Pre-oxidized coating-free press hardening steel

Abstract

A pre-oxidized steel sheet for press hardening, a press-hardened pre-oxidized steel sheet for vehicle components, and a method of forming a pre-oxidized steel sheet. The pre-oxidized steel sheet includes a surface, a body region below the surface, and an oxide layer disposed on the surface. The oxide layer has a thickness in a range of 20 nm to 60 nm, and includes at least one of manganese oxide, iron oxide, ferromanganese oxide, chromium oxide, and amorphous silicon oxide.

Description

Technical Field

[0001] This invention relates to the automotive field, and more particularly to pre-oxidized, uncoated, mold-hardened steel for vehicle components. Background Technology

[0002] Molded hardened steel sheets are commonly used in a variety of applications, including structural components for vehicle bodies. Applications include bumper beams, struts, panels, rails / components, and door sills. The steel is supplied in thin coils, typically ranging in thickness from 0.5 mm to 4.0 mm. The coils are cut, trimmed, and hot-stamped into the desired shapes.

[0003] Various coatings have been used to protect steel sheets from corrosion and high-temperature oxidation during transport and molding hardening. Examples of such coatings include hot-dip galvanizing and aluminosilicate coatings. Hot-dip galvanizing provides a zinc layer on the surface of the steel sheet, which bonds to the iron in the steel near the zinc-to-steel interface. This coating protects the steel from corrosion and improves abrasion resistance during stamping or other manufacturing processes. Aluminosilicate coatings provide a layer of aluminum and silicon on the surface of the steel sheet. Aluminosilicate coatings can also be applied to the steel sheet prior to molding hardening, providing corrosion resistance and protecting the steel from oxidation during molding hardening. Similarly, aluminum-zinc coatings, optionally including silicon to increase adhesion, can be used to protect the steel in the steel sheet from oxidation. However, when such a coating is present on the steel surface during welding or brazing, the coating introduces impurities into the weld joint and can negatively affect the mechanical properties of the joint. Therefore, in some applications, it is preferable to remove the coating from the joint surface using mechanical abrasion, chemical removal, laser ablation, or other methods prior to welding or brazing.

[0004] Therefore, while existing coatings used on steel plates have achieved their intended purpose, new and improved steel plates and methods for forming them are needed to protect the steel plate surface from oxidation and corrosion before and during the molding and hardening process. Summary of the Invention

[0005] According to various aspects, this disclosure relates to a pre-oxidized steel sheet for compression molding. The pre-oxidized steel sheet includes a surface, a body region below the surface, and an oxide layer disposed on the surface. The body region includes a steel composition comprising carbon (C) present in the range of 0.05 to 0.35 wt% of the total weight of the composition, manganese (Mn) present in the range of 0.5 to 5.0 wt% of the total weight of the composition, silicon (Si) present in the range of 0.5 to 2.0 wt% of the total weight of the composition, chromium (Cr) present in the range of 0.6 to 4.0 wt% of the total weight of the composition, niobium (Nb) optionally present in the range of 0.02 to 0.05 wt% of the total weight of the composition, and iron present in 100% of the total weight of the remaining portion of the composition. Furthermore, the oxide layer includes at least one of manganese oxide, iron oxide, ferromanganese oxide, chromium oxide, and amorphous silicon oxide. Further, the thickness of the oxide layer is in the range of 20 nanometers to 60 nanometers.

[0006] In the above-described embodiments, the pre-oxidized steel sheet further includes a subsurface depletion zone between the body region and the surface, wherein the thickness of the subsurface depletion zone is less than 0.7 micrometers and includes manganese present in the range of 25% to 50% of the amount of manganese in the body region.

[0007] In any of the above embodiments, the oxide layer comprises a relative weight percentage of MnFe2O4 to Fe2O3 in the range of 1:1 to 5:1.

[0008] In any of the above embodiments, the oxide layer comprises a relative weight percentage of Mn2O3 to MnFe2O4 in the range of 0.35:1 to 3:1.

