High toughness press hardened steel parts and manufacturing method thereof

Abstract

The present invention provides steel plates and press-hardened steel parts having a composition, by weight percent, of 0.05-0.4% C, 0.5-4% Mn, 0.1-1.3% Si, 0.01-0.1% Al, 0.01-1.0% Cr, 0.01-0.1% Ti, 0.0005-0.08% B, 0.05-0.4% Cu, 0.002-0.1% Sn, P≦0.020%, S≦0.010%, and N≦0.02%, with the remainder being iron and unavoidable impurities resulting from the smelting process and depending on the process route. The press-hardened steel parts have a microstructure containing more than 95% martensite at the surface, with the remainder being optional bainite and retained austenite.

Description

[Technical Field]

[0001] The present invention relates to high strength press hardened steel parts having high toughness. [Background technology]

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

[0003] In such applications, it is desirable to manufacture steel parts that combine high mechanical strength, high impact resistance, and good corrosion resistance. Furthermore, one of the major challenges in the automotive industry is to reduce the weight of vehicles in order to improve fuel efficiency from the perspective of protecting the global environment, without neglecting safety requirements, including those in harsh environments.

[0004] This weight reduction can be achieved in particular by the use of steel components with a martensite or bainite / martensite microstructure.

[0005] WO2016163469 relates to a martensitic heat-treated steel plate member having good scale characteristics, high yield strength, and excellent toughness. This steel part has a toughness of 35 J / cm2 measured in a Charpy impact test at -80°C. 2 (0.35J / mm 2 ) is considered to have excellent toughness. 2 (0.55J / mm 2 No steel component will ever reach a toughness value greater than 100%. This can result in components failing at high stresses. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. 2016 / 163469 Summary of the Invention

[0007] Therefore, the object of the present invention is to solve the above-mentioned problems and to provide a steel sheet having an average Charpy impact energy value of 0.90 J / mm, calculated as the average of the Charpy impact energy values ​​measured at 20°C, -40°C, -60°C and -80°C. 2 The above provides a press hardened steel part with high toughness.

[0008] Preferably, the press hardened steel part according to the invention has a Charpy impact energy of 0.75 J / mm measured at -80°C. 2 That's all.

[0009] Preferably, press hardened steel parts according to the present invention have a ductility loss Δ between the Charpy impact energy measured at 20°C and the Charpy impact energy measured at -80°C of less than 25%.

[0010] Another object of the present invention is to obtain a steel sheet that can be transformed into such press-hardened steel parts by hot forming. [Means for solving the problem]

[0011] The object of the present invention is achieved by providing a steel sheet as set forth in claim 1. Another object is achieved by providing a steel part as set forth in claim 2. The steel part may also include the properties set forth in any one of claims 3 to 5. Another object is achieved by providing a method as set forth in claim 6. Another object is achieved by providing a method as set forth in claim 7. DETAILED DESCRIPTION OF THE INVENTION

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

[0013] Next, the composition of the steel sheet according to the present invention will be explained, and the contents are expressed in weight percent (wt%).

[0014] According to the present invention, the carbon content is 0.05% to 0.4% to ensure satisfactory strength. If the carbon content exceeds 0.4%, the weldability and bendability of the steel may be reduced. If the carbon content is less than 0.05%, the tensile strength is too low.

[0015] The manganese content is 0.5% to 4%. If the addition amount exceeds 4%, the risk of center segregation increases and toughness is impaired. If it is less than 0.5%, the hardenability of the steel decreases. Preferably, the manganese content is 0.8% to 2%, more preferably 0.8% to 1.6%.

[0016] According to the present invention, the silicon content is 0.1% to 1.3%. Silicon is an element involved in hardening in solid solution. Silicon is added to limit the formation of carbides. Above 1.3%, silicon is detrimental to toughness. Furthermore, silicon oxide is formed on the surface, which can impair the coatability of the steel and reduce the weldability of the steel sheet and steel parts. Preferably, the silicon content is 0.1% to 1%, more preferably 0.1% to 0.5%, and even more preferably 0.1% to 0.4%.

[0017] The aluminum content is 0.01% to 0.1% because it is a very effective element for deoxidizing steel in the liquid phase during smelting. When the titanium content is insufficient, aluminum can protect boron. The aluminum content is less than 0.1% to avoid oxidation problems and the formation of ferrite during press hardening. Preferably, the aluminum content is 0.01% to 0.05%.

