High strength press hardened steel parts and manufacturing method thereof

Abstract

The present invention deals with a press hardened steel part having a composition, in weight percent, including 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 the following elements: Al≦0.2%, Cr≦0.8%, Nb≦0.06%, Ti≦0.06%, B≦0.005%, Mo≦0.35%, the balance of the composition being iron and unavoidable impurities resulting from smelting, the press hardened part having a microstructure including at least 95% tempered martensite and at least 5% bainite, austenite and ferrite in total at the surface fraction.

Description

[Technical Field]

[0001] The present invention relates to high strength press hardened steel parts with excellent bendability and weldability. [Background technology]

[0002] High strength press hardened parts can be used as structural elements in automobiles for anti-intrusion or energy absorbing functions.

[0003] In these types of applications, it is desirable to produce steel parts that combine high mechanical strength, high impact resistance and good corrosion resistance.

[0004] Furthermore, one of the major challenges in the automotive industry is to reduce the weight of vehicles to improve fuel efficiency in an environmentally friendly manner, without neglecting safety requirements.

[0005] This weight reduction can be achieved, inter alia, through the use of steel components with tempered martensite or bainite / martensite microstructures.

[0006] These types of parts can be welded, with car manufacturers specifying that the weld joint should not constitute the weakest point of the welded steel part.

[0007] In fact, the presence of spot welds on structural components of the vehicle body can lead to failure during a crash due to strain concentrations in the softened heat affected zone (HAZ). Summary of the Invention [Problem to be solved by the invention]

[0008] The object of the present invention is therefore to solve the above-mentioned problems and to provide a press-hardened steel part having a combination of high mechanical properties with a tensile strength TS of 1000 MPa or more, a uniform elongation loss ΔUE1 of 25% or less in the spot weld area and a bending angle of 55° or more.

[0009] Preferably, press hardened steel parts according to the present invention have a strain to failure of 0.50 or greater.

[0010] Preferably, the press hardened steel part according to the invention has a yield strength YS of 980 MPa or greater. [Means for solving the problem]

[0011] The object of the present invention is achieved by providing a steel part according to claim 1. The steel part may also include the features of any 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 of claims 6 to 8. DETAILED DESCRIPTION OF THE INVENTION

[0012] The present invention will now be described in detail and illustrated by way of examples, without introducing any limitations.

[0013] The composition of the steel of the present invention will now be described, with the contents expressed as weight percent.

[0014] According to the present invention, the carbon content is 0.2% to 0.34% to ensure satisfactory strength. If the carbon content exceeds 0.34%, the fracture strain and bending angle of the steel plate will not reach the target values. In addition, the weldability of the steel plate may be reduced. If the carbon content is less than 0.2%, the tensile strength and yield strength will not reach the target values.

[0015] The manganese content is 0.50% to 1.24%. Addition of more than 1.24% increases the risk of center segregation, which can impair bendability and reduce fracture strain. If the content is less than 0.50%, the hardenability of the steel sheet decreases, and the tensile strength and yield strength do not reach the target values.

[0016] The silicon content is 0.5% to 2%. Silicon is an element involved in hardening in solid solution. Silicon is added to limit the formation of carbides and ensure a high level of tensile strength. If the silicon content exceeds 2%, silicon oxide will form on the surface, impairing the coatability of the steel. There is also a risk of the weldability of the steel sheet being 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%.

[0017] Some elements can be optionally added.

[0018] The aluminum content is a very effective element for deoxidizing the steel in the liquid phase during refining, so 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.

[0019] Optionally, chromium content can be added up to 0.8% to improve hardening in solid solution. The chromium content is not more than 0.8% to limit workability issues and cost. Preferably, the chromium content is not more than 0.6%.

[0020] Niobium content can be optionally added up to 0.06% to refine the prior austenite grain size and improve the ductility of the steel, with additions above 0.06% increasing the risk of forming NbC or Nb(C,N) carbides, which impairs bendability.

[0021] 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%.

[0022] Boron content can be optionally added up to 0.005%. Boron improves the hardenability of the steel. The boron content is not more than 0.005% to avoid the risk of breaking the slab during continuous casting.

[0023] Molybdenum may be added optionally up to 0.35%. Like boron, molybdenum improves the hardenability of the steel. To limit costs, molybdenum is limited to 0.35%.

[0024] The remainder of the steel composition is iron and impurities resulting from smelting. In this respect, P, S and N are considered to be residual elements, at least unavoidable impurities. Their contents are limited to 0.020% for P, 0.010% for S and 0.010% for N.

[0025] The microstructure of the press hardened steel part according to the present invention will now be described.

