High-strength press-hardened steel parts and methods for manufacturing the same

Abstract

The present invention relates to a press-hardened steel part having a composition comprising, in weight percent, C 0.2 - 0.34%, Mn 0.50 - 1.24%, Si 0.5 - 2%, P ≤ 0.020%, S ≤ 0.010%, N ≤ 0.010%, and optionally one or more of Al ≤ 0.2%, Cr ≤ 0.8%, Nb ≤ 0.06%, Ti ≤ 0.06%, B ≤ 0.005%, and Mo ≤ 0.35%, wherein the remainder of the composition consists of iron and unavoidable impurities resulting from smelting. The press-hardened steel part has a microstructure comprising, in surface fraction, 95% or more of tempered martensite and 5% or less of bainite, austenite, or ferrite.

Description

Technology Field

[0001] The present invention relates to a high-strength press-hardened steel part with good bendability and weldability. Background Technology

[0002] High-strength press-hardened parts can be used as structural elements of automobiles for intrusion prevention or energy absorption functions.

[0003] In such types of applications, it is desirable to produce steel parts that combine high mechanical strength, high impact resistance, and good corrosion resistance. Furthermore, one of the major challenges of the automotive industry is to reduce the weight of vehicles to improve fuel efficiency in terms of preserving the global environment without neglecting safety requirements.

[0004] This weight reduction can be achieved, in particular, thanks to the use of steel parts with a tempered martensite or bainite / martensite microstructure.

[0005] These types of parts can be welded, and automobile manufacturers stipulate that welded joints must not constitute the weakest area of ​​the welded steel part.

[0006] In fact, the presence of spot welds on structural components of the vehicle body can lead to failure during a collision due to the localization of deformation in the softened heat-affected zone (HAZ). The problem to be solved

[0007] Accordingly, the object of the present invention is to solve the aforementioned problem and provide a press-hardened steel part having a combination of high mechanical properties, including a tensile strength (TS) of 1000 MPa or more, a uniform elongation loss (ΔUEl) in the spot weld area of ​​25% or less, and a bending angle of 55° or more.

[0008] Preferably, the press-hardened steel part according to the present invention has a fracture deformation of 0.50 or more.

[0009] Preferably, the press-hardened steel part according to the present invention has a yield strength (YS) of 980 MPa or more. means of solving the problem

[0010] The object of the present invention is achieved by providing a steel part according to claim 1. The steel part may also include features according to any one of claims 2 to 4. Another object is achieved by providing a method according to claim 5. Another object is achieved by providing a method according to any one of claims 6 to 8.

[0011] The present invention will now be described in detail and illustrated by examples without introducing limitations.

[0012] The composition of the steel according to the present invention will now be described, and its content is expressed in weight percent.

[0013] According to the present invention, the carbon content is 0.2% to 0.34% to ensure satisfactory strength. With a carbon content of 0.34% or more, the fracture deformation and bending angle of the steel plate did not achieve the target values. Furthermore, the weldability of the steel plate may be reduced. If the carbon content is less than 0.2%, the tensile and yield strengths will not reach the target values.

[0014] The manganese content is 0.50% to 1.24%. If added in excess of 1.24%, the risk of center segregation increases due to impaired bendability, and fracture deformation may be reduced. If it is less than 0.50%, the hardenability of the steel sheet decreases, and the tensile strength and yield strength will not reach the target values.

[0015] The silicon content is 0.5% to 2%. Silicon is an element that participates in solid solution hardening. Silicon is added to limit carbide formation and ensure a high level of tensile strength. If it exceeds 2%, silicon oxide forms on the surface, which impairs the coating properties of the steel. Furthermore, the weldability of the steel sheet may be reduced. Preferably, the silicon content is 0.5% to 1.8%. More preferably, the silicon content is 0.6% to 1.8%, and even more preferably 0.6% to 1.6%.

[0016] Some elements may be added selectively.

[0017] Since aluminum is a very effective element for deoxidizing the liquid steel during elaboration, it can be optionally added up to 0.2%. Preferably, the aluminum content is 0.1% or less. More preferably, the aluminum content is 0.06% or less.

