Coated steel sheet and high-strength press-hardened steel component and method for producing same

Abstract

The invention relates to a coated steel sheet and a press-hardened steel component having a composition comprising: 0.15 to 0.25 wt% of C, 0.5 to 1.8 wt% of Mn, 0.1 to 1.25 wt% of Si, 0.01 to 0.1 wt% of Al, 0.1 to 1.0 wt% of Cr, 0.01 to 0.1 wt% of Ti, 0.001 to 0.004 wt% of B, P < = 0.020 wt%, S < = 0.010 wt%, N < = 0.010 wt%, with the balance being Fe and unavoidable impurities. The remainder of the composition is iron and unavoidable impurities resulting from smelting. The press-hardened steel component comprises: a body having a microstructure comprising more than 95% martensite and less than 5% bainite in surface fraction; a coating at a surface of the steel component; a ferritic interdiffusion layer between the coating and the body, and a ratio between a ferritic grain width GW interdiffusion layer in the interdiffusion layer and a primal austenitic grain size PAGS body in the body satisfy the following equation (GW interdiffusion layer / PAGS body)-1 > = 30%.

Description

[0001] This invention patent application is a divisional application of the invention patent application filed on December 3, 2021, with application number 202180082416.0 and invention title "Coated steel plate and high-strength press-hardened steel component and manufacturing method thereof". Technical Field

[0002] This invention relates to coated steel sheets, and to high-strength press-hardened steel components with good bending properties. Background Technology

[0003] High-strength press-hardened components can be used in automobiles as structural elements for intrusion prevention or energy absorption functions.

[0004] In this type of application, the goal is to produce steel components that combine high mechanical strength, high impact resistance, and high corrosion resistance. Furthermore, given global environmental protection efforts, one of the main challenges for the automotive industry is to reduce vehicle weight to improve fuel efficiency without neglecting safety requirements.

[0005] Weight reduction can be achieved, in particular, by using steel components with a martensitic or bainitic / martensitic microstructure.

[0006] Publication WO2016104881 relates to hot-pressed components used as structural parts for vehicles, etc., which require impact resistance and, more particularly, tensile strength of 1300 MPa or greater; and a method for manufacturing said hot-pressed components by heating steel to a temperature that allows the formation of a single austenitic phase, quenching it, and hot-forming it using a die. To obtain such properties, the base steel sheet includes a thin ferrite layer of less than 50 μm at its surface, and the carbide size and density should be controlled. The ferrite layer in the matrix allows for the suppression of the propagation of fine cracks formed on the coating to the base, resulting in low bending performance with a bending angle of less than 70°.

[0007] Publication WO2018179839 relates to hot-pressed components obtained by hot pressing steel sheets having a microstructure varying in the thickness direction, comprising a soft layer of at least 90% ferrite, a transition layer of ferrite and martensite, and a hard layer primarily composed of martensite, and exhibiting both high strength and high flexibility. To obtain these properties, the cold-rolled steel sheet is annealed in an atmosphere with a dew point temperature of 50°C to 90°C, which may be detrimental to the aluminum alloy coating. Summary of the Invention

[0008] Therefore, the object of the present invention is to solve the above-mentioned problems and provide a press-hardened steel component that combines a tensile strength TS greater than or equal to 1350 MPa with a bending angle greater than 70°. Preferably, the press-hardened steel component according to the present invention has a yield strength YS greater than or equal to 1000 MPa.

[0009] Another object of the present invention is to obtain coated steel sheets that can be thermoformed into press-hardened steel parts.

[0010] The object of the present invention is achieved by providing the steel plate according to claim 1. Another object is achieved by providing the method according to claim 2. Another object of the present invention is achieved by providing a press-hardened steel component according to claim 3. The steel component may also include the characteristics of any one of claims 4 to 6. Another object is achieved by providing the method according to claim 7. Attached Figure Description

[0011] The invention will now be described and illustrated in detail with reference to the accompanying drawings, by way of examples without limitation:

[0012] - Figure 1a A schematic cross-section of a coated steel plate not according to Test 4 of the present invention is shown;

[0013] - Figure 1b A schematic cross-section showing a press-hardened steel component not according to Test 4 of the present invention;

[0014] - Figure 2a A schematic cross-section of a coated steel plate not according to Test 5 of the present invention is shown;

[0015] - Figure 2b A schematic cross-section showing a press-hardened steel component not conforming to Test 5 of the present invention;

