Press-hardened steel part with high toughness

Abstract

Steel sheet made from a steel having a composition comprising in weight percent: C: 0.05-0.3% Mn: 0.5-4% Si: 0.24-1.7% Al: 0.01-0.1% Cr: 0.01-1.0% B: 0.0005-0.08% Ti: 0.01-0.1% Cu: 0.05-0.4% P ≤ 0.020% S ≤ 0.010% N ≤ 0.02% and optionally comprising one or more of the following elements in weight percent: Sn ≤ 0.1% Ni ≤ 0.4% Mo ≤ 0.40% Nb ≤ 0.08% Ca ≤ 0.1% wherein the remainder of the composition consists of iron and unavoidable impurities resulting from melting, wherein the steel sheet has a structure comprising, in area proportions, 50% or more of ferrite, the remainder being pearlite or cementite.

Description

[0001] The present invention relates to a high-strength press-hardened steel part with high toughness.

[0002] High-strength press-hardened parts can be used as structural elements in motor vehicles for anti-burglary or energy absorption functions.

[0003] For these 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 biggest challenges in the automotive industry is reducing vehicle weight to improve fuel efficiency and protect the global environment, while maintaining safety requirements, even under the harshest conditions.

[0004] This weight reduction can be achieved in particular by using steel parts with martensitic or bainitic / martensitic microstructure.

[0005] Publication WO2016163469 relates to a martensitic heat-treated steel sheet element that exhibits good scaling properties and a high yield strength, as well as excellent toughness. Steel parts with a toughness of more than 35 J / cm 2 (0.35 J / mm 2 ), measured in the Charpy impact test at -80 °C, are considered particularly tough. However, none of the steel parts achieves a toughness value of more than 55 J / mm 2 (0.55 J / mm 2 ), which could lead to parts that break under high loads.

[0006] The purpose of the invention is therefore to solve the above-mentioned problem and to provide a press-hardened steel part which has a high toughness, with average Charpy notched impact strengths, calculated as the average of the Charpy notched impact strengths measured at 20 °C, -40 °C, -60 °C and -80 °C, greater than or equal to 0.90 J / mm 2 .

[0007] Preferably, the press-hardened steel part according to the invention has a Charpy notched impact strength, measured at -80°C, of ​​0.75 J / mm 2 or more.

[0008] Preferably, the press-hardened steel part according to the invention has a ductility loss Δ between the Charpy notched impact strength measured at 20 °C and the Charpy notched impact strength measured at -80 °C of less than 25%.

[0009] A further object of the invention is to obtain a steel sheet which can be converted into such a press-hardened steel part by hot forming.

[0010] The purpose of the present invention is achieved by providing a steel sheet according to claim 1. A further object is achieved by providing a steel part according to claim 2. The steel part may also comprise features according to any one of claims 3 to 5.

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

[0012] The composition of the steel sheet according to the invention is described below, wherein the content is given in weight percent (wt%).

[0013] According to the invention, the carbon content is 0.05% to 0.3% to ensure sufficient strength. A carbon content above 0.3% may impair the weldability and bendability of the steel. A carbon content below 0.05% results in insufficient tensile strength.

[0014] The manganese content is 0.5% to 4%. Adding more than 4% increases the risk of central segregation, which impairs toughness. Below 0.5%, the hardenability of the steel is reduced. The manganese content is preferably 0.8% to 2%, more preferably 0.8% to 1.6%.

[0015] According to the invention, the silicon content is 0.24% to 1.7%. Silicon is an element involved in solid-solution hardening. Silicon is added to limit the formation of carbides. More than 1.7% silicon is detrimental to toughness. In addition, silicon oxides form on the surface, which impair the coatability of the steel, and the weldability of the steel sheet and steel part may be reduced. The silicon content is preferably 0.24% to 1%, more preferably 0.24% to 0.5%, even more preferably 0.24% to 0.4%.

[0016] The aluminum content is 0.01% to 0.1% because aluminum is a very effective element for deoxidizing steel in the liquid phase during processing. Aluminum can protect boron when the titanium content is insufficient. The aluminum content is less than 0.1% to avoid oxidation problems and the formation of ferrite during press hardening. The aluminum content is preferably 0.01% to 0.05%.

[0017] According to the invention, the chromium content is 0.01% to 1.0%. Chromium is an element involved in the hardenability of the steel sheet and must be more than 0.01%. The chromium content is below 1.0% to limit processability issues and reduce costs.