[0009] According to various additional aspects, this disclosure relates to a mold-hardened pre-oxidized steel sheet for vehicle components. The mold-hardened pre-oxidized steel sheet includes a surface, a body region below the surface, and an oxide layer disposed on the surface. The body region below the surface includes a steel composition comprising carbon (C) present in the range of 0.05 to 0.35 wt% of the total weight of the composition, manganese (Mn) present in the range of 0.5 to 5.0 wt% of the total weight of the composition, silicon (Si) present in the range of 0.5 to 2.0 wt% of the total weight of the composition, chromium (Cr) present in the range of 0.6 to 4.0 wt% of the total weight of the composition, optionally niobium (Nb) present in the range of 0.02 to 0.05 wt% of the total weight of the composition, and iron present in 100% of the total weight of the remaining portion of the composition. Furthermore, the oxide layer includes at least one of manganese oxide, iron oxide, ferromanganese oxide, chromium oxide, and amorphous silicon oxide. Further, the thickness of the oxide layer is in the range of 60 nanometers to 1 micrometer.

[0010] In the above embodiments, the body region has a martensitic structure ranging from 85 vol% to 99.8 vol% of the total volume of the body region, austenite ranging from 0.1 vol% to 8 vol% of the total volume of the body region, and a carbide phase ranging from 0.1 vol% to 7 vol% of the total volume of the body region, wherein the total volume of the body region is 100%. In other embodiments, the carbide phase is rich in chromium, and the chromium content of the carbide phase ranges from 5 wt% to 51 wt% of the carbide phase, with the remainder, totaling 100 wt%, being carbon.

[0011] In any of the above embodiments, the compression-hardened pre-oxidized steel sheet component has a yield strength in the range of 1,000 MPa to 1,500 MPa, an ultimate tensile strength in the range of 1,400 MPa to 1,900 MPa, and a total elongation in the range of 5% to 10%.

[0012] According to various additional aspects, this disclosure relates to a method for forming a pre-oxidized steel sheet for vehicle components. The method includes heating the steel sheet in a first process environment at a pre-oxidation temperature ranging from 600°C to 850°C for a pre-oxidation period ranging from 0.1 seconds to 1,000 seconds. The method also includes forming an oxide layer on the surface of the steel sheet, the oxide layer having a thickness ranging from 20 nanometers to 60 nanometers on the surface of the steel sheet. The steel sheet comprises the following steel composition: carbon (C) present in the range of 0.05 to 0.35 wt% of the total weight percentage of the composition; manganese (Mn) present in the range of 0.5 to 5.0 wt% of the total weight percentage of the composition; silicon (Si) present in the range of 0.5 to 2.0 wt% of the total weight percentage of the composition; chromium (Cr) present in the range of 0.6 to 4.0 wt% of the total weight percentage of the composition; niobium (Nb) optionally present in the range of 0.02 to 0.05 wt% of the total weight percentage of the composition; and iron present in 100% of the total weight percentage of the composition. The oxide layer comprises at least one of manganese oxide, iron oxide, ferromanganese oxide, chromium oxide, and amorphous silicon oxide. This method provides a pre-oxidized steel sheet.

[0013] In the above implementation, the method further includes exposing the steel plate to air in a first process environment.

[0014] In any of the above embodiments, the method further includes exposing the steel plate in the first process environment to dry air having a first dew point in the range of -40 degrees Celsius to 50 degrees Celsius.

[0015] In any of the above embodiments, the method further includes exposing the steel plate in a first process environment to moisture, and at least one of nitrogen, hydrogen and carbon monoxide, wherein the moisture in the process environment provides oxygen to form an oxide layer.

[0016] In any of the above embodiments, the method further includes forming an under-surface depletion region between the body region below the surface and the surface. In other embodiments, the method includes forming the under-surface depletion region to have a thickness in the range of 0.01 micrometers to 0.7 micrometers and to include manganese present in the range of 25% to 50% of the amount of manganese in the body region.

[0017] In any of the above embodiments, the method further includes annealing the steel plate while heating it.

[0018] In any of the above embodiments, the steel plate includes an oil layer, and the method further includes burning off the oil layer on the steel plate.