[0016] - Figure 3a A schematic cross-section of the coated steel plate according to Tests 1 and 2 of the present invention is shown;

[0017] - Figure 3b A schematic cross-section showing the press-hardened steel component according to Tests 1 and 2 of the present invention;

[0018] - Figure 4a A schematic cross-section of a coated steel plate according to Test 3 of the present invention is shown;

[0019] - Figure 4b A schematic cross-section showing the press-hardened steel component according to Test 3 of the present invention;

[0020] - Figure 5a A schematic cross-section of a coated steel plate not according to Test 9 of the present invention is shown;

[0021] - Figure 5b A schematic cross-section showing a pressed hardened steel component not according to Test 9 of the present invention. Detailed Implementation

[0022] The composition of the steel according to the present invention will now be described, with the content expressed as a weight percentage.

[0023] According to the present invention, the carbon content is 0.15% to 0.25% to ensure satisfactory strength. When the carbon content is greater than 0.25%, the weldability and bendability of the steel sheet may decrease. If the carbon content is less than 0.15%, the tensile strength will not reach the target value.

[0024] The manganese content is between 0.5% and 1.8%. Adding more than 1.8% increases the risk of center segregation, which is detrimental to bendability. Below 0.5%, the hardenability of the steel plate decreases. Preferably, the manganese content is between 0.8% and 1.5%.

[0025] According to the present invention, the silicon content is from 0.1% to 1.25%. Silicon is an element involved in solid solution hardening. Adding silicon limits carbide formation. Above 1.25%, silicon oxide forms on the surface, which impairs the coatability of the steel. Furthermore, the weldability of the steel sheet may be reduced. Preferably, the silicon content is from 0.2% to 1.25%. More preferably, the silicon content is from 0.3% to 1.25%.

[0026] The aluminum content is 0.01% to 0.1%, as aluminum is a very effective element for deoxidizing steel in its liquid phase during processing. Aluminum can protect boron if the titanium content is insufficient. An aluminum content below 0.1% avoids oxidation problems and ferrite formation during press hardening. Preferably, the aluminum content is 0.01% to 0.05%.

[0027] According to the present invention, the chromium content is from 0.1% to 1.0%. Chromium is an element involved in solid solution hardening and must be above 0.1%. A chromium content below 1.0% limits processing performance issues and costs.

[0028] The titanium content is 0.01% to 0.1% to protect boron from forming BN. The titanium content is limited to 0.1% to avoid TiN formation.

[0029] According to the present invention, the boron content is from 0.001% to 0.004%. Boron improves the hardenability of steel. The boron content is not higher than 0.004% to avoid the risk of slab breakage during continuous casting.

[0030] You can add elements as you like.

[0031] The molybdenum content can optionally be added up to 0.40%. Like boron, molybdenum improves the hardenability of steel. Molybdenum content should not exceed 0.40% to limit costs.

[0032] According to the invention, niobium can optionally be added up to 0.08% to improve the ductility of the steel. Adding more than 0.08% increases the risk of forming NbC or Nb(C,N) carbides, which is detrimental to flexibility. Preferably, the niobium content is less than or equal to 0.05%.

[0033] Calcium can also be added as an optional element up to 0.1%. Adding calcium in the liquid stage allows for the production of fine oxides, which improves the castability of continuous casting.

[0034] The remaining components of steel are iron and impurities produced during smelting. In this respect, at least P, S, and N are considered residual elements, which are unavoidable impurities. Their content is less than 0.010% for S, less than 0.020% for P, and less than 0.010% for N.

[0035] The microstructure of the coated steel plate according to the present invention will now be described.

[0036] The cross-section of the coated steel plate of the present invention is schematically shown in... Figure 3a and Figure 4a The coated steel plate comprises a body (2); a decarburized layer (3) on top of the body (2), the decarburized layer (3) comprising a ferrite layer (4) with a thickness of 1 μm to 100 μm on its upper part; and a coating (1). Preferably, the thickness of the ferrite layer is 20 μm to 100 μm. More preferably, the thickness of the ferrite layer is 25 μm to 100 μm. More preferably, the thickness of the ferrite layer is 25 μm to 80 μm.

[0037] The microstructure of the main body (2) of the coated steel plate contains 60% to 90% ferrite by surface fraction, with the remainder being martensite-austenite islands, pearlite or bainite.