[0018] According to the invention, the boron content is between 0.0005% and 0.08%. Boron improves the hardenability of the steel. The boron content is not greater than 0.08% to avoid the risk of slab breakage during continuous casting.

[0019] The titanium content is 0.01% to 0.1% to protect the boron from the formation of BN. The titanium content is limited to 0.1% to prevent the formation of TiN. In a preferred embodiment, Ti / N is >3.42 for boron protection. Preferably, the tin content is 0.02% to 0.05%.

[0020] According to the invention, the copper content is 0.05 to 0.4% to increase the toughness of the steel part. The copper content is limited to 0.4% to limit the risk of hot cracking, which could weaken the slab. Preferably, the copper content is 0.05 to 0.25%, more preferably 0.07 to 0.25%. Preferably, the copper content is 0.08 to 0.25%, even more preferably 0.08 to 0.20%. Preferably, the copper content is 0.08 to 0.18%.

[0021] Some elements can be added optionally.

[0022] Tin can be added up to 0.1% to improve the hardenability of the steel. At levels above 0.1%, tin can increase the risk of hot cracking and limit the processability of slabs.

[0023] Preferably the sum of the copper and tin content is 0.08% to 0.3%.

[0024] Nickel can be added up to 0.4% to limit the hydrogen absorption of the steel during its production and reduce the risk of delayed fracture due to hydrogen embrittlement.

[0025] The nickel content is considered a residual element up to 0.020%. Preferably, the nickel content, if added, is up to 0.1%, more preferably up to 0.05%.

[0026] Optionally, molybdenum can be added up to 0.40%. Like boron, molybdenum improves the hardenability of the steel. The molybdenum content is limited to 0.40% to limit costs.

[0027] 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 NbC or Nb(C,N) carbides forming, which adversely affects bendability. The niobium content is preferably less than or equal to 0.05%.

[0028] Calcium can also be added as an optional element up to 0.1%, and preferably in a minimum amount of 0.0001%. Adding Ca in the liquid phase allows for the formation of fine oxides that promote castability. Furthermore, calcium can help limit the formation of harmful MnS by promoting the formation of CaO-CaS.

[0029] The remainder of the steel's composition is iron and unavoidable impurities resulting from the smelting process and depending on the process route. In blast furnace production, the proportion of unavoidable impurities is very low. In arc furnace production loaded with scrap, the steel sheet may also contain residual elements from this scrap, such as antimony, arsenic, and lead, which amount to up to 0.03% and are considered unavoidable impurities.

[0030] P, S, and N are also part of the unavoidable impurities, regardless of the process route. Their content is less than or equal to 0.010% for S, less than or equal to 0.020% for P, and less than or equal to 0.02% for N.

[0031] In a particular embodiment, the steel sheet has a chemical composition comprising the following elements, expressed in weight %: C: 0.062-0.095% Mn: 1.4-1.9% Si: 0.24-0.5% Al: 0.020-0.070% Cr: 0.02-0.1% where 1.5% ≤ (C+Mn+Si+Cr) ≤ 2.7 Nb 0.040-0.060% Ti: 0.01-0.1% B: 0.0005-0.004% Cu: 0.05-0.4% S ≤ 0.003% P ≤ 0.020% N ≤ 0.009% and optionally comprising one or more of the following elements in weight percent: Sn: 0.002-0.1% 0.0001 ≤ Ca ≤ 0.003% the remainder of the composition being iron and unavoidable impurities resulting from the melting process and depending on the process route.

[0032] In another particular embodiment, the steel sheet has a chemical composition comprising the following elements, expressed in weight%: C: 0.15-0.3% Mn: 0.5-3% Si: 0.24-0.5% Cr 0.01-1% Ti 0.01-0.1% Al 0.01-0.1% B: 0.0005-0.08% Cu: 0.05-0.4% S ≤ 0.010% P ≤ 0.020% N ≤ 0.02% and optionally comprising one or more of the following elements in weight percent: Sn: 0.002-0.1% the remainder of the composition being iron and unavoidable impurities resulting from the melting process and depending on the process route.

[0033] In another particular embodiment, the steel sheet has a chemical composition comprising the following elements, expressed in weight%: C: 0.15-0.25% Mn: 0.5-1.8% Si: 0.24-1.25% Cr 0.1-1% Ti 0.01-0.1% Al 0.01-0.1% B: 0.001-0.004% Cu: 0.05-0.4% S ≤ 0.010% P ≤ 0.020% N ≤ 0.02% and optionally comprising one or more of the following elements in weight percent: Sn: 0.002-0.1% Mo ≤ 0.40% Nb ≤ 0.08% Ca ≤ 0.1% the remainder of the composition being iron and unavoidable impurities resulting from the melting process and depending on the process route.

