] The chromium content ranges from 0.14% to 0.25%. Chromium is used to provide strength through solid solution hardening and to improve the hardenability of steel sheets during hot stamping. Chromium is limited to 0.25% to limit costs and avoid processing problems. Below 0.14%, the target mechanical properties cannot be met.

[0046] The aluminum content is in the range of 0.02% to 0.06% because it is a very effective element for deoxygenating steel in the liquid phase during refining. If the titanium content is insufficient, aluminum can protect boron. The aluminum content is less than 0.06% to avoid oxidation problems and ferrite formation during press hardening. Below 0.02%, the desired deoxygenation properties of aluminum in the liquid phase are not achieved.

[0047] The titanium content is in the range of 0.02% to 0.06% to protect boron; otherwise, boron would be trapped within the BN precipitate. To avoid excessive TiN formation, the titanium content is limited to 0.06%. Below 0.02%, the desired boron-protecting properties of titanium are not achieved.

[0048] The boron content is in the range of 0.0015% to 0.0040%. Boron improves the hardenability of steel. To avoid the problem of semi-finished products breaking immediately after casting, the boron content is 0.0040% or less. Below 0.0015%, the target mechanical properties cannot be met.

[0049] The presence of sulfur in molten steel can lead to the formation of MnS precipitates, which are detrimental to flexibility; therefore, the sulfur content is controlled to 0.005% or less, preferably 0.003%.

[0050] Phosphorus causes problems with brittleness and weldability, so it is controlled to 0.04% or less. In certain embodiments, the P content is controlled to 0.02% or less to further avoid problems with brittleness and weldability.

[0051] The nitrogen content ranges from 0.008% to 0.020%, preferably from 0.010% to 0.020%, preferably from 0.010% to 0.015%. The inventors have found that by controlling the minimum nitrogen content, a large number of small-sized TiN precipitates are formed in the skin portions of the steel sheet and the resulting steel parts. If the amount of nitrogen exceeds 0.020%, preferably 0.015%, the amount of precipitates formed can be harmful to flexibility, so the amount is limited. In certain embodiments, the product Ti 2 ×N of the square of the Ti composition and the nitrogen composition is 10.0×10 -6 wt% 3 or more, preferably 12.0×10 -6 wt% 3 or more. The inventors have found that by controlling the minimum Ti 2 ×N level, a large number of small-sized TiN precipitates are formed in the skin portions of the steel sheet and the resulting steel parts.

[0052] Molybdenum is optionally added up to a maximum of 0.3%. Molybdenum improves the hardenability of the steel. Molybdenum is limited to 0.3% to limit costs and avoid processing problems.

[0053] Niobium is optionally added up to a maximum of 0.1%. Niobium improves the ductility of the steel. Niobium is limited to 0.1% to limit costs and avoid processing problems.

[0054] Vanadium is optionally added up to a maximum of 0.3%. Vanadium improves the hardenability of the steel. Vanadium is limited to 0.3% to limit costs and avoid processing problems.

[0055] If one or more of the above elements are added, the following formula is further validated to limit costs and avoid processing problems: Cr + Mo + Nb + V ≤ 0.5%.

[0056] The remainder of steel composition consists of iron and impurities arising from the refining process. The level of impurities arising from the refining process depends on the manufacturing route used and the level of scrap used in the molten steel. For example, when using a basic oxygen furnace route containing low-level steel scrap (recycled steel), the level of impurities remains very low. However, a large amount of scrap in the converter may be added to the pig iron produced in the basic oxygen furnace, which increases the level of impurities. Furthermore, for example, in the case of steel refining using an electric furnace with a very high proportion of recycled scrap steel, the level of impurities increases significantly. When using high-level scrap, the level of Cu can reach 0.25%, Ni can reach 0.25%, Sn can reach 0.05%, As can reach 0.03%, Sb can reach 0.03%, and Pb can reach 0.03%.

[0057] The steel component according to the present invention can be manufactured by any suitable method.

[0058] A preferred method involves first providing a semi-finished product by casting steel having the chemical composition according to the present invention. The casting is carried out continuously or in batches. The semi-finished product has a thickness in the range of, for example, 40 mm to 120 mm, preferably 50 mm to 70 mm. During the casting process, a casting speed of 3.0 m / min or more, preferably 4 m / min or more, is maintained.