[0018] According to the present invention, the chromium content is 0.01% to 1.0%. Chromium is an element that contributes to the hardenability of the steel sheet and must be higher than 0.01%. The chromium content is less than 1.0% to limit workability issues and costs.

[0019] According to the invention, the boron content is between 0.0005% and 0.08%. Boron improves the hardenability of the steel. To avoid the risk of slab breakage during continuous casting, the boron content is not more than 0.08%.

[0020] To protect boron from the formation of BN, the titanium content is 0.01% to 0.1%. To avoid the formation of TiN, the titanium content is limited to 0.1%. In a preferred embodiment, Ti / N>3.42 for boron protection.

[0021] According to the present invention, the copper content is 0.05-0.4% to increase the toughness of the steel part. To limit the risk of hot shorts that could weaken the slab, the copper content is limited to 0.4%. Preferably, the copper content is 0.05-0.25%, more preferably 0.07-0.25%. More preferably, the copper content is 0.08-25%, even more preferably 0.08-0.20%. More preferably, the copper content is 0.08-0.18%.

[0022] The tin content is 0.002% to 0.1% to improve the hardenability of the steel. Above 0.1%, tin may increase the risk of hot short circuits and limit the workability of the slab. Preferably, the tin content is 0.002% to 0.050%.

[0023] Preferably, the total content of copper and tin is 0.08% to 0.3%, more preferably 0.09% to 0.3%, and even more preferably 0.1% to 0.3%.

[0024] Some elements can be optionally added.

[0025] Nickel may be added in amounts up to 0.4% to limit hydrogen uptake in the steel during its manufacture and limit the risk of delayed fracture due to hydrogen embrittlement.

[0026] The nickel content is considered a residual element up to 0.020%. Preferably, when added, the nickel content is max 0.1%, more preferably max 0.05%.

[0027] Molybdenum content can be optionally added up to 0.40%. Like boron, molybdenum improves the hardenability of steel. Molybdenum is limited to 0.40% or less to limit costs.

[0028] Niobium can optionally be added up to a maximum of 0.08% to improve the ductility of the steel. Additions above 0.08% increase the risk of forming NbC or Nb(C,N) carbides, which impairs bendability. Preferably, the niobium content is not more than 0.05%.

[0029] Calcium may also be added as an optional element, up to 0.1%, preferably at a minimum of 0.0001%. The addition of calcium at the liquid stage allows for the formation of fine oxides that promote castability in continuous casting. Furthermore, calcium can help limit the formation of harmful MnS by promoting the formation of CaO-CaS.

[0030] The remainder of the steel composition is iron and unavoidable impurities resulting from the smelting process and depending on the process route. In the case of the production route using a blast furnace, the level of unavoidable impurities is very low. In the case of the production route using an electric arc furnace charged with scrap, the steel plate may further contain residual elements derived from such scrap, such as antimony, arsenic and lead, up to 0.03%, which are considered unavoidable impurities.

[0031] P, S and N are also unavoidable impurities, regardless of the processing route. Their contents are limited to 0.010% or less for S, 0.020% or less for P and 0.02% or less for N.

[0032] In a particular embodiment, the steel sheet comprises the following, expressed in weight percent: C: 0.062 to 0.095% Mn: 1.4 to 1.9% Si: 0.2 to 0.5% Al: 0.020 to 0.070% Cr: 0.02 to 0.1% It has a chemical composition including the element 1.5%≦(C+Mn+Si+Cr)≦2.7, Nb: 0.040 to 0.060% Ti: 0.01 to 0.1% B: 0.0005 to 0.004% Cu: 0.05 to 0.4% Sn: 0.002 to 0.1% S≦0.003% P≦0.020% N≦0.009% and optionally, in weight percent, the following elements: 0.0001≦Ca≦0.003% and The remainder of the composition is iron and unavoidable impurities resulting from the smelting process and depending on the processing route.

[0033] In another particular embodiment, the steel sheet comprises the following, expressed in weight percent: C: 0.15 to 0.4% Mn: 0.5 to 3% Si: 0.1 to 0.5% Cr: 0.01 to 1% Ti 0.01~0.1% Al 0.01 to 0.1% B: 0.0005 to 0.08% Cu: 0.05 to 0.4% Sn: 0.002 to 0.1% S≦0.010% P≦0.020% N≦0.02% It has a chemical composition including the element The remainder of the composition is iron and unavoidable impurities resulting from the smelting process and depending on the processing route.