[0038] This ferrite forms during critical zone annealing of cold-rolled steel sheets. The remaining portion of the microstructure at the end of soaking is austenite, which transforms into martensite-austenite islands, pearlite, or bainite during the cooling of the steel sheet.

[0039] By controlling the furnace atmosphere to set the dew point temperature strictly above -10°C and below or equal to 20°C, a decarburized layer is obtained on the top of the body during the annealing of cold-rolled steel sheets.

[0040] The coated steel sheet according to the invention can be produced by any suitable manufacturing method, and those skilled in the art can determine suitable manufacturing methods. However, it is preferred to use the method according to the invention, which includes the following steps:

[0041] A semi-finished product having the above-described steel composition is provided, which can be further hot-rolled. The semi-finished product is then reheated at a temperature of 1150°C to 1300°C.

[0042] The steel plate is then hot-rolled at a hot rolling termination temperature of 800°C to 950°C.

[0043] The hot-rolled steel is then cooled at a temperature below 670°C. 卷取 The material is then wound up and optionally acid-washed to remove oxides.

[0044] The coiled steel sheet is then optionally cold-rolled to obtain cold-rolled steel sheet. The cold rolling reduction is preferably 20% to 80%. Below 20%, recrystallization during subsequent heat treatment is unfavorable, which may impair the ductility of the steel sheet. Above 80%, there is a risk of edge cracking during cold rolling.

[0045] The steel sheet is then annealed in an HNx atmosphere containing 0% to 15% H2 to an annealing temperature T of 700°C to 850°C. A At the annealing temperature T A The holding time t ranges from 10 seconds to 1200 seconds. A To obtain annealed steel sheets. Below 700°C, the kinetics of decarburization layer formation are too slow for a ferrite layer to be formed on top. Holding time t A The time is greater than or equal to 10 seconds to allow the ferrite layer to form, and less than or equal to 1200 seconds to limit the thickness of the ferrite layer.

[0046] During this annealing process, the atmosphere inside the furnace is controlled to have a dew point temperature T that is strictly above -10°C and below or equal to +20°C. DP1 To form a decarburized layer according to the invention. If T DP1 Below or equal to -10°C, the formation of the decarburized layer slows down, and a ferrite layer cannot form on top of the decarburized layer. The bending flexibility of the steel component will be too low. If T DP1 Above 20°C, the surface of the steel plate may be completely oxidized, damaging its coatability and mechanical properties.

[0047] In one embodiment of the invention, the annealed steel sheet is heated to an annealing temperature T2 of 700°C to 850°C and held at said temperature T2 for a holding time t2 of 10 seconds to 1200 seconds, wherein the atmosphere has a dew point temperature T strictly above -10°C and below or equal to +20°C. DP2 Then, an aluminum alloy coating is applied to the steel plate.

[0048] The microstructure of the press-hardened steel component according to the present invention will now be described. Figure 3b and Figure 4b The diagram schematically shows the cross-section of a press-hardened steel component.

[0049] The steel component continuously comprises, from its body to its surface:

[0050] -The main body (7), the microstructure of which comprises more than 95% martensite and less than 5% bainite by surface fraction,

[0051] - Ferrite interdiffusion layer (6),

[0052] - Aluminum-based coating (5).

[0053] During the heating of the steel billet cut from the steel sheet according to the invention, all microstructural elements of the bulk are transformed into austenite, and the ferrite in the decarburized layer is transformed into austenite with a wider grain size than the austenite in the bulk. After hot forming, the steel part is then die-hardened. The interdiffusion layer grows from the previous wide-grain-size austenite layer, and therefore has a larger grain width than the original austenite grain size in the bulk. To improve the flexibility of the steel sheet without deteriorating mechanical properties, the ferrite grain width GW in the interdiffusion layer is... 相互扩散层 PAGS (original austenite grain size in the main body) 主体 The ratio between them satisfies the following equation:

[0054] (GW 相互扩散层 / PAGS 主体 )-1≥30%

[0055] The ferrite grain width is the average distance between two parallel grain boundaries, which are oriented along the thickness direction of the steel plate. The annealing temperature T according to the present invention... A Annealing time t A and dew point temperature T DP1 The combination allows for large grain widths in the interdiffusion layers. Furthermore, heating the billet prior to pressing allows for small PAGS in the bulk.