[0026] Press-hardened steel parts have a microstructure containing 95% or more of tempered martensite at the surface. This tempered martensite is formed by heating the steel part to a temperature T in the range of 390°C to 510°C. temp Holding time t between 1 second and 1000 seconds temp It is formed during heating at

[0027] Some bainite, ferrite, and austenite may optionally be present, the sum of which is no more than 5% in surface fraction.

[0028] Preferably, the microstructure of the press hardened steel part is made up of 100% tempered martensite.

[0029] The press-hardened steel parts according to the invention can be produced by any suitable production method, which can be defined by a person skilled in the art. However, it is preferred to use a method according to the invention, which comprises the following steps:

[0030] A steel plate having a composition according to the present invention is prepared and cut into a predetermined shape to obtain a steel blank.

[0031] The steel blank is heated at a temperature T comprised between 810°C and 960°C, preferably between 850°C and 950°C, more preferably between 880°C and 950°C.HF The heating time is between 5 s and 1200 s to obtain a heated steel blank with a fully austenitic microstructure. HF During HF The heated steel blank is transferred to a forming press and hot formed to obtain the steel part.

[0032] The steel part is then die hardened to a temperature of up to 200°C.

[0033] Steel parts are subjected to temperatures T between 390°C and 510°C. temp and the holding time t is between 1 and 1000 seconds. temp During this time, the temperature T temp to obtain tempered steel parts and ensure temperature homogeneity on all steel parts.

[0034] Above 510°C, the tensile strength of the steel part decreases. Below 390°C, the uniform elongation loss ΔUE1 in the spot weld area exceeds 25%. The tempered steel part is then cooled to room temperature.

[0035] For each tempered product, the HAZ susceptibility is assessed by the uniform elongation loss of the welded JIS tensile specimen compared to the unwelded reference. The uniform elongation loss ΔUEl is calculated as follows:

[0036] The uniform elongation UE1 of steel is measured for a tensile test piece in accordance with JIS standard Z2241. A welding spot is made on the tensile test piece, centered on the deformation region of the test piece. The uniform elongation UE1 of this welded tensile test piece is w is measured according to JIS standard Z2241.

[0037] The uniform elongation loss ΔUEl can be calculated using the following formula: ΔUEl = [(UEl - UEl w ) / UEl]*100

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

[0039] A steel slab having the above composition is cast and heated to a temperature T in the range of 1100°C to 1300°C. reheat After being reheated to , the steel sheet is hot rolled at a finish hot rolling temperature in the range of 800°C to 950°C to obtain a hot rolled steel sheet.

[0040] Next, the hot-rolled steel sheet is heated to a temperature T lower than 670°C. coil and wind it up.

[0041] The hot rolled steel sheet may optionally be pickled to remove oxidation.

[0042] Hot-rolled steel sheets are optionally rolled at temperatures T included between 500°C and 750°C. HBA The temperature is heated to 300 seconds and the holding time is t HBA During HBA The temperature can be maintained.

[0043] Next, the steel sheet is cold-rolled to obtain a cold-rolled steel sheet. The cold-rolling reduction is preferably between 20% and 80%. If it is less than 20%, recrystallization during the subsequent heat treatment is unfavorable, which may impair the ductility of the steel sheet. If it exceeds 80%, edge cracking may occur during cold rolling.

[0044] The cold-rolled steel sheet is optionally annealed at an annealing temperature T comprised between 650 ° C and 900 ° C. A Annealing is performed for a holding time t between 10 seconds and 1200 seconds. A During the temperature T A The steel sheet is then heated to a temperature of 1000 K to obtain an annealed steel sheet, which reduces the tensile strength and makes the steel easier to cut. Finally, the steel sheet is cooled to room temperature.

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

[0046] In a second embodiment of the invention, a steel sheet prepared for manufacturing a steel part is manufactured by the following successive steps:

[0047] A steel slab having the composition according to the invention is cast at a temperature T comprised between 1100 ° C and 1300 ° C. reheat After being reheated to 800°C to 950°C, the steel sheet is hot rolled at a finish hot rolling temperature in the range of 800°C to 950°C to obtain a hot rolled steel sheet.

[0048] Next, the hot-rolled steel sheet is heated to a temperature T lower than 670°C. coil and wind it up.

[0049] The hot rolled steel sheet can be optionally pickled to remove oxidation. The hot rolled steel sheet can be optionally pickled at a temperature T HBA Heat to 300 seconds to 50 hours, t HBA Between the T HBA The temperature can be maintained.

[0050] The steel sheet is then cold-rolled to obtain a cold-rolled steel sheet. The cold-rolling reduction is preferably 20% to 80%. If the reduction is less than 20%, recrystallization during the subsequent heat treatment is unfavorable, which may impair the ductility of the steel sheet. If the reduction exceeds 80%, edge cracking may occur during cold rolling.