[0034] In another particular embodiment, the steel sheet comprises the following, expressed in weight percent: C: 0.15 to 0.25% Mn: 0.5 to 1.8% Si: 0.1 to 1.25% Cr 0.1-1% Ti 0.01~0.1% Al 0.01 to 0.1% B: 0.001 to 0.004% Cu: 0.05 to 0.4% Sn: 0.002 to 0.1% S≦0.010% P≦0.020% N≦0.02% and optionally, in weight percent, the following elements: Mo≦0.40% Nb≦0.08% Ca≦0.1% and The remainder of the composition is iron and unavoidable impurities resulting from the smelting process and depending on the processing route.

[0035] In another particular embodiment, the steel sheet comprises the following, expressed in weight percent: C: 0.24 to 0.4% Mn: 0.5 to 3% Si: 0.10 to 1.3% Al: 0.015 to 0.070 Cr: 0.1 to 1.0% Ni: 0.25 to 0.4% Nb: 0 to 0.060% B: 0.0005 to 0.0040 Cu: 0.05 to 0.4% Sn: 0.002 to 0.1% 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% It has a chemical composition including the element Optionally, in weight percent, the following elements: Mo: 0.05 and 0.40% Ca 0.0005 and 0.005% and The remainder of the composition is iron and unavoidable impurities resulting from the smelting process and depending on the processing route.

[0036] In another particular embodiment, the steel sheet comprises the following, expressed in weight percent: C: 0.3 to 0.4% Mn: 0.5 to 1.0% Si: 0.4 to 0.8% Cr: 0.1 to 1.0% Mo: 0.1 to 0.4% Nb: 0.01 to 0.08% Al: 0.01 to 0.1% Cu: 0.05 to 0.4% Sn: 0.002 to 0.1% Ti: 0.01 to 0.03% B: 0.0005 to 0.003% P≦0.020% Ca≦0.0010% S≦0.010% N≦0.02% and optionally having a chemical composition comprising the elements Ni<0.4% The remainder of the composition is iron and unavoidable impurities resulting from the smelting process and depending on the process route.

[0037] The steel sheet according to the invention can be manufactured by any suitable manufacturing method, which can be determined by a person skilled in the art. However, it is preferable to use a method according to the invention, which comprises the following steps:

[0038] The above-mentioned steel composition is provided in a semi-finished product which can be further hot rolled. Such a semi-finished product can be, for example, a slab.

[0039] The semi-finished product is obtained by casting liquid steel, which can be produced by a steelmaking process using, for example, the Basic Oxygen Furnace (BOF) route. In the BOF route, hot metal or pig iron, obtained, for example, in a blast furnace or smelting furnace, is decarburized to form liquid steel. Optionally, iron scrap containing elements such as copper, nickel, chromium, molybdenum, tin, arsenic, antimony, or lead is charged into the furnace together with the hot metal or pig iron. Direct reduced iron (DRI) can also be charged.

[0040] Liquid steel can also be produced in an electric arc furnace (EAF) by melting scrap iron to produce liquid steel directly. DRI can also be charged with scrap iron in the EAF.

[0041] The semi-finished product is heated to a temperature of 1100°C to 1300°C. The steel sheet is then hot rolled at a finish hot rolling temperature (FRT) of 830°C to 950°C. Preferably, the FRT is comprised between 850°C and 950°C. The hot rolled steel is then cooled and coiled at a temperature below 670°C, and optionally pickled to remove oxidation.

[0042] In one preferred embodiment of the present invention, the hot-rolled steel sheet is then cooled to room temperature.

[0043] In another preferred embodiment, the hot-rolled steel sheet is subjected to an annealing temperature T of 700°C to 850°C. A and annealed for a holding time t of 10 to 1200 seconds. A over the annealing temperature T A The material is maintained at 400° C., 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.

[0044] In another preferred embodiment of the present invention, the hot-rolled steel sheet is cold-rolled and annealed at an annealing temperature T A and annealed for a holding time t of 10 to 1200 seconds. A over the annealing temperature T AThe material is maintained at 400° C., 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.

[0045] In another preferred embodiment of the present invention, the hot-rolled steel sheet is cold-rolled and annealed at an annealing temperature T A The holding time t is 300 seconds to 80 hours to obtain annealed steel sheet. A over the annealing temperature T A is maintained.