[0056] In one embodiment, the press-hardened steel component may further include a martensitic layer with a carbon gradient between the body and the interdiffusion layer, such as... Figure 4b As indicated by (8). During the heating of the billet, carbon diffuses from the bulk to the surface. The upper ferrite of the decarburized layer then transforms into an austenite layer with a carbon gradient. During die quenching, this austenite layer with a carbon gradient transforms into a martensite layer with a carbon gradient.

[0057] The press-hardened steel component according to the invention has a tensile strength TS greater than or equal to 1350 MPa and a bending angle greater than 70°. The bending angle is determined on the press-hardened component according to the bending standard of method VDA238-100 (normalized to a thickness of 1.5 mm).

[0058] In a preferred embodiment of the present invention, the yield strength YS is greater than or equal to 1000 MPa.

[0059] TS and YS are measured according to ISO standard ISO 6892-1.

[0060] The press-hardened steel component according to the invention can be produced by any suitable manufacturing method, and those skilled in the art can determine suitable manufacturing methods. However, it is preferred to use the method according to the invention, which includes the following steps:

[0061] The coated steel sheet according to the invention is cut into a predetermined shape to obtain a steel billet. The steel billet is then heated to a temperature of 880°C to 950°C over a period of 10 to 900 seconds to obtain a heated steel billet. The heated steel billet is then transferred to a forming machine, followed by thermoforming and die-hardening.

[0062] The invention will now be illustrated by the following examples, which are by no means limiting.

[0063] Example

[0064] The seven categories of components summarized in Table 1 are cast into semi-finished products, processed into steel plates, and then processed into steel parts according to the process parameters summarized in Table 2.

[0065] Table 1 - Composition

[0066] The tested composition is summarized in the table below, where the element content is expressed as a weight percentage.

[0067] steel C Mn Si Al Cr Nb Ti B Mo P S N A 0.18 1.00 0.7 0.02 0.82 0.03 0.03 0.003 0.20 0.012 0.001 0.005 B 0.17 1.01 1.0 0.03 0.52 0.03 0.03 0.003 0.20 0.001 0.001 0.005 C 0.21 1.20 0.3 0.02 0.15 - 0.04 0.002 0.02 0.012 0.002 0.006 D 0.23 1.20 0.3 0.03 0.20 - 0.04 0.003 - 0.012 0,002 0.006 <![CDATA[ E ]]> <![CDATA[ 0.14 ]]> 1.19 0.2 0.05 0.48 - 0.03 0.002 0.32 0.006 0.001 0.006 <![CDATA[ F ]]> <![CDATA[ 0.08 ]]> 1.59 0.4 0.04 0.06 0.05 0.02 0.004 - 0.010 0.002 0.006 <![CDATA[ G ]]> <![CDATA[ 0.33 ]]> 0.60 0.5 0.03 0.33 0.05 0.01 0.003 0.17 0.013 0.001 0.004

[0068] Steels A to D are according to the present invention.

[0069] The underlined value does not correspond to this invention.

[0070] Table 2 - Process Parameters

[0071] The steel semi-finished product, intended as a casting, is reheated at 1200°C, hot-rolled to a hot-rolling finish temperature of 800°C to 950°C, coiled at 550°C, and cold-rolled with a reduction of 60%. The steel sheet is then heated to temperature T. A And maintained at the stated temperature in an HNx atmosphere containing 5% H2 with a controlled dew point for a duration of t. A The steel plate is then cooled to a temperature of 560°C to 700°C, and then hot-dip coated with an aluminum-silicon coating containing 10% silicon.

[0072] Prior to coating, sample 3 underwent a second annealing at temperature T2, and the steel sheet was held at said temperature in an HNx atmosphere with 5% H2 and a controlled dew point for a duration of t2. The following specific conditions were applied:

[0073]

[0074] The underlined value does not correspond to this invention.

[0075] The coated steel plates were analyzed, and the corresponding characteristics of the decarburized layer are summarized in Table 3.

[0076] Table 3 - Characteristics of the decarburized layer on coated steel sheets

[0077] test The presence of a decarburized layer Thickness (μm) of the upper ferrite portion of the decarburized layer. 1 yes 35 2 yes 30 3 yes 60 <![CDATA[ 4 ]]> <![CDATA[ no ]]> <![CDATA[ - ]]> <![CDATA[ 5 ]]> yes <![CDATA[ - ]]> <![CDATA[ 6 ]]> <![CDATA[ no ]]> <![CDATA[ - ]]> <![CDATA[ 7 ]]> yes 25 <![CDATA[ 8 ]]> yes <![CDATA[ - <!-- 5 -->]]> <![CDATA[ 9 ]]> yes <![CDATA[ 130 ]]> <![CDATA[ 10 ]]> <![CDATA[ no ]]> <![CDATA[ - ]]>

[0078] The underlined value does not correspond to this invention.