[0019] In any of the above embodiments, the method further includes heating the pre-oxidized steel sheet to a molding hardening temperature in the range of 850°C to 1,200°C in a second process environment. In other embodiments, the method includes exposing the pre-oxidized steel sheet to air in the second process environment. In still other embodiments, the method includes exposing the pre-oxidized steel sheet to dry air having a first dew point in the range of -40°C to 10°C in the second process environment.

[0020] In any of the above embodiments, the method further includes pressing the pre-oxidized steel sheet in a mold after heating and quenching the pre-oxidized steel sheet at a cooling rate in the range of 20 degrees Celsius to 200 degrees Celsius per second. Attached Figure Description

[0021] The accompanying drawings described herein are for illustrative purposes only and are not intended to limit the scope of this disclosure in any way.

[0022] Figure 1 A vehicle according to an embodiment of this disclosure is shown;

[0023] Figure 2 A welded joint according to an embodiment of this disclosure is shown;

[0024] Figure 3A A method for subsequently molding hardening a pre-oxidized steel sheet according to an embodiment of this disclosure is shown;

[0025] Figure 3B The embodiments of the present disclosure are shown in Figure 3A The relative temperature T and oxide layer thickness O formed during the method;

[0026] Figure 4A A scanning electron microscope image of the cross-section of a pickled steel plate is shown, with a scale of 300 nanometers (nm).

[0027] Figure 4B A scanning electron microscope image of a cross-section of an uncoated, pre-oxidized steel sheet is shown, with a scale of 300 nanometers (nm).

[0028] Figure 4C A scanning electron microscope image of a cross section of an uncoated pre-oxidized steel sheet after molding hardening is shown, with a scale of 1 micrometer (μm) in length.

[0029] Figure 4D A scanning electron microscope image of a cross section of a steel sheet that has not been pre-oxidized and has been mold-hardened without any coating is shown, with a scale of 5 micrometers (μm) in length.

[0030] Figure 5 An embodiment of the compression molding process according to an embodiment of the present disclosure is shown;

[0031] Figure 6A A scanning electron microscope image of a cross section of a pre-oxidized steel sheet after compression curing, with an underdeveloped pre-oxidized layer prior to compression curing, is shown, at a scale of 5 micrometers.

[0032] Figure 6B A scanning electron microscope image of a cross section of a pre-oxidized steel sheet after compression curing, with an overdeveloped pre-oxidized layer prior to compression curing, is shown, at a scale of 5 micrometers. Detailed Implementation

[0033] The following description is merely exemplary in nature and is not intended to limit this disclosure, its application, or its uses. Furthermore, it is not intended to be bound by any express or implied theory presented in the foregoing background, summary of the invention, or the following detailed description. It should be understood that throughout the drawings, corresponding reference numerals denote similar or corresponding parts and features.

[0034] Reference will now be made in detail to several embodiments of the present disclosure illustrated in the accompanying drawings. Wherever possible, the same or similar reference numerals are used in the drawings and description to refer to the same or similar parts or steps. Unless otherwise stated, the drawings are in simplified form and not drawn to exact scale.

[0035] This disclosure relates to pre-oxidized uncoated steel sheets for compression-hardening steel applications, compression-hardened pre-oxidized uncoated steel sheets for vehicle components, methods for forming pre-oxidized uncoated steel sheets, and methods for compression-hardening pre-oxidized uncoated steel sheets. It should be understood that uncoated steel sheets are understood to be steel sheets that do not include a coating containing at least one of aluminum, zinc, and silicon applied prior to the compression-hardening process. However, oil may be present on the steel sheet prior to pre-oxidation to inhibit rusting. Alternatively, oil may be applied to the steel sheet after pre-oxidation to inhibit rusting prior to compression-hardening. It should also be understood that compression-hardening refers to hot stamping, hot pressing, or hot forming die hardening, as well as hot blow forming. Pre-oxidized uncoated steel sheets, after compression-hardening, can be used to form various structural and non-structural components for vehicle applications, including but not limited to bumper beams, pillars (such as A, B, or C pillars), panels, rails / components, door bumper beams, and battery casings. Furthermore, this disclosure relates to methods and systems for forming pre-oxidized uncoated steel sheets and compression-hardening pre-oxidized uncoated steel sheets.