[0046] The microstructure of the steel sheet according to the present invention contains ferrite at a surface fraction of 50% or more, with the remainder being pearlite or cementite.

[0047] The steel part according to the invention can be manufactured by any suitable manufacturing method, which can be determined by a person skilled in the art. However, it is preferable to use a method according to the invention, which comprises the following steps:

[0048] A steel plate having the above chemical composition and microstructure is prepared and cut into a predetermined shape to obtain a steel blank.

[0049] The steel blank is then heated to a temperature T1 of 800°C to 980°C and maintained 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 then die quenched.

[0050] Next, the microstructure of the steel part according to the present invention will be described.

[0051] During heating of a steel blank cut from a steel plate, all microstructural elements transform to austenite. The heated blank is then transferred to a forming press and hot formed. After hot forming, the steel part is then die-quenched, and the austenite transforms to greater than 95% martensite, with the remainder being optional bainite and retained austenite. Preferably, the microstructure comprises greater than 98% martensite, with the remainder being optional bainite and retained austenite.

[0052] The press hardened steel part according to the present invention has a Charpy impact energy of 0.90 J / mm, calculated as the average of the Charpy impact energy values ​​measured at 20°C, -40°C, -60°C and -80°C. 2 The average Charpy impact energy is equal to or greater than 0.95 J / mm 2 That's all.

[0053] The toughness is measured by Charpy impact energy at 20°C, 40°C, 60 and 80°C according to ISO Standards 148 and 1:2006(F) and ISO Standards 148 and 1:2017(F).

[0054] Preferably, the press hardened steel parts have a hardness of 0.75 J / mm at -80°C. 2 It has a higher Charpy impact energy.

[0055] Preferably, press hardened steel parts according to the present invention have a ductility loss Δ between the Charpy impact energy measured at 20°C and the Charpy impact energy measured at -80°C of less than 25%.

[0056] Preferably, the press-hardened steel part has a tensile strength TS of at least 950 MPa. More preferably, the press-hardened steel part has a TS of at least 1350 MPa. TS is measured according to standard ISO 6892-1.

[0057] In a preferred embodiment of the present invention, a martensitic steel sheet can be produced by a method comprising the following steps: preparing a hot-rolled steel sheet having the above-mentioned chemical composition, optionally annealing it to a temperature T of 500°C to 750°C, maintaining it at said annealing temperature for a holding time t of 300 seconds to 80 hours, and optionally cold-rolling it. The steel sheet is then annealed to a temperature T1 of 800°C to 980°C for a holding time 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, maintained at said temperature for a holding time of 1 second to 600 seconds, and then cooled to room temperature to obtain a martensitic steel sheet having a microstructure comprising more than 95% martensite, with the remainder being optional bainite and retained austenite.

[0058] Preferably, the martensitic steel sheet has an average Charpy impact energy value, calculated as the average of the Charpy impact energy values ​​measured at 20°C, -40°C, -60°C and -80°C, of ​​0.90 J / mm 2 Preferably, this average value is 0.95 J / mm 2 That's all.

[0059] Preferably, the martensitic steel sheet has a thermal resistance of 0.75 J / mm at -80°C. 2 It has a higher Charpy impact energy.

[0060] Preferably, the martensitic steel sheet has a ductility loss Δ of less than 25% between the Charpy impact energy measured at 20°C and the Charpy impact energy measured at -80°C.

[0061] Preferably, the martensitic steel sheet has a tensile strength TS of 950 MPa or more, and more preferably, the martensitic steel sheet has a TS of 1350 MPa or more.

[0062] The present invention will now be illustrated by the following examples, which should not be construed as limiting in any way. [Example]

[0063] Five 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 processing parameters summarized in Table 3.

[0064] Table 1 - Composition The compositions tested are summarized in the table below, with elemental contents expressed as weight percent (wt%).

[0065] [Table 1] Steels A to B are according to the invention, and C to E are references. Underlined values: not corresponding to the present invention

[0066] Table 2 - Microstructure of steel plate The cast steel semi-finished products were reheated at 1200° C., hot rolled at a finish hot rolling temperature of 890° C. and coiled at 550° C. The microstructure of the steel sheets, expressed in terms of surface fraction, is summarized in the table below.

[0067] [Table 2]

[0068] The surface fraction is determined by the following method: specimens are cut from the steel plate, polished and etched with reagents known per se to expose the microstructure, after which the sections are examined optically.