[0079] The coated steel sheet was then cut to obtain a billet, heated at 900°C for 6 minutes, and hot-formed. The steel component was analyzed, and the corresponding microstructure and ferrite grain width (GW) in the interdiffusion layer were determined. 相互扩散层 and the original austenite grain size PAGS in the main body 主体 The results are summarized in Table 4. The mechanical properties are summarized in Table 5.

[0080] Table 4 - Microstructure of Press-hardened Steel Components

[0081]

[0082] The underlined value does not correspond to this invention.

[0083] nd: Undetermined

[0084] The surface fraction, ferrite grain width in the interdiffusion layer, and PAGS were determined by cutting the specimen from a press-hardened steel component, polishing it, and etching it with reagents known per se to expose the microstructure. The section was then examined at magnification greater than 5000x using an optical microscope or a scanning electron microscope, such as a scanning electron microscope with a field emission gun connected to a BSE (backscattered electron) device (“FEG-SEM”).

[0085] Table 5 - Mechanical properties of press-hardened steel components

[0086] The mechanical properties of the test samples were determined and summarized in the table below:

[0087]

[0088] Underlined values: do not match the target value

[0089] The embodiments show that, due to their specific composition and microstructure, the steel components according to the invention (i.e., embodiments 1 to 3) are the only steel components that exhibit all the target properties.

[0090] Figure 3a Schematic cross-sections of the coated steel plates from experiments 1 and 2. The process parameters of this invention, including the annealing temperature T. A Annealing time t A and dew point temperature T DP1 The combination allows for the formation of a decarburized layer (3), in which a ferrite layer (4) is formed on the upper part.

[0091] The coated steel sheet is then thermoformed. Figure 3b A schematic cross-section showing the press-hardened steel components of tests 1 and 2.

[0092] The grain width of the ferrite formed in the interdiffusion layer (6) is generated by the pure ferrite layer, in which austenite with a larger grain size forms during heating. The interdiffusion layer grows on this large austenite grain size. Then the grain width of the ferrite in the interdiffusion layer (6) is larger than the original austenite grain size in the body (7), resulting in good flexibility with a bending angle greater than 70°.

[0093] Figure 4a A schematic cross-section of the coated steel plate from Test 3. The process parameters of this invention, including the annealing temperature T. A Annealing time t A and dew point temperature T DP1 The combination leads to the formation of a decarburized layer (3), which has a deeper ferrite layer on top than in experiments 1 and 2 (4) due to the longer annealing time.

[0094] The coated steel sheet is then thermoformed. Figure 4b This represents a schematic cross-section of the press-hardened steel used in Experiment 3.

[0095] The ferrite grain size in the interdiffusion layer (6) is generated from a pure ferrite layer in which austenite with a larger grain size is formed during the heating of the steel sheet. The interdiffusion layer grows on these larger austenite grain sizes. Then the ferrite grain width in the interdiffusion layer (6) is larger than the original austenite grain size in the body (7), resulting in good flexibility with a bending angle greater than 70°. In addition, due to the thick ferrite layer (3) in the coated steel sheet, a martensite layer with a carbon gradient is formed between the body of the press-hardened steel component and the interdiffusion layer, resulting in a tensile strength greater than 1350 MPa.

[0096] In Experiment 4, the composition of the steel plate was the same as that in Experiment 1 according to the invention. Compared with Experiment 1, the dew point temperature during the annealing of the steel plate was too low to obtain a decarburized layer with a ferrite upper portion in the coated steel plate. Figure 1a A schematic cross-section of a coated steel plate with a coating (1) and a body (2) representing these tests.

[0097] The coated steel sheet is then thermoformed. Figure 1b The schematic cross section represents the press-hardened steel component of test 4. Due to the absence of a ferrite layer, the ferrite grain width in the interdiffusion layer (6) is equal to the original austenite grain size in the body (7), resulting in a low bending angle of less than 70°.

[0098] In Experiment 5, the coated steel plate had a decarburized layer, and there was no ferrite layer on top of the decarburized layer, such as... Figure 2a This is shown schematically. The absence of a ferrite layer is due to the low dew point temperature of -10°C, which slows down the decarburization kinetics.