[0036] As used herein, the term "vehicle" is not limited to automobiles. While this document primarily describes the technology in the context of automobiles, the technology is not limited to automobiles. These concepts can be applied to a wide range of applications, such as motorcycles, mopeds, locomotives, aircraft, ships, and other vehicles, as well as other structural and non-structural components and thermal bonding applications.

[0037] Figure 1A non-limiting embodiment of a vehicle 100 including a body frame 102 is shown. The body frame 102 is formed from one or more components 104 assembled together using molded hardened steel billets of pre-oxidized uncoated steel sheets. The components 104 forming the body frame 102 include, but are not limited to, for example, A-pillars 106, B-pillars 108, floor panels 110, and roof 112, as well as, for example, bumper beams, door impact beams, and battery housings.

[0038] To assemble component 104, one or more thermal processes (such as welding or brazing) can be used to join component 104. Figure 2 An embodiment of a joint 200 formed using a thermal process is shown. The joint 200 includes a joint interface 202 formed by thermally bonding the two steel plates 204, 206 at interface surfaces 208, 210 on each steel plate 204, 206. Interface surfaces 208, 210 are those surfaces that form part of the joint 200.

[0039] Figure 3AA general method 300 for forming pre-oxidized, uncoated, die-hardened steel is shown. In an embodiment, the method includes casting and rolling a steel sheet at frame 302. The steel composition includes iron, carbon, manganese, silicon, chromium, and optionally niobium. In an embodiment, the steel composition includes carbon (C) present in the range of 0.05 to 0.35 wt% of the total weight of the composition, manganese (Mn) present in the range of 0.5 to 5.0 wt% of the total weight of the composition, silicon (Si) present in the range of 0.5 to 2.0 wt% of the total weight of the composition, chromium (Cr) present in the range of 0.6 to 4.0 wt% of the total weight of the composition, niobium (Nb) optionally present in the range of 0.02 to 0.05 wt% of the total weight of the composition, and iron present in 100% of the total weight of the remaining portion of the composition. In other embodiments, the steel plate is substantially composed of carbon (C) present in the range of 0.05 to 0.35 wt% of the total composition, manganese (Mn) present in the range of 0.5 to 5.0 wt% of the total composition, silicon (Si) present in the range of 0.5 to 2.0 wt% of the total composition, chromium (Cr) present in the range of 0.6 to 4.0 wt% of the total composition, niobium (Nb) optionally present in the range of 0.02 to 0.05 wt% of the total composition, and the remainder of the composition comprising 100% of the total weight percentage. The steel sheet is composed of iron, or alternatively, carbon (C) present in the range of 0.05 to 0.35 wt% of the total composition, manganese (Mn) present in the range of 0.5 to 5.0 wt% of the total composition, silicon (Si) present in the range of 0.5 to 2.0 wt% of the total composition, chromium (Cr) present in the range of 0.6 to 4.0 wt% of the total composition, niobium (Nb) optionally present in the range of 0.02 to 0.05 wt% of the total composition, and iron present in 100% of the total composition in the remainder. It should be understood that in any of the above embodiments, unavoidable trace elements may be present and introduced, for example, in the process environment (i.e., the atmosphere surrounding the steel sheet during processing), through process equipment, or in the raw materials. In embodiments, the elements of the steel sheet formulation are combined, melted in a furnace, and formed into an ingot, which is pressed and formed into a steel sheet. The steel plate is then passed through a series of rollers to stretch it and reduce its thickness until the desired thickness is achieved.

[0040] At frame 304, the steel sheet may optionally be pickled by passing it through one or more tanks containing pickling solutions to remove impurities, rust, scale, or other contaminants from the steel sheet surface. Pickling solutions include acids such as hydrochloric acid, sulfuric acid, hydrochloric acid-sulfuric acid, and phosphoric acid pickling. In an alternative embodiment, eco-friendly surface pickling (EPS) and automatic brushing are used to remove abrasive oxides. Figure 3B As shown, in this embodiment, pickling can be performed at temperatures below 100 degrees Celsius, such as in the range of 21 degrees Celsius to 100 degrees Celsius. At the end of pickling, the steel sheet may optionally be oiled to form an oil layer, which helps prevent rusting. Figure 4A A scanning electron microscope image of a cross-section of the pickled steel sheet 400 is shown. The micrograph shows that there is almost no oxidation (if any) on the surface 404 of the pickled steel sheet 400.