[0069] Table 3 - Processing parameters The steel plate was then cut to obtain steel blanks, which were heated to a temperature T1, maintained at said temperature for a dwell time t1 and hot formed. The following specific conditions were applied:

[0070] [Table 3] Underlined values: not corresponding to the present invention

[0071] The steel parts were analyzed and the corresponding microstructures are summarized in Table 4. The mechanical properties are summarized in Table 5.

[0072] Table 4 - Microstructure of press-hardened steel parts

[0073] [Table 4]

[0074] The surface fraction is determined by the following method: a specimen is cut from the press-hardened steel part, polished, and etched with reagents known per se to expose the microstructure, after which the section is examined by optical microscopy or by a scanning electron microscope, for example a scanning electron microscope equipped with a field emission electron gun ("FEG-SEM") at a magnification of more than 5000 times, coupled to an EBSD (Electron Back Scattered Diffraction) device.

[0075] Table 5 - Mechanical properties of press-hardened steel parts The toughness of the part is T 試験 The toughness values ​​were measured by Charpy impact tests at four temperatures: 20°C, -40°C, -60°C, and -80°C, and are summarized in the table below. The average Charpy impact energy value is calculated by averaging the four toughness values. The ductility loss Δ between 20°C and -80°C is calculated by the difference between the Charpy impact energy measured at 20°C and the Charpy impact energy measured at -80°C.

[0076] [Table 5] Underlined values: do not match target values

[0077] The examples show that the steel parts according to the invention, namely Tests 1 and 2, are the only ones that exhibit the targeted properties thanks to their specific composition and microstructure.

[0078] The steel part of Test 3 has a similar chemical composition to the steel part of Test 1, except for a lower level of copper. At the same Charpy test temperature, the toughness of Test 3 is much lower than that of Test 1. The lower the temperature at which the Charpy impact is measured, the greater the difference in toughness between the two tests. This can be evidenced by the value of the average Charpy impact energy, calculated by averaging the four toughness values. The higher the average value, the higher the toughness.

[0079] Furthermore, the copper content according to the invention allows the ductile-brittle transition temperature (DBTT) to be shifted to a lower temperature. This DBTT is the temperature at which a ductile material becomes brittle. This DBTT shift can be evidenced by the value of the average Charpy impact energy. The higher the average value, the lower the DBTT shift.

[0080] Tests 4 and 5 involve steel parts with low levels of copper. The average Charpy impact energy was 0.90 J / mm 2 Less than this means that the steel part has low toughness and a high DBTT.

Claims

1. A steel plate made of steel, in weight percent: C: 0.05-0.4% Mn: 0.5 to 4% Si: 0.1-1.3% Al: 0.01~0.1% Cr:0.01~1.0% B: 0.0005-0.08% Ti: 0.01~0.1% Cu: 0.05-0.25% Sn: 0.002-0.1% Cu+Sn: 0.08% to 0.3% P≦0.020% S≦0.010% N≦0.02% and optionally, in weight percent, the following elements: Ni≦0.4% Mo≦0.40% Nb≦0.08% Ca≦0.1% wherein the remainder of the composition is iron and unavoidable impurities resulting from refining, and the steel sheet has a microstructure containing 50% or more ferrite in terms of surface fraction, and the remainder is pearlite or cementite.

2. 10. A press-hardened steel part made from the steel of claim 1, wherein the steel part has a microstructure comprising a surface fraction of greater than 95% martensite, the remainder being optional bainite and retained austenite.

3. 0.90 J / mm, calculated as the average of the Charpy impact energies measured at 20°C, -40°C, -60°C and -80°C ２ 3. The press hardened steel part of claim 2 having an average Charpy impact energy value of at least 100 kJ / cm.

4. 0.75 J / mm ２ 4. The press-hardened steel part of claim 2 or 3, having a Charpy impact energy measured at -80°C higher than that of the steel part.

5. 4. The press hardened steel part of claim 2 or 3, having a ductility loss Δ between the Charpy impact energy measured at 20°C and the Charpy impact energy measured at -80°C that is less than 25%.

6. A method for producing a press-hardened steel part according to claim 2, comprising the following successive steps: Providing a steel sheet 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 １ and a residence time t １ During this time, the T １ maintaining the temperature to obtain a heated steel blank; transferring the heated steel blank to a forming press; hot forming the heated steel blank in the forming press to obtain a formed part; and die-quenching the molded part.

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