[0099] The coated steel sheet is then thermoformed. Figure 2b A schematic cross-section of the press-hardened steel component from test 5 is shown. Due to the absence of a ferrite layer, the ferrite grain width in the interdiffusion layer (6) is equal to the original austenite grain size in the body (7), resulting in a low bending angle of less than 70°.

[0100] In tests 6 and 7, the steel plate had a low carbon level of 0.14%. In test 6, the low dew point temperature T was -35°C. DP1 The growth of decarburized and ferrite layers in coated steel sheets is not permitted. In Test 7, compared to Test 6, the steel sheets were annealed at the same temperature and time, but at a dew point temperature of -10°C. The higher dew point temperature, due to the low carbon level of the steel sheet, allowed for the acquisition of a decarburized layer with a ferrite layer. However, the low carbon level did not allow for the desired mechanical properties of press-hardened steel components. In particular, the tensile strength was below 1350 MPa.

[0101] In Experiment 8, the steel sheet had a low carbon level of 0.08%. This low carbon level, combined with the process parameters, resulted in a decarburized layer in the coated steel sheet lacking a ferrite layer. However, due to the low carbon level, the yield strength and tensile strength of the press-hardened steel component could not be achieved.

[0102] In Experiment 9, the steel sheet was held at a homogenization temperature for 3600 seconds, which resulted in a thicker ferrite layer in the decarburized layer of the coated steel sheet than in previous experiments. Figure 5a The schematic cross section of the coated steel plate of test 9 is shown, which has a coating (1), a decarburized layer (3), a thicker ferrite layer (4) with a coarser grain size, and a body (2).

[0103] The coated steel sheet is then thermoformed, and Figure 5bThe diagram shows a schematic cross-section of the press-hardened steel component from Experiment 9. During the heating of the steel component, the bulk microstructure is austenite, and the thick ferrite layer transforms into an austenite layer with a carbon gradient. However, since the thickness of the ferrite layer is greater than 100 μm, a ferrite layer still exists between the interdiffusion layer and the austenite layer with a carbon gradient.

[0104] During the die-hardening of steel components, the ferrite layer remains, and the austenite layer with a carbon gradient transforms into a martensite layer with a carbon gradient, resulting in a multiphase layer. This causes a decrease in yield strength and tensile strength.

[0105] In Test 10, the steel plate had a carbon content higher than 0.25%. The low dew point temperature T was -40°C. DP1 The growth of the decarburized layer is not allowed, resulting in the absence of a ferrite layer in the coated steel sheet and a low bending angle of less than 70° in the press-hardened parts.

[0106] According to embodiments of the present invention, the following notes are also disclosed:

[0107] Note 1. A coated steel sheet, said coated steel sheet being made of steel having a composition comprising:

[0108] C: 0.15% to 0.25% by weight

[0109] Mn: 0.5 wt% to 1.8 wt%

[0110] Si: 0.1 wt% to 1.25 wt%

[0111] Al: 0.01% to 0.1% by weight

[0112] Cr: 0.1% to 1.0% by weight

[0113] Ti: 0.01 wt% to 0.1 wt%

[0114] B: 0.001% by weight to 0.004% by weight

[0115] P ≤ 0.020 wt%

[0116] S≤0.010% by weight

[0117] N≤0.010% by weight

[0118] and optionally include one or more of the following elements:

[0119] Mo ≤ 0.40% by weight

[0120] Nb≤0.08% by weight

[0121] Ca ≤ 0.1% by weight

[0122] The remaining component is iron and unavoidable impurities produced during smelting.

[0123] The coated steel sheet includes, from its body to its surface, the following:

[0124] -The body, the microstructure of which comprises 60% to 90% ferrite by surface fraction, with the remainder being martensite-austenite islands, pearlite or bainite.

[0125] - The top of this main body is a decarburized layer, which includes a ferrite layer with a thickness of 1 μm to 100 μm in the upper part.

[0126] - A coating made of aluminum or aluminum alloy.

[0127] Appendix 2. A method for producing coated steel sheets, the method comprising the following sequential steps:

[0128] - Cast steel to obtain slabs, said steel having a composition according to Appendix 1,

[0129] -At temperatures between 1100℃ and 1300℃ T 再加热 The slab will then be reheated.

[0130] - Hot rolling of the reheated slab at a hot rolling termination temperature of 800°C to 950°C.