[0034] In another particular embodiment, the steel sheet has a chemical composition comprising the following elements, expressed in weight%: C: 0.24-0.3% Mn: 0.5-3% Si: 0.24-1.7% Al: 0.015-0.070 Cr: 0.1-1.0% Ni: 0.25-0.4% Nb: 0 - 0.060% B: 0.0005-0.0040 Cu: 0.05-0.4% S ≤ 0.010% P ≤ 0.020% N ≤ 0.02% Ti 0.01-0.1% 2.6 + (Mn / 5.3) + (Cr / 13) + (Si / 15) ≥1.1% and optionally comprising one or more of the following elements in weight percent: Sn: 0.002-0.1% Mo: 0.05-0.40% Approx. 0.0005-0.005% the remainder of the composition being iron and unavoidable impurities resulting from the manufacturing process.

[0035] The steel sheet according to the invention can be produced by any suitable manufacturing method that can be defined by a person skilled in the art. However, it is preferred to use the method according to the invention, comprising the following steps: A semi-finished product that can be further hot-rolled, with the steel composition described above, is provided. Such a semi-finished product can be, for example, a slab.

[0036] The semi-finished product is obtained by pouring liquid steel, which can be produced in a steelmaking process, e.g., a blast furnace (BOF). In the BOF process, pig iron obtained, for example, in a blast furnace or a melting furnace is decarburized to convert it into liquid steel. Optionally, ferrous waste, comprising elements such as copper, nickel, chromium, molybdenum, tin, arsenic, antimony, or lead, is loaded into the furnace along with this pig iron. Direct reduced iron (DRI) can also be loaded.

[0037] Liquid steel can also be produced in an electric arc furnace (EAF) by melting iron scrap to directly produce liquid steel. DRI can also be loaded into the electrolytic furnace along with iron scrap. The semi-finished product is heated to a temperature of 1100 °C to 1300 °C.

[0038] The steel sheet is then hot rolled to a finishing hot rolling temperature (FRT) of 830 °C to 950 °C. Preferably, the FRT is from 850 °C to 950 °C, more preferably from 880 °C to 950 °C. The hot-rolled steel is then cooled and coiled at a temperature below 670 °C, and optionally pickled to remove oxidation.

[0039] In a preferred embodiment of the invention, the hot-rolled steel sheet is then cooled to room temperature.

[0040] In another preferred embodiment, the hot-rolled steel sheet is heated to an annealing temperature T A annealed from 700 °C to 850 °C and held for a holding time t A from 10 sec to 1200 sec at this annealing temperature T A held and optionally coated with an aluminum coating or an aluminum alloy coating or a zinc coating or a zinc alloy coating and cooled to room temperature.

[0041] In a further preferred embodiment of the invention, the hot-rolled steel sheet is cold-rolled and heated to an annealing temperature T A annealed from 700 °C to 850 °C and held for a holding time t A from 10 sec to 1200 sec at this annealing temperature T A held and optionally coated with an aluminum coating or an aluminum alloy coating or a zinc coating or a zinc alloy coating and cooled to room temperature.

[0042] In a further preferred embodiment of the invention, the hot-rolled steel sheet is cold-rolled and heated to an annealing temperature T A annealed from 500 °C to 750 °C and held for a holding time t A from 300 sec to 80 h at this annealing temperature T A held.

[0043] The structure of the steel sheet according to the invention consists of 50% or more of ferrite in area proportions, the rest being pearlite or cementite.

[0044] The steel part according to the invention can be manufactured by any suitable manufacturing method that can be defined by a person skilled in the art. However, it is preferred to use the method according to the invention, comprising the following steps: A steel sheet having the above-mentioned chemical composition and structure is provided and cut into a predetermined shape to obtain a steel blank.

[0045] The steel blank is then heated to a temperature T1 of 800 °C to 980 °C and held at this temperature T1 for a residence time t1 of 10 seconds to 900 seconds to obtain a heated steel blank. The heated steel blank is then transferred to a forming press and hot formed. After hot forming, the steel part is subsequently quenched in the press.

[0046] The structure of the steel part according to the invention will now be described.

[0047] When the steel blank cut from the steel sheet is heated, all structural elements are converted to austenite. The heated blank is then placed in a forming press and hot formed. After hot forming, the steel part is quenched in the press, during which more than 95% of the austenite is converted to martensite, with the remainder being optional bainite and retained austenite.