[0059] For example, a semi-finished product having the above-mentioned chemical composition is manufactured by continuous casting, and the semi-finished product is optionally subjected to a light reduction process directly during the continuous casting process in order to avoid central segregation.

[0060] The semi-finished products provided by casting can be used directly at high temperatures after the casting step, or they may be first cooled to a low temperature and then reheated for hot rolling.

[0061] Prior to the subsequent hot rolling step, the semi-finished product is maintained or reheated to a reheating temperature of 1075°C to 1200°C, preferably 1100°C to 1200°C. By controlling the temperature of the semi-finished product within this range before hot rolling, the mechanical force required to reduce the thickness of the semi-finished product throughout the hot rolling process can be controlled.

[0062] The manufacturing process further includes the following steps:

[0063] - A step of pickling the hot-rolled steel sheet,

[0064] -Optionally, the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet.

[0065] - A step of applying an annealing step to a hot-rolled or cold-rolled steel sheet by heating it in an annealing furnace to an annealing temperature TA expressed in degrees Celsius in the range of 700°C to 850°C, wherein the furnace further includes a soaking section in which the steel sheet is maintained at the temperature TA for a holding time tA of 10 seconds to 20 minutes.

[0066] -Optionally, the step of cooling the annealed steel sheet to a temperature range of 400°C to 700°C.

[0067] -Optionally, the step of coating the annealed steel sheet with a metal coating,

[0068] - In order to obtain a steel blank, the steel plate is cut into a predetermined shape.

[0069] - The steel blank is heated to a temperature of Ac3 or higher and 950°C or lower for 10 seconds to 15 minutes to obtain a heated steel blank.

[0070] - A step of transferring the heated steel blank to a forming press,

[0071] - A step of obtaining a molded part by hot forming the heated blank in a forming press,

[0072] - A step of die-quenching the molded part to obtain a steel part,

[0073] -Optionally, the steel part is painted and baked at a temperature of 150°C to 250°C for 10 minutes to 2 hours.

[0074] Optionally, a steel part can be obtained by hot stamping a steel blank having a metal coating on at least one of its outer surfaces. For example, the coating is an aluminum-based coating containing at least 50% aluminum. When the steel blank is hot stamped, the heat from the austenitization step induces interdiffusion between the elements of the steel substrate beneath the coating and the elements of the coating. As a result, the coating on the steel part has an increased iron content, in particular, compared to the amount of iron present in the metal coating on the steel blank.

[0075] Next, the microstructure of the steel component according to the present invention will be described.

[0076] The steel component contains 95% or more martensite by surface fraction, with the remainder being optionally bainite, ferrite, and / or retained austenite. Bainite, ferrite, and retained austenite are optional phases, and in a preferred embodiment, the microstructure of the component consists of 100% martensite.

[0077] The inventors have found that on a sample according to the present invention, the surface fraction of TiN particles in the skin is 200 × 10⁻⁶. -6 particles / mm 2The above findings were observed. Furthermore, the average equivalent diameter of the TiN particles in the skin portion is 2.0 microns or less, preferably 1.5 microns or less. These specific combinations of skin TiN density and average equivalent diameter size were observed in the sample according to the present invention. While we do not wish to be bound by theory, the inventors believe that these precipitates may be related to the good bending results observed in the parts according to the present invention.

[0078] In certain embodiments, the average prior austenite particle size PAGS_S is 6.0 μm or less.

[0079] The inventors have found that steel parts having the aforementioned properties exhibit excellent strength and flexibility. This provides parts with very good impact resistance and impact absorption capabilities.

[0080] In particular, the inventors have found that the steel part has an ultimate tensile strength of 1350 MPa or more, preferably 1400 MPa or more, measured in the transverse direction, and a bending angle α1.5 of 57° or more, measured in the transverse direction and normalized to a thickness of 1.5 mm. [Examples]

[0081] The present invention will now be explained using examples.

[0082] Samples R1 and R2 are reference samples not according to the present invention. Samples I1 to I3 are inventive samples according to the present invention.

[0083] Tables 1a and 1b show the chemical composition of the samples, expressed in weight percent, as well as the calculated Ac3 temperature (°C) and associated Ti 2 ×N(weight% 3 This is a compilation of (represented by ).

[0084] [Table 1]

[0085] [Table 2]

[0086] Table 2 summarizes the process parameters used to produce the samples. Underlined values ​​in the table are not according to the present invention.

[0087] [Table 3]

[0088] Table 3 summarizes the characteristics measured for samples after hot stamping and paint curing.