[0018] Optionally, the chromium content may be added up to 0.8% to improve hardening in the solid solution. To limit processability issues and costs, the chromium content is 0.8% or less. Preferably, the chromium content is 0.6% or less.

[0019] Niobium content can be optionally added up to 0.06% to refine the austenite grain size and improve the ductility of the steel. Adding more than 0.06% increases the risk of forming NbC or Nb(C,N) carbides, which impairs bendability.

[0020] Titanium content can be optionally added up to 0.06% to protect boron from the formation of BN. Preferably, the titanium content is higher than 0.01%.

[0021] The boron content may be optionally added up to 0.005%. Boron improves the hardenability of steel. To avoid the risk of slab failure during continuous casting, the boron content is 0.005% or less.

[0022] Molybdenum can be optionally added up to 0.35%. Like boron, molybdenum improves the hardenability of steel. To limit costs, the amount of molybdenum is 0.35% or less.

[0023] The remainder of the steel composition consists of iron and impurities resulting from smelting. In this regard, P, S, and N are considered as residual elements that are at least unavoidable impurities. Their content is P 0.020% or less, S 0.010% or less, and N 0.010% or less.

[0024] Now, the microstructure of a press-hardened steel part according to the present invention will be described.

[0025] The press-hardened steel part has a microstructure comprising at least 95% tempered martensite as a surface fraction. This tempered martensite is retained for a holding time t consisting of 1 s to 1000 s temp During this time, a temperature T consisting of 390℃ to 510℃ temp It is formed while heating the steel parts.

[0026] Some bainite, ferrite, and austenite may be optionally present, and their sum is 5% or less of the surface fraction.

[0027] Preferably, the microstructure of the press-hardened steel portion is 100% tempered martensite.

[0028] The press-hardened steel part according to the present invention can be manufactured by any suitable manufacturing method, and those skilled in the art can define it. However, it is preferable to use the method according to the present invention comprising the following steps:

[0029] A steel plate having the composition according to the present invention is provided and cut into a predetermined shape to obtain a steel blank.

[0030] The steel blank is at a temperature T of 810°C to 960°C, preferably 850°C to 950°C, more preferably 880°C to 950°C. HF Heated to, and holding time t from 5 s to 1200 s HF During the above T HF A heated steel blank having a completely austenitic structure is obtained by maintaining it at a certain temperature. The heated steel blank is transferred to a forming press and hot forming to obtain a steel part.

[0031] After that, the steel parts are die-quenched until they reach a temperature of 200°C or lower.

[0032] To ensure temperature homogeneity for the entire steel part, the steel part is at a temperature T between 390℃ and 510℃. temp Reheated to a holding time t of 1 s to 1000 s temp During the above temperature T temp It is maintained in such a way that tempered steel parts are obtained.

[0033] At temperatures above 510°C, the tensile strength of the steel part decreases. At temperatures below 390°C, the uniform elongation loss (ΔUEl) in the spot-welded areas exceeds 25%. Subsequently, the tempered steel part is cooled to room temperature.

[0034] For each tempered product, HAZ sensitivity is evaluated through the uniform elongation loss of the welded JIS tensile specimen compared to a weldless reference. The uniform elongation loss (ΔUEl) is calculated as follows:

[0035] The uniform elongation (UEl) of the steel is measured on a tensile test specimen according to the standard JIS Z2241. A weld spot is formed on the tensile test specimen centered on the deformation region of the specimen. The uniform elongation (UEl) of this welded tensile test specimenw ) is measured according to the standard JIS Z2241.

[0036] The uniform elongation loss (ΔUEl) is determined by the following equation:

[0037] ΔUEl = [(UEl-UEl w ) / UEl]*100

[0038] In a preferred first embodiment of the present invention, a steel sheet provided for manufacturing a steel part is produced by the following successive steps:

[0039] A steel slab having the aforementioned composition is, before being hot-rolled at a finishing hot-rolling temperature of 800°C to 950°C, at a temperature T of 1100°C to 1300°C. reheat It is cast and reheated to obtain hot-rolled steel sheets.