[0048] Preferably, the microstructure consists of more than 98% martensite, the remainder being optional bainite and retained austenite.

[0049] The press-hardened steel part according to the invention has an average Charpy impact strength, calculated as the average of the Charpy impact strengths measured at 20 °C, -40 °C, -60 °C and -80 °C, greater than or equal to 0.90 J / mm 2 Preferably, this average value is greater than or equal to 0.95 J / mm 2 .

[0050] Toughness is measured by Charpy impact strength at 20 °C, -40 °C, -60 °C, and -80 °C according to ISO 148-1:2006 (F) and ISO 148-1:2017 (F). Preferably, the press-hardened steel part has a Charpy impact strength at -80 °C of more than 0.75 J / mm 2 .

[0051] Preferably, the press-hardened steel part according to the invention has a ductility loss Δ between the Charpy notched impact strength measured at 20 °C and the Charpy notched impact strength measured at -80 °C of less than 25%.

[0052] Preferably, the press-hardened steel part has a tensile strength TS greater than or equal to 950 MPa. Preferably, the press-hardened steel part has a tensile strength TS greater than or equal to 1350 MPa. TS is measured according to ISO 6892-1.

[0053] In a preferred embodiment of the invention, a martensitic steel sheet can be produced by a process comprising the following steps: the hot-rolled steel sheet having the above chemical composition is provided and optionally annealed to a temperature T of 500°C to 750°C and held at this annealing temperature for a holding time t of 300 seconds to 80 hours, and optionally cold-rolled. The steel sheet is then annealed to a temperature T1 of 800°C to 980°C for t1 of 10 seconds to 900 seconds and cooled to below Ms. The steel sheet is optionally reheated to a temperature of 150 °C to 270 °C and held at that temperature for a holding time of 1 sec to 600 sec before being cooled to room temperature to obtain a martensitic steel sheet with a microstructure comprising more than 90% martensite, the remainder optionally being bainite and retained austenite.

[0054] Preferably, this martensitic steel sheet has an average Charpy impact strength, calculated as the average of the Charpy impact strengths measured at 20 °C, -40 °C, -60 °C and -80 °C, greater than or equal to 0.90 J / mm 2 Preferably, this average value is greater than or equal to 0.95 J / mm 2 .

[0055] Preferably, the martensitic steel sheet has a Charpy impact strength at -80 °C of more than 0.75 J / mm 2 on.

[0056] Preferably, the martensitic steel sheet has a ductility loss Δ between the Charpy notched impact strength measured at 20 °C and the Charpy notched impact strength measured at -80 °C of less than 25%.

[0057] Preferably, the martensitic steel sheet has a tensile strength TS greater than or equal to 950 MPa. More preferably, the martensitic steel sheet has a tensile strength TS greater than or equal to 1350 MPa.

[0058] The invention will now be illustrated by the following examples, which are in no way limiting. Example

[0059] Five alloys, the compositions of which are listed in Table 1, were cast into semi-finished products and processed into steel sheets and subsequently into steel parts, in compliance with the process parameters listed in Table 3. Table 1 - Compositions

[0060] The tested compositions are summarized in the following table, where the element contents are given in weight percent (wt%).

[0061] The steels AB are according to the invention, CE are comparative compositions. Underlined values: do not correspond to the inventionTable 2 - Structure of steel sheets

[0062] The cast steel semi-finished products were reheated at 1200 °C, hot-rolled to a final hot rolling temperature of 890 °C, and coiled at 550 °C. The microstructures of the steel sheets are summarized in the following table: Attempt Steel Ferrite (%) Perlite (%) 1 A 75 25 2 B 70 30 3 C 73 27 4 D 73 27 5 E 73 27

[0063] The surface area is determined using the following procedure: a sample is cut from the steel sheet, polished, and etched with a known reagent to visualize the microstructure. The sample is then visually examined. Table 3 - Process parameters

[0064] The steel sheets were then cut to obtain a steel blank, heated to a temperature T1, held at that temperature for a residence time t1, and hot-formed. The following specific conditions applied: Attempt Steel T1(°C) t1(s) 1 A 920 575 2 B 920 575 3 C 920 540 4 D 900 540 5 E 900 600 Underlined values: do not correspond to the invention

[0065] The steel parts were analyzed and the corresponding microstructure is summarized in Table 4. The mechanical properties are summarized in Table 5. Table 4 - Structure of the press-hardened steel part Attempt Martensite 1 100 % 2 100 % 3 100 % 4 100 % 5 100 % Underlined values: do not correspond to the invention