[0041] At frame 306, the pickled and oiled steel sheet may optionally be pre-oxidized. For example... Figure 3B As shown, the pre-oxidation process at frame 306 is performed by heating an uncoated steel sheet at a pre-oxidation temperature PT below the transformation temperature Ac1 of the steel composition. The transformation temperature Ac1 is understood as the lower critical temperature of the steel sheet, i.e., the temperature at which the steel begins to transform from ferrite to austenite. In an embodiment, the steel sheet is heated at a temperature sufficient to anneal it, which is understood to cause a rearrangement of the crystal structure within the steel sheet. In other embodiments, the pre-oxidation process is performed at a pre-oxidation temperature PT in the range of 600°C to 850°C (inclusive of all values ​​and ranges therein, such as 760°C). Further, the steel sheet is exposed to the pre-oxidation temperature for a pre-oxidation period (Pp) in the range of 0.1 seconds to 1,000 seconds (inclusive of all values ​​and ranges therein). In an embodiment, the cooling rate of the steel sheet after pre-oxidation is in the range of 1°C per second to 100°C per second (inclusive of all values ​​and ranges therein).

[0042] The pre-oxidation process is carried out in an oven or other semi-enclosed or preferably enclosed environment that provides the first process environment. The first process environment during pre-oxidation includes at least one of the following: air from the ambient atmosphere, dry air (such as in the range of 20 to 30 degrees Celsius) with a dew point in the range of -40 to 50 degrees Celsius (inclusive). In alternative embodiments, nitrogen, hydrogen, carbon monoxide, and mixtures thereof may be used in the first process environment. When oxygen is absent in the first process environment, moisture provides a sufficient amount of oxygen for oxidation. In embodiments, the process is continuous. If the steel sheet is oiled during the pickling process at frame 304, the oil is burned off during the pre-oxidation process. As described above, the pre-oxidation process can be combined with annealing or other heat treatment methods, which can be performed simultaneously with hot / cold rolling and optionally after pickling and oiling.

[0043] During the pre-oxidation process, such as Figure 4B As shown, an oxide layer 406 is formed on the surface 404 of the steel plate 402. In an embodiment, the thickness 408 of the oxide layer 406 is in the range of 20 nm to 60 nm, including all values ​​and ranges therein, spanning at least 90% of the area of ​​the surface 404. The oxide structures in the oxide layer 406 include, but are not limited to, manganese oxide including Mn2O3, iron oxide including Fe2O3, manganese iron oxide including MnFe2O4, chromium oxide including CrO and Cr2O3, and amorphous silicon oxide including SiO2. The relative weight percentage of MnFe2O4 to Fe2O3 is 1:1 or greater, such as in the range of 1:1 to 5:1, including all values ​​and ranges therein. The relative weight percentage of Mn2O3 to MnFe2O4 is 0.35:1 or greater, such as in the range of 0.35:1 to 3:1, including all values ​​and ranges therein. The relative weight percentages are determined by energy-dispersive X-ray spectroscopy.

[0044] Furthermore, in this embodiment, a subsurface depletion region 410 is formed below the surface 404 of the steel plate 402. The subsurface depletion region 410 has a thickness 412 less than 0.7 micrometers from the surface 404 of the steel plate 402, including all values ​​and ranges from 0.01 to 0.7 micrometers, wherein the manganese content is less than 50% of the manganese content of the body region 414 of the steel plate 402 below the subsurface depletion region 410, including all values ​​and ranges from 25% to 50% by weight of the manganese content present in the body region 414. The body region 414 comprises the initial steel composition used to form the steel plate 402 described above at frame 302. Figure 3B The development of oxide layer 406 grown to a thickness of PO during the pre-oxidation process at box 306 is illustrated. As shown, during the pre-oxidation process, oxide layer 406 is grown to a given thickness of PO until the temperature drops to less than 200 degrees Celsius, including all values ​​and ranges therein, such as from 21 degrees Celsius to 200 degrees Celsius.