[0040] Subsequently, hot-rolled steel sheets at a temperature T lower than 670℃ coil It is coiled.

[0041] Hot-rolled steel sheets can be optionally pickled to remove oxides.

[0042] Hot-rolled steel sheets are optionally at a temperature T of 500℃ to 750℃ HBA It can be heated to a holding time t of 300 s to 50 h. HBA During the above T HBA It can be maintained at a certain temperature.

[0043] Next, the steel sheet is cold-rolled to obtain a cold-rolled steel sheet. The cold reduction rate is preferably 20% to 80%. Below 20%, recrystallization during subsequent heat treatment is undesirable and may impair the ductility of the steel sheet. Above 80%, there is a risk of edge cracking during cold rolling.

[0044] To lower the tensile strength and facilitate the cutting of the steel, the cold-rolled steel sheet is optionally annealed at a temperature T of 650°C to 900°C. AAnnealed at the above temperature T A A retention time t of 10 s to 1200 s A The annealed steel sheet is obtained by maintaining it for a period of time. Finally, the steel sheet is cooled to room temperature.

[0045] Preferably, the annealed steel sheet is coated with aluminum or an aluminum alloy coating or zinc or a zinc alloy coating before being cooled to room temperature.

[0046] In a second embodiment of the present invention, a steel plate provided for manufacturing a steel part is produced by the following successive steps:

[0047] A steel slab having the composition according to the present invention is, before being hot rolled at a finishing hot rolling temperature of 800°C to 950°C, at a temperature T of 1100°C to 1300°C reheat It is cast and reheated to obtain hot-rolled steel sheets.

[0048] Subsequently, hot-rolled steel sheets at a temperature T lower than 670℃ coil It is coiled.

[0049] Hot-rolled steel sheets may be optionally pickled to remove oxides. Hot-rolled steel sheets may be optionally subjected to a temperature T of 500°C to 750°C. HBA It can be heated to a holding time t of 300 s to 50 h. HBA During the above T HBA It can be maintained at a certain temperature.

[0050] Next, the steel sheet is cold-rolled to obtain a cold-rolled steel sheet. The cold reduction rate is preferably 20% to 80%. Below 20%, recrystallization during subsequent heat treatment is undesirable and may impair the ductility of the steel sheet. Above 80%, there is a risk of edge cracking during cold rolling.

[0051] To lower the tensile strength and facilitate the cutting of the steel, the cold-rolled steel sheet is optionally annealed at a temperature T of 500°C to 750°C. A Annealed at the above temperature T A A retention time t of 300 s to 50 h A The annealed steel sheet is obtained by maintaining it for a period of time. Finally, the steel sheet is cooled to room temperature.

[0052] The press-hardened steel part according to the present invention has a tensile strength (TS) of 1000 MPa or more, a uniform elongation loss (ΔUEl) of 25% or less in the spot weld area, and a bending angle of 55° or more.

[0053] In a preferred embodiment of the present invention, the press-hardened steel part has a yield strength (YS) of 980 MPa or more.

[0054] In another preferred embodiment, the press-hardened steel part according to the present invention has a fracture deformation of 0.50 or more. Specific details for implementing the invention

[0055] The present invention is now described in a non-limiting manner by the following examples.

[0056] yes

[0057] The eight grades listed in Table 1 with their compositions were cast as semi-finished products and processed into steel plates, and then processed into steel parts according to the process parameters listed in Table 2.

[0058] Table 1 - Composition

[0059] The tested compositions are listed in the table below, where the elemental content is expressed as weight percentage.

[0060]

[0061] Table 2 - Process Parameters

[0062] The cast steel semi-finished product was reheated at 1250°C, hot-rolled at a finishing hot-rolling temperature of 800 to 950°C, coiled at 580°C, and cold-rolled with a reduction rate of 58%. Subsequently, the steel plate was at a temperature T of 790°C A Heat to 180 s for a holding time t A During the above temperature T A Maintain in.