[0066] The surface areas are determined using the following procedure: a sample is cut from the press-hardened steel part, polished, and etched with a known reagent to visualize the microstructure. The sample is then examined with a light or scanning electron microscope, e.g., a field emission gun scanning electron microscope (“FEG-SEM”) at a magnification of over 5000x, coupled with an EBSD (electron backscattered diffraction) device. Table 5 - Mechanical properties of the press-hardened steel part

[0067] The toughness of the parts was determined in the Charpy impact test at four temperatures T test The impact strengths were measured at 20°C, -40°C, -60°C, and -80°C and are summarized in the table below. The average Charpy impact strength is calculated using the average of the four toughness values. The ductility loss Δ between 20°C and -80°C is calculated using the difference between the Charpy impact strength measured at 20°C and the Charpy impact strength measured at -80°C. Attempt T test (°C) Charpy value (J / mm 2 ) Average Charpy toughness (J / mm 2 ) Δ (%) TS (MPa) 1 20 1,06 1,00 19 1491 -40 1,08 -60 0,98 -80 0,87 2 20 1,09 0,98 24 1486 -40 1,00 -60 0,96 -80 0,85 3 20 1,04 0,86 45 1513 -40 0,92 -60 0,89 -80 0,59 4 20 0,91 0,71 49 1501 -40 0,84 -60 0,65 -80 0,42 5 20 1,01 0,87 29 1541 -40 0,93 -60 0,82 -80 3,72 Underlined values: do not match the target value

[0068] The examples show that the steel parts according to the invention, i.e. tests 1 and 2, are the only ones that have the desired properties thanks to their specific composition and structure.

[0069] The steel part from Test 3 has a similar chemical composition to the steel part from Test 1, except for a lower copper content. At the same Charpy test temperature, the toughness of Test 3 is significantly lower than that of Test 1. The lower the temperature at which the Charpy impact strength is measured, the greater the difference in toughness between the two tests. This is demonstrated by the average Charpy impact strength value, which is calculated from the average of the four toughness values. The higher the average, the greater the toughness.

[0070] Furthermore, the copper content according to the invention enables a shift in the ductile-brittle transition temperature (DBTT) to lower temperatures. This DBTT is the temperature at which a ductile material becomes brittle. This shift in the DBTT can be demonstrated by the average value of the Charpy notched impact strength. The higher the average value, the more the DBTT shifts to lower temperatures.

[0071] Tests 4 and 5 involve steel parts with low copper content. The average Charpy impact strength is below 0.90 J / mm 2 , which means that the toughness of the steel part is low and the DBTT is high. QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] WO 2016163469

[0005] Cited non-patent literature

[0000] ISO 6892-1

[0052]

Claims

[1] Steel sheet made from a steel having a composition comprising in weight percent: C: 0.05-0.3% Mn: 0.5-4% Si: 0.24-1.7% Al: 0.01-0.1% Cr: 0.01-1.0% B: 0.0005-0.08% Ti: 0.01-0.1% Cu: 0.05-0.4% P ≤ 0.020% S ≤ 0.010% N ≤ 0.02% and optionally comprising one or more of the following elements in weight percent: Sn ≤ 0.1% Ni ≤ 0.4% Mo ≤ 0.40% Nb ≤ 0.08% Ca ≤ 0.1% the remainder of the composition being iron and unavoidable impurities resulting from melting, the steel sheet having a structure comprising, in area proportions, 50% or more of ferrite, the remainder being pearlite or cementite. [2] A press-hardened steel part made from a steel according to claim 1, wherein the steel part has a microstructure comprising, in area proportions, more than 95% martensite, the remainder being optional bainite and retained austenite. [3] Press-hardened steel part according to claim 2, wherein the press-hardened steel part has an average Charpy notched impact strength calculated as the average of the Charpy notched impact strengths measured at 20 °C, -40 °C, -60 °C and -80 °C and greater than or equal to 0.90 J / mm 2 is. [4] Press-hardened steel part according to one of claims 2 to 3, wherein the press-hardened steel part has a Charpy impact strength measured at -80 °C of more than 0.75 J / mm 2 has. [5] Press-hardened steel part according to one of claims 2 to 4, wherein the press-hardened steel part has a ductility loss Δ between the Charpy notched impact strength measured at 20 °C and the Charpy notched impact strength measured at -80 °C of less than 25%.