[0089] [Table 4]

[0090] Table 4 summarizes the microstructural characteristics of the samples after hot stamping. The average prior austenite grain size PAGS_S in the skin is expressed in microns. The prior austenite grain surface refinement ratio is calculated by dividing two values ​​having the same units, and is therefore a dimensionless quantity. Underlined values ​​in the table are not according to the present invention.

[0091] [Table 5]

[0092] The steel component according to the present invention has a prior austenite skin grain refinement ratio of 1.2 or higher and the chemical properties according to the present invention, thereby achieving extremely high mechanical properties with an ultimate tensile strength of 1350 MPa or higher in the transverse direction, and at the same time being able to maintain a high bending angle of 57° or higher in the transverse direction, normalized to a thickness of 1.5 mm.

Claims

1. Steel part (1), having the following characteristics: - By weight %, 0.18 ≤ C ≤ 0.27 0.18 ≤ Si ≤ 0.30 1.0 ≤ Mn ≤ 1.5 0.14 ≤ Cr ≤ 0.25 0.02 ≤ Al ≤ 0.06 0.02 ≤ Ti ≤ 0.06 0.0015 ≤ B ≤ 0.0040 0 ≤ S ≤ 0.005 0 ≤ P ≤ 0.04 0.008% ≤ N ≤ 0.020 0 ≤ Mo ≤ 0.3 0 ≤ Nb ≤ 0.1 0 ≤ V ≤ 0.3, Includes, The remaining components of the composition are iron and unavoidable impurities resulting from the refining process. - A microstructure containing 95% or more martensite by surface fraction, with the remainder being optionally selected bainite, ferrite, and / or retained austenite. - The steel part (1) comprises a bulk portion (3) that occupies the central 80% of its thickness, and skins (2) that occupy the outermost 10% of its thickness on both sides of the bulk portion (3), - The surface fraction of TiN particles in the skin portion is 200 × 10 －６ Inclusions / mm ２ The above is true, and the average equivalent diameter of the TiN particles in the skin portion is 2.0 microns or less. A steel part having [a certain characteristic].

2. Product Ti ２ ×N is 10.0 × 10 －６ weight% ３ The steel part according to claim 1, wherein each element is expressed in weight percent.

3. The steel part according to claim 1 or 2, wherein the average prior austenite grain size PAGS_S in the skin is 6.0 μm or less.

4. A steel component according to any one of claims 1 to 3, having an ultimate tensile strength of 1350 MPa or more as measured in the lateral direction.

5. A steel part according to any one of claims 1 to 4, having a bending angle α1.5 of 57° or more, measured in the lateral direction and normalized to a thickness of 1.5 mm.

6. A steel part according to any one of claims 1 to 5, which is covered with a metal coating containing 50% or more aluminum by weight.

7. A method for manufacturing steel parts, comprising the following steps: - A step of providing the steel composition described in claim 1, - A step of casting a semi-finished product having the above composition at a casting speed of 3.0 m / min or more, -Optionally, a step of cooling the semi-finished product after casting, - A step of maintaining or reheating the semi-finished product in a temperature range of 1075°C to 1200°C. The steps of obtaining a hot-rolled steel sheet by hot-rolling the semi-finished product using a finishing temperature of -850°C or higher and a winding temperature of 525°C or higher, - The step of pickling the hot-rolled steel sheet, -Optionally, the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet. - A step of applying the step of annealing the hot-rolled or cold-rolled steel sheet by heating it in an annealing furnace to an annealing temperature TA expressed in °C, which is 700 °C to 850 °C, wherein the furnace further includes a soaking section in which the steel sheet is maintained at the temperature TA for a holding time tA of 10 seconds to 20 minutes. -Optionally, a step of cooling the annealed steel sheet to a temperature range of 400°C to 700°C. -Optionally, the step of coating the annealed steel sheet with a metal coating, - In order to obtain a steel blank, the steel plate is cut into a predetermined shape. - The steel blank is heated to a temperature of Ac3 or higher and 950°C or lower for 10 seconds to 15 minutes to obtain a heated steel blank. - A step of transferring the heated steel blank to a forming press, - A step of obtaining a molded part by hot forming the heated blank in the molding press, - A step of die-quenching the molded part to obtain a steel part, -Optionally, the steel part is painted and baked at a temperature of 150°C to 250°C for 10 minutes to 2 hours. A method that includes this.

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