[0045] At frame 308, the pre-oxidized steel sheet is molded and hardened. Again, the steel sheet remains uncoated at this stage. Figure 5 An embodiment of process 500 for molding hardened steel sheet is shown. At frame 502, steel sheet, typically supplied in coil form, is cut into blanks 512, which are generally of a flat geometry. Although an elliptical shape is shown, blank 512 can have any shape, including but not limited to circular, elongated circular, or polygonal shapes with three or more sides. Optionally, in the embodiment, blank 512 may also be processed by at least one of the following processes: cold forming, trimming, and piercing. Optionally, pre-oxidized coils are roll-formed, welded, and cut into tubular shapes.

[0046] At box 504, as follows Figure 3B As shown, the steel sheet billet 512 is heated to a molding hardening temperature PHT above the transformation temperature Ac3, which is equal to or higher than the upper critical temperature of the steel sheet, at which the steel sheet completely transforms from ferrite to austenite during heating. In an embodiment, the molding hardening temperature is in the range of 850 degrees Celsius to 1,200 degrees Celsius, including all values ​​and ranges therein, such as 930 degrees Celsius. In an embodiment, the heating is carried out for a molding hardening period (PHp) in the range of 10 seconds to 1,000 seconds (inclusive). In an embodiment, a tunnel furnace or muffle furnace can be used to heat the steel sheet billet 512 at a heating rate of 1 degree Celsius to 20 degrees Celsius per second. In an alternative or additional embodiment, induction heating can be used to obtain a heating rate between 20 degrees Celsius per second and 1,000 degrees Celsius per second. The second process environment includes air, such as air from the ambient atmosphere or treated air with a reduced dew point (such as in the range of 40 degrees Celsius to 10 degrees Celsius (inclusive)).

[0047] At frame 506, a heated steel sheet blank 512 is conveyed to the die 518 of the press using a robot or conveyor 516. In the die 518, at frame 508, the heated steel sheet blank 512 is pressed into the general shape of the cavity 520 of the die 518, forming the general shape of a component such as an A-pillar, bumper beam, etc. Additionally, the steel sheet blank 512 is quenched during pressing. The cooling rate of the steel sheet blank 512 is in the range of 20 degrees Celsius per second to 200 degrees Celsius per second, inclusive. The die 518 is cooled by circulating a heat transfer fluid through channels or other openings in the die 518. The heat transfer fluid includes, for example, water or oil. Pressing and quenching are applied for a time period in the range of 3 seconds to 15 seconds, inclusive. At frame 510, the component 522 formed from the steel sheet blank 512 is removed from the die 518. Alternatively, the part can be formed by air hardening without using a mold.

[0048] Figure 3B The relative growth of the oxide layer 406 thickness PHO of the pre-oxidized steel sheet after the molding hardening process of frame 308 is shown. As shown, the oxide layer 406 can continue to grow by heating and cooling. Figure 4CAn embodiment of the oxide layer 406 after compression molding is shown. The thickness 408 of the oxide layer 406 is less than 1 micrometer, including all values ​​and ranges from 60 nanometers to 1 micrometer, and in a preferred embodiment, the oxide layer 406 spans at least 90% of the area of ​​surface 404 in the range of 60 nanometers to 0.4 micrometers. At this thickness 408, the oxide layer 406 is not considered to interfere with spot welding or painting. Furthermore, after compression molding, the steel sheet of component 522 has a martensitic structure, austenite, and carbide phase in the body region, wherein the martensitic structure is present in the range of 85 vol% to 99.8 vol% of the total volume, the austenite is present in the range of 0.1 vol% to 8 vol% of the total volume, and the carbide phase is present in the range of 0.1 vol% to 7 vol% of the total volume, wherein the total volume is 100%, and the austenite is measured by X-ray diffraction and the carbide is measured by transmission electron microscopy. The carbide phase includes transition metal carbides, wherein the transition metal may include, for example, iron, manganese, silicon, carbon, and chromium. In an embodiment, the carbide phase is rich in chromium, and the chromium content is in the range of 5% to 51% by weight of the carbide phase, including all values ​​and ranges therein, such as 25% to 50% by weight of the carbide phase.