[0051] The cold-rolled steel sheet is optionally annealed at an annealing temperature T in the range of 500°C to 750°C. A Annealing is performed for a holding time of t between 300 seconds and 50 hours. A During the temperature T A The steel sheet is then heated to a temperature of 1000 K to obtain an annealed steel sheet, which reduces the tensile strength and makes the steel easier to cut. Finally, the steel sheet is cooled to room temperature.

[0052] The press hardened steel part according to the invention has a tensile strength TS of 1000 MPa or more, a uniform elongation loss ΔUE1 in the spot weld area of ​​25% or less, and a bend 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 greater.

[0054] In another preferred embodiment, the press hardened steel part according to the present invention has a strain to failure of 0.50 or greater.

[0055] The present invention is illustrated by the following examples, which are not intended to be limiting in any way. [Example]

[0056] Eight grades, whose compositions are summarized in Table 1, were cast into semi-finished products and processed into steel plates and then steel parts according to the process parameters summarized in Table 2.

[0057] The compositions tested are summarized in the table below, with elemental contents expressed as weight percent.

[0058] [Table 1]

[0059] The cast steel semi-finished products were reheated at 1250 °C, hot rolled at a finish hot rolling temperature comprised between 800 and 950 °C, coiled at 580 °C, and cold rolled with a reduction of 58%. The steel sheets were then rolled at a temperature T of 790 °C. A and hold for 180 seconds, t A During the temperature T A Maintain it.

[0060] The steel plate was cut into a predetermined shape to obtain a steel blank. Then, the steel blank was heated to a temperature T HF Hold time of 120 seconds until t HF The blank was then heated at 100°C and transferred to a forming press. The heated blank was hot formed in the forming press to obtain a steel part, which was then die-hardened until a temperature of 80°C was reached.

[0061] The steel parts are then heated to a temperature T tempReheat to 1000 s for a holding time of t temp Between the T temp The temperature was maintained and cooled to room temperature.

[0062] [Table 2]

[0063] The steel parts were analyzed and the corresponding microstructures and properties are summarized in Tables 3 and 4, respectively.

[0064] [Table 3]

[0065] The surface fraction is determined by the following method: specimens are cut from the press-hardened steel parts, polished, and etched with a reagent known per se, for example Nital reagent, to reveal the microstructure. The sections are then examined by optical or scanning electron microscopy, for example using a scanning electron microscope with a field emission gun ("FEG-SEM") at a magnification of more than 5000 times, coupled to an electron backscatter diffraction (EBSD) device. Tempered martensite can be distinguished from martensite due to its lower dislocation density compared to martensite.

[0066] <Table 4: Properties of steel parts> TS and YS are measured according to ISO standard ISO6892-1.

[0067] The bending angle was determined on press-hardened parts according to method VDA238-100 bending standard (normalized to a thickness of 1.5 mm).

[0068] The term fracture strain refers to the fracture strain criterion defined by Pascal Dietsch et al. in "Methodology to assess fracture during crash simulation: fracture strain criteria and their calibration" in Metallurgical Research Technology, Vol. 114, No. 6, 2017. The fracture strain is the equivalent strain in the material at the deformation point when the critical bending angle is reached. The fracture strain value is determined under plane strain conditions, which are the most severe conditions from the perspective of vehicle crashworthiness, and is obtained by finite element analysis.

[0069] [Table 4]

[0070] Weld spots are made on the tensile specimens, centered on the deformation zone of the specimen. The corresponding uniform elongation losses ΔUEl of the resistance spot welds are summarized in Table 5.

[0071] [Table 5]

[0072] As a result of their particular composition and process parameters used, the examples according to the invention, i.e., Examples 1-7, are the only ones that exhibit a combination of high mechanical properties with TS greater than 1000 MPa, bend angle greater than 55°, and uniform elongation loss less than 25%. Examples 1-7 also have a strain to break greater than 0.50.

[0073] The tempering temperatures applied to the steel parts in Trials 8 and 9 are too low to limit the deleterious effect of HAZ softening on uniform elongation, as indicated by the uniform elongation loss of greater than 25%.

[0074] Also, compared to Trial 2, which has the same steel composition, the lower tempering temperature of Trial 8 results in a higher uniform elongation loss and a lower strain to failure value than Trial 2.

[0075] The steel part in trial 10 was not tempered, which means a uniform elongation loss of more than 25%.

[0076] In Trial 11, the carbon content of the steel part was too high to achieve the target failure strain and bendability values.