[0063] A steel plate was cut into a predetermined shape to obtain a steel blank. Subsequently, a holding time t of 120 s was performed before transferring the steel blank to a forming press. HF During temperature T HF The heated blank was hot-formed in a forming press to obtain a steel part before die-quenching until it reached a temperature of 80°C.

[0064] Next, the steel part at a temperature T of 390℃ to 510℃ temp Reheat, and before cooling to room temperature, hold for a time t of 1 s to 1000 s. temp During the above T temp It was maintained at the temperature.

[0065]

[0066] Steel parts were analyzed, and the corresponding microstructures and properties are listed in Tables 3 and 4, respectively.

[0067] Table 3 - Microstructure of steel parts

[0068]

[0069] The surface fraction is determined by the following method: a specimen is cut from a press-hardened steel part, polished to expose the microstructure, and etched with a reagent known in itself, such as Nital reagent. The section is then examined via optical or scanning electron microscopy, for example, with a scanning electron microscope ("FEG-SEM") equipped with a field emission electron gun at a magnification greater than 5000x, coupled to an electron backscatter diffraction (EBSD) device. Tempered martensite can be distinguished from martensite due to its lower dislocation density compared to martensite.

[0070] Table 4 - Characteristics of Steel Parts

[0071] TS and YS are measured according to the ISO standard ISO 6892-1.

[0072] The bending angle was determined in press-hardened parts according to the VDA238-100 bending standard (normalized to a thickness of 1.5 mm).

[0073] The term fracture deformation refers to the fracture deformation criterion defined by Pascal Dietsch et al. in "Methods for Evaluating Fracture in Impact Simulation: Fracture Deformation Criteria and Their Calibration" in Metallurgical Research Technology Volume 114, Number 6, 2017. Fracture deformation is the equivalent deformation within the material at the point of deformation when the critical bending angle is reached. Fracture deformation values ​​were determined under plane deformation conditions, which are the most severe conditions during a vehicle collision, and were obtained thanks to finite element analysis.

[0074]

[0075] Table 5 - Spot Welding Characteristics of Press-Hardened Steel Parts

[0076] The welding spot is formed on the tensile test specimen centered on the deformation region of the specimen. The corresponding uniform elongation loss (ΔUEl) of the resistance spot weld is listed in Table 5.

[0077]

[0078] Due to the specific composition and process parameters of those used, the examples according to the present invention, namely Examples 1–7, are the only ones that exhibit a combination of high mechanical properties with a TS greater than 1000 MPa, a bending angle greater than 55°, and a uniform elongation loss of less than 25%. Furthermore, Examples 1–7 have a fracture strain greater than 0.50.

[0079] The tempering temperature applied to the steel parts of Test 8 and Test 9 is too low to limit the harmful effects of HAZ softening on uniform elongation, as indicated by a uniform elongation loss higher than 25%.

[0080] In addition, compared to Test 2 which has the same steel composition, the lower tempering temperature of Test 8 leads to higher uniform elongation loss and lower fracture deformation values ​​than Test 2.

[0081] Tempering is not performed on the steel portion of Test 10, which implies a uniform elongation loss higher than 25%.

[0082] In Test 11, the carbon content of the steel part is too high to achieve the target fracture deformation and bending values.