[0049] The compression-hardened pre-oxidized steel sheet has a yield strength ranging from 1,000 MPa to 1,500 MPa, inclusive. Furthermore, the ultimate tensile strength of the compression-hardened pre-oxidized steel sheet ranges from 1,400 MPa to 1,900 MPa, inclusive. Further, the total elongation of the compression-hardened pre-oxidized steel sheet ranges from 5% to 10%, inclusive. The test procedure used in this paper is ASTM E8 standard using A50 samples.

[0050] In comparison, Figure 4D The diagram illustrates the oxide layer obtained if no pre-oxidation is performed at frame 306 prior to molding hardening at frame 308, and no coating such as a zinc coating, aluminum coating, or silicon coating is applied prior to molding hardening. As shown, steel sheet 402 has an oxide layer 406 with a thickness 408 of approximately 5 micrometers on surface 404 of steel sheet 402. This oxide layer 406 may have peeling or other undesirable properties. Furthermore, a relatively thick internal oxide layer or intergranular oxide layer is formed in region 418 below surface 404, as indicated by a mesh-like or similar mesh feature below surface 404. Internal oxidation is understood to potentially have an adverse effect on the spot weldability of steel sheet 402. Figure 4C In the region 420 below the surface 404 of the molded hardened pre-oxidized steel sheet shown, there is almost no or no internal oxidation, and if any oxidation is present, its thickness is less than 1 micrometer.

[0051] It has been found that the formation of an insufficient oxide layer with a thickness of less than 20 nm or an overdeveloped oxide layer with a thickness of more than 60 nm during the pre-oxidation period at frame 306 results in the formation of a significantly thicker oxide layer 406 during and after the molding curing period at frame 308. Figure 6A An oxide layer 606 is shown following a compression-hardened steel sheet 602, which has a pre-oxidized oxide layer of less than 20 nanometers formed at frame 306. After compression hardening at frame 308, the thickness 608 of the oxide layer 606 is in the range of 3 to 4 micrometers. Furthermore, a relatively thick internal oxide layer or intergranular oxide layer is formed in region 618 below surface 604, as shown by the mesh-like or similar mesh-like features below surface 604, which is understood to potentially have an adverse effect on the spot weldability of the steel sheet 602. Figure 6B The oxide layer 606 is shown after the steel sheet 602 with a pre-oxidized layer greater than 60 nanometers is molded and hardened at frame 306. After molding hardening at frame 308, the thickness 608 of the oxide layer 606 is in the range of 3 micrometers to 4 micrometers. Furthermore, with the overdevelopment of the oxide layer 606 during the pre-oxidation at frame 306, the region 618 below surface 604 includes internal oxidation or intergranular oxidation, as shown by the mesh or quasi-mesh features below surface 604, which is again understood to potentially have an adverse effect on the spot weldability of the steel sheet. Insufficient development of the oxide layer 606 during the pre-oxidation at frame 306 may be due to insufficient pre-oxidation temperature (PT), insufficient pre-oxidation time (Pp), or both. Overdevelopment of the oxide layer 606 during the pre-oxidation at frame 306 may be due to the use of too high a pre-oxidation temperature (PT), too long a pre-oxidation time (Pp), or both.

[0052] The pre-oxidized steel sheet, the pressure-cured pre-oxidized steel sheet, the pressure-cured pre-oxidized steel sheet components for vehicles, and the method described herein offer numerous advantages. These advantages include high-temperature oxidation resistance and the elimination of the need for an inert environment (such as a nitrogen atmosphere) during the pressure curing process. Furthermore, these advantages include the ability to spot weld and paint the pressure-cured steel sheet without interference from the oxide layer. These advantages also include, for example, the ability to omit coatings on the steel sheet, such as aluminum-silicon or zinc coatings, which could weaken weld joints if not removed. Another advantage is the ability to combine the pre-oxidation process with annealing, which can be performed while the steel sheet is still in the steel mill. Other advantages include the ability to place the steel into an induction furnace without removing coatings that might melt in the furnace. Yet another advantage is the option to use air or dry air in the pressure curing process environment, and the option to use an inert nitrogen or hydrogen process environment.