Claims

1. 1. A press-hardened steel part comprising, in weight percent: C: 0.2-0.34% Mn: 0.50-1.24% Si: 0.5 to 2% P≦0.020% S≦0.010% N≦0.010% and optionally, in weight percent, the following elements: Al≦0.2% Cr≦0.8% Nb≦0.06% Ti≦0.06% B≦0.005% Mo≦0.35% The remainder of the composition is iron and unavoidable impurities resulting from smelting, and in a surface fraction: - 95% or more tempered martensite, and - Bainite, austenite and ferrite, the sum of which does not exceed 5% made of a steel having a microstructure including 1. A press-hardened steel part having a tensile strength TS of 1000 MPa or greater, a uniform elongation loss ΔUE1 in the spot weld area of ​​25% or less, and a bend angle of 55° or greater.

2. 10. The press hardened steel part of claim 1 having a strain to failure of 0.50 or greater.

3. 3. A press-hardened steel part according to claim 1 or 2, having a yield strength YS of 980 MPa or greater.

4. 1. A method of manufacturing a press hardened steel part, comprising the following successive steps: - providing a steel sheet having the composition according to claim 1, - cutting said steel plate into a predetermined shape to obtain a steel blank; - the steel blank is heated to a temperature T comprised between 810 ° C and 960 ° C ＨＦ and a holding time t included in 5 seconds to 1200 seconds ＨＦ During the T ＨＦ maintaining the temperature to obtain a heated steel blank; - transferring the heated blank to a forming press; - hot forming said heated blank in a forming press to obtain a steel part; - hardening the steel part to a temperature of not more than 200°C; - Temperature T included in 390℃ to 510℃ ｔｅｍｐ Reheat the steel part to a temperature of 1000° C. for a holding time t ｔｅｍｐ During the T ｔｅｍｐ maintaining the temperature to obtain a tempered steel part; - cooling the tempered steel part to room temperature Including, 1. A method for producing a press-hardened steel part, wherein the press-hardened steel part has a tensile strength TS of 1000 MPa or more, a uniform elongation loss ΔUE1 of 25% or less in the spot weld area, and a bend angle of 55° or more.

5. 5. The method of claim 4, wherein the steel sheet is subjected to the following successive steps: - casting steel to obtain slabs, said steel having a composition according to claim 1; - Temperature T included in the range of 1100°C to 1300°C ｒｅｈｅａｔ reheating the slab with - hot rolling the reheated slab at a finish hot rolling temperature comprised between 800°C and 950°C to obtain a hot rolled steel sheet; - The hot-rolled steel sheet is wound at a coiling temperature T ｃｏｉｌ and winding the steel sheet to obtain a wound steel sheet. - optionally pickling the coiled steel sheet; Optionally, the hot-rolled steel sheet is subjected to a temperature T comprised between 500°C and 750°C. ＨＢＡ and holding time t included in 300 seconds to 50 hours ＨＢＡ During the T ＨＢＡ maintaining at temperature; - cold rolling the steel sheet to obtain a cold-rolled steel sheet; Optionally, the cold-rolled steel sheet is subjected to an annealing temperature T comprised between 650 ° C and 900 ° C. Ａ and a holding time t included in 10 seconds to 1200 seconds Ａ The steel plate is heated to the temperature T Ａ and maintaining the temperature at 400° C. to obtain an annealed steel sheet. - cooling the steel plate to room temperature 1. A method for producing press-hardened steel parts, the method comprising:

6. 5. The method of claim 4, wherein the steel sheet is subjected to the following successive steps: - casting steel to obtain slabs, said steel having a composition according to claim 1; - Temperature T included in the range of 1100°C to 1300°C ｒｅｈｅａｔ reheating the slab with - hot rolling the reheated slab at a finish hot rolling temperature comprised between 800°C and 950°C to obtain a hot rolled steel sheet; - The hot-rolled steel sheet is wound at a coiling temperature T ｃｏｉｌ and winding the steel sheet to obtain a wound steel sheet. - optionally pickling the coiled steel sheet; Optionally, the hot-rolled steel sheet is subjected to a temperature T comprised between 500°C and 750°C. ＨＢＡ and holding time t included in 300 seconds to 50 hours ＨＢＡ During the T ＨＢＡ maintaining at temperature; - cold rolling the steel sheet to obtain a cold-rolled steel sheet; Optionally, the cold-rolled steel sheet is subjected to an annealing temperature T comprised between 500 ° C and 750 ° C. Ａ and holding time t included in 300 seconds to 50 hours Ａ The steel plate is heated to the temperature T Ａ and maintaining the temperature at 400° C. to obtain an annealed steel sheet. - cooling the steel plate to room temperature 1. A method for producing press-hardened steel parts, the method comprising:

7. The method for producing a press-hardened steel part according to claim 5, wherein the annealed steel sheet is coated with an aluminum coating or an aluminum alloy coating, or a zinc coating or a zinc alloy coating.

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