Claims

Claim 1 A press-hardened steel part made of steel having a composition in weight percent comprising: C: 0.2 - 0.34%, Mn: 0.50 - 1.24%, Si: 0.5 - 2%, P ≤ 0.020%, S ≤ 0.010%, N ≤ 0.010%, and optionally, in weight percent comprising: Al ≤ 0.2%, Cr ≤ 0.8%, Nb ≤ 0.06%, Ti ≤ 0.06%, B ≤ 0.005%, Mo ≤ 0.35%; the remainder of the composition being iron and unavoidable impurities from smelting; the steel part has a microstructure comprising, in surface fraction, - 95% or more of tempered martensite, and - bainite, austenite, and ferrite totaling 5% or less; and the press-hardened steel part has a tensile strength (TS) of 1000 MPa A press-hardened steel part having the above, a uniform elongation loss (ΔUEl) in the spot weld area of ​​25% or less, and a bending angle of 55° or more. Claim 2 delete Claim 3 In claim 1, a press-hardened steel part having a fracture deformation of 0.50 or more. Claim 4 In claim 1, the press-hardened steel part is a press-hardened steel part having a yield strength (YS) of 980 MPa or more. Claim 5 A method for manufacturing a press-hardened steel part, comprising the following consecutive steps: - providing a steel plate having a composition according to claim 1; - cutting the steel plate into a predetermined shape to obtain a steel blank; - heating the steel blank at a temperature T between 810°C and 960°C. HF Heating with, and the above T HF Holding time t at temperature from 5 s to 1200 s HF A step of obtaining a heated steel blank by maintaining it for a period of time; a step of transferring the heated steel blank to a forming press; a step of hot forming the heated steel blank in the forming press to obtain a steel part; a step of die-quenching the steel part until it reaches a temperature of 200°C or lower; a step of the steel part at a temperature T between 390°C and 510°C temp Reheat with, and the above T temp Holding time t at temperature from 1 s to 1000 s temp A method for manufacturing a press-hardened steel part comprising the steps of: maintaining for a period of time to obtain a tempered steel part; and cooling the tempered steel part to room temperature, wherein the press-hardened steel part has a tensile strength (TS) of 1000 MPa or more, a uniform elongation loss (ΔUEl) of 25% or less in the spot weld area, and a bending angle of 55° or more. Claim 6 In claim 5, the steel plate comprises the following continuous steps: - casting steel having the composition according to claim 1 to obtain a slab; - the slab at a temperature T between 1100°C and 1300°C. reheat A step of reheating; - a step of hot rolling the reheated slab at a finishing hot rolling temperature of 800°C to 950°C to obtain a hot-rolled steel sheet; - a step of coiling the hot-rolled steel sheet at a coiling temperature T of less than 670°C. coil A step of coiling to obtain a coiled steel plate, - optionally a step of pickling the coiled steel plate, - optionally a step of heating the hot-rolled steel plate at a temperature T of 500℃ to 750℃. HBA Heating, and holding time t from 300 s to 50 h HBA During the above T HBA A step of maintaining at a temperature, - a step of cold rolling the steel plate to obtain a cold-rolled steel plate, - optionally annealing the cold-rolled steel plate at an annealing temperature T of 650℃ to 900℃. A Heating with, and the above temperature T A A retention time t of 10 s to 1200 s A A method for manufacturing a press-hardened steel part, produced by the steps of: maintaining for a period of time to obtain an annealed steel sheet; and cooling the steel sheet to room temperature. Claim 7 In claim 5, the steel plate comprises the following continuous steps: - casting steel having the composition according to claim 1 to obtain a slab; - the slab at a temperature T between 1100°C and 1300°C. reheat A step of reheating; - a step of hot rolling the reheated slab at a finishing hot rolling temperature of 800°C to 950°C to obtain a hot-rolled steel sheet; - a step of coiling the hot-rolled steel sheet at a coiling temperature T of less than 670°C. coil A step of coiling to obtain a coiled steel plate, - optionally a step of pickling the coiled steel plate, - optionally a step of heating the hot-rolled steel plate at a temperature T of 500℃ to 750℃. HBA Heating, and holding time t from 300 s to 50 h HBA During the above T HBA A step of maintaining at a temperature, - a step of cold rolling the steel plate to obtain a cold-rolled steel plate, - optionally annealing the cold-rolled steel plate at an annealing temperature T of 500℃ to 750℃. A Heating with, and the above temperature T A A retention time t of 300 s to 50 h A A method for manufacturing a press-hardened steel part, produced by the steps of: maintaining for a period of time to obtain an annealed steel sheet; and cooling the steel sheet to room temperature. Claim 8 In claim 6, the method for manufacturing a press-hardened steel part wherein the annealed steel sheet is coated with aluminum or an aluminum alloy coating or zinc or a zinc alloy coating.

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