[0053] The description in this disclosure is merely exemplary in nature, and variations thereof without departing from the spirit and scope of this disclosure are intended to fall within its scope. Such variations should not be considered as departing from the spirit and scope of this disclosure.

Claims

1. A pre-oxidized steel sheet for compression molding hardening, comprising: surface; The body region below the surface, the body region comprising a steel composition, the steel composition comprising: Carbon (C) present in the range of 0.05% to 0.35% by weight of the total weight of the composition. Manganese (Mn) present in the range of 0.5% to 5.0% by weight of the total weight of the composition. The composition contains silicon (Si) in the range of 0.5 to 2.0 wt% of total weight, chromium (Cr) in the range of 0.6 to 4.0 wt% of total weight, and optionally niobium (Nb) in the range of 0.02 to 0.05 wt% of total weight, and Iron present in the remainder of the composition in 100% of the total weight percentage; and An oxide layer disposed on the surface, the oxide layer comprising at least one of manganese oxide, iron oxide, ferromanganese oxide, chromium oxide, and amorphous silicon oxide, the thickness of the oxide layer being in the range of 20 nanometers to 60 nanometers.

2. The pre-oxidized steel plate according to claim 1 further includes a subsurface depletion zone between the body region and the surface, wherein, The thickness of the depleted region beneath the surface is less than 0.7 micrometers and includes manganese present in the range of 25% to 50% of the manganese content in the bulk region.

3. The pre-oxidized steel plate according to claim 1, wherein, The oxide layer comprises a relative weight percentage of MnFe2O4 to Fe2O3 in the range of 1:1 to 5:

1.

4. The pre-oxidized steel plate according to claim 1, wherein, The oxide layer comprises a relative weight percentage of Mn2O3 to MnFe2O4 in the range of 0.35:1 to 3:

1.

5. A method for forming a pre-oxidized steel sheet for vehicle components, comprising: In a first process environment, the steel plate is heated to a pre-oxidation temperature ranging from 600°C to 850°C for a pre-oxidation period ranging from 0.1 seconds to 1,000 seconds, wherein the steel plate comprises the following steel composition: Carbon (C) present in the range of 0.05% to 0.35% by weight of the total weight of the composition. Manganese (Mn) present in the range of 0.5% to 5.0% by weight of the total weight of the composition. The composition contains silicon (Si) in the range of 0.5 to 2.0 wt% of total weight, chromium (Cr) in the range of 0.6 to 4.0 wt% of total weight, and optionally niobium (Nb) in the range of 0.02 to 0.05 wt% of total weight, and Iron present in the remainder of the composition at 100% of the total weight percentage of the composition; and an oxide layer formed on the surface of the steel plate to provide a pre-oxidized steel plate, the oxide layer having a thickness in the range of 20 nanometers to 60 nanometers on the surface of the steel plate, the oxide layer comprising at least one of manganese oxide, iron oxide, ferromanganese oxide, chromium oxide and amorphous silicon oxide.

6. The method of claim 5, further comprising exposing the steel plate to air in the first process environment.

7. The method of claim 5, further comprising exposing the steel plate in the first process environment to dry air having a first dew point in the range of -40 degrees Celsius to 50 degrees Celsius.

8. The method of claim 5, further comprising exposing the steel plate in the first process environment to moisture, and at least one of nitrogen, hydrogen, and carbon monoxide, wherein, The moisture in the process environment provides oxygen to form the oxide layer.

9. The method of claim 5, further comprising forming an under-surface depletion region between the body region below the surface and the surface.

10. The method of claim 9, further comprising forming the subsurface depletion region to a thickness in the range of 0.01 micrometers to 0.7 micrometers and including manganese present in the range of 25% to 50% of the amount of manganese in the body region.

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