[0131] - At a winding temperature T below 670°C 卷取 The hot-rolled steel sheet is then coiled to obtain a coiled steel sheet.

[0132] -Optionally, the coiled steel sheet is pickled.

[0133] -Optionally, the coiled steel sheet is cold-rolled to obtain a cold-rolled steel sheet.

[0134] - Heating the hot-rolled steel sheet or the cold-rolled steel sheet to an annealing temperature T of 700°C to 850°C. A and the steel plate is placed at temperature T A The holding time t ranges from 10 seconds to 1200 seconds. A To obtain annealed steel sheets, the atmosphere contains 0% to 15% H2 and has a dew point T strictly above -10°C and below or equal to +20°C. DP1 ,

[0135] - Cool the annealed steel sheet to a temperature range of 560°C to 700°C.

[0136] - The annealed steel sheet is coated with aluminum or an aluminum alloy coating.

[0137] - Cool the coated steel sheet to room temperature.

[0138] Appendix 3. A press-hardened steel component, the steel component having a composition comprising the following:

[0139] C: 0.15% to 0.25% by weight

[0140] Mn: 0.5 wt% to 1.8 wt%

[0141] Si: 0.1 wt% to 1.25 wt%

[0142] Al: 0.01% to 0.1% by weight

[0143] Cr: 0.1% to 1.0% by weight

[0144] Ti: 0.01 wt% to 0.1 wt%

[0145] B: 0.001% by weight to 0.004% by weight

[0146] P ≤ 0.020 wt%

[0147] S≤0.010% by weight

[0148] N≤0.010% by weight

[0149] and optionally include one or more of the following elements:

[0150] Mo ≤ 0.40% by weight

[0151] Nb≤0.08% by weight

[0152] Ca ≤ 0.1% by weight

[0153] The remaining component is iron and unavoidable impurities produced during smelting.

[0154] The steel component continuously comprises, from its body to its surface:

[0155] -The body, the microstructure of which comprises more than 95% martensite and less than 5% bainite by surface fraction.

[0156] - Ferrite interdiffusion layer

[0157] -Aluminum-based coating,

[0158] The ferrite grain width GW in the interdiffusion layer 相互扩散层 Compared with the original austenite grain size PAGS in the main body 主体 The ratio between them satisfies the following equation:

[0159] (GW 相互扩散层 / PAGS 主体 -1 ≥ 30%.

[0160] Note 4. The press-hardened steel component according to Note 3, wherein the press-hardened steel component includes a martensite layer with a carbon gradient between the body and the ferrite interdiffusion layer.

[0161] Note 5. The press-hardened steel component according to any one of Notes 3 and 4, wherein the press-hardened steel component has a tensile strength TS greater than or equal to 1350 MPa and a bending angle greater than 70°.

[0162] Note 6. The press-hardened steel component according to Note 5, wherein the press-hardened steel component has a yield strength YS greater than or equal to 1000 MPa.

[0163] Appendix 7. A method for manufacturing a press-hardened steel component according to any one of Appendices 3 to 6, comprising the following sequential steps:

[0164] - Provide a steel plate having the composition according to Appendix 1, or produced by the method according to Appendix 2.

[0165] - The steel plate is cut into a predetermined shape to obtain a steel billet.

[0166] - The steel billet is heated to a temperature of 880°C to 950°C over a period of 10 to 900 seconds to obtain a heated steel billet.

[0167] - Transfer the heated slab to the forming machine.

[0168] - The heated slab is thermoformed in the forming machine to obtain the formed part.

[0169] - The molded part is subjected to molding and quenching.

Claims

1. A coated steel sheet, said coated steel sheet being made of steel having a composition comprising: C: 0.15% to 0.25% by weight Mn: 0.5 wt% to 1.8 wt% Si: 0.1 wt% to 1.25 wt% Al: 0.01% to 0.1% by weight Cr: 0.1% to 1.0% by weight Ti: 0.01 wt% to 0.1 wt% B: 0.001% by weight to 0.004% by weight P ≤ 0.020 wt% S≤0.010% by weight N≤0.010% by weight and optionally include one or more of the following elements: Mo ≤ 0.40% by weight Nb≤0.08% by weight Ca ≤ 0.1% by weight The remaining component is iron and unavoidable impurities produced during processing. The coated steel sheet includes, from its body to its surface, the following: -The body, the microstructure of which comprises 60% to 90% ferrite by surface fraction; - A decarburized layer on top of the main body, the decarburized layer comprising, in its upper portion, a ferrite layer with a thickness of 1 μm to 100 μm; and - A coating made of aluminum or aluminum alloy.

2. A method for producing coated steel sheets, the method comprising the following sequential steps: - Cast steel to obtain slabs, said steel having a composition comprising the following: C： 0.15% by weight to 0.25% by weight Mn: 0.5 wt% to 1.8 wt% Si: 0.1 wt% to 1.25 wt% Al: 0.01% to 0.1% by weight Cr: 0.1% to 1.0% by weight Ti: 0.01 wt% to 0.1 wt% B: 0.001% by weight to 0.004% by weight P ≤ 0.020 wt% S≤0.010% by weight N≤0.010% by weight and optionally include one or more of the following elements: Mo ≤ 0.40% by weight Nb≤0.08% by weight Ca ≤ 0.1% by weight The remaining portion of the composition consists of iron and unavoidable impurities resulting from processing; -At temperatures between 1100℃ and 1300℃ T 再加热 The slab will then be reheated. - Hot rolling of the reheated slab at a hot rolling termination temperature of 800°C to 950°C; - At a winding temperature T below 670°C 卷取 The hot-rolled steel sheet is coiled to obtain a coiled steel sheet; -Optionally, the coiled steel sheet is pickled; -Optionally, the coiled steel sheet is cold-rolled to obtain a cold-rolled steel sheet; - Heating the hot-rolled steel sheet or the cold-rolled steel sheet to an annealing temperature T of 700°C to 850°C. A and the steel plate is placed at temperature T A The holding time t ranges from 10 seconds to 1200 seconds. A To obtain annealed steel sheets, the atmosphere contains 0% to 15% H2 and has a dew point T strictly above -10°C and below or equal to +20°C. DP1 ; - Cool the annealed steel sheet to a temperature range of 560°C to 700°C; - The annealed steel sheet is coated with aluminum or an aluminum alloy coating; as well as - Cool the coated steel sheet to room temperature. The coated steel sheet includes, from its body to its surface, the following: -The body, the microstructure of which comprises 60% to 90% ferrite by surface fraction; - A decarburized layer on top of the main body, the decarburized layer comprising, in its upper portion, a ferrite layer with a thickness of 1 μm to 100 μm; and - A coating made of aluminum or aluminum alloy.

3. A press-hardened steel component, the steel component comprising the following components: C: 0.15% to 0.25% by weight Mn: 0.5 wt% to 1.8 wt% Si: 0.1 wt% to 1.25 wt% Al: 0.01% to 0.1% by weight Cr: 0.1% to 1.0% by weight Ti: 0.01 wt% to 0.1 wt% B: 0.001% by weight to 0.004% by weight P ≤ 0.020 wt% S≤0.010% by weight N≤0.010% by weight and optionally include one or more of the following elements: Mo ≤ 0.40% by weight Nb≤0.08% by weight Ca ≤ 0.1% by weight The remaining component is iron and unavoidable impurities produced during processing. The steel component continuously comprises, from its body to its surface: -The body, the microstructure of which comprises more than 95% martensite by surface fraction. - Ferrite interdiffusion layer -Aluminum-based coating, The ferrite grain width GW in the interdiffusion layer 相互扩散层 Compared with the original austenite grain size PAGS in the main body 主体 The ratio between them satisfies the following equation: (GW 相互扩散层 / PAGS 主体 )-1≥30%。 4. The press-hardened steel component according to claim 3, wherein the press-hardened steel component comprises a martensite layer with a carbon gradient between the body and the ferrite interdiffusion layer.

5. The press-hardened steel component according to claim 3, wherein the press-hardened steel component has a tensile strength TS greater than or equal to 1350 MPa and a bending angle greater than 70°.

6. The press-hardened steel component according to claim 3, wherein the press-hardened steel component has a yield strength YS greater than or equal to 1000 MPa.

7. A method for manufacturing a press-hardened steel component according to claim 5, the method comprising the following sequential steps: Provide the steel plate according to claim 1; The steel plate is cut into a predetermined shape to obtain a steel billet; The steel billet is heated to obtain a heated steel billet; The heated steel billet is transferred to a forming machine; The heated steel billet is thermoformed in the forming machine to obtain the formed part; as well as The formed part is then subjected to molding and quenching.

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