High-toughness press-hardened steel parts and their manufacturing method

Abstract

We provide press-hardened steel parts that exhibit excellent Charpy impact toughness at low temperatures. [Solution] The present invention deals with steel sheets and press-hardened steel parts having a composition by weight percent containing C0.05~0.4%, Mn0.5~4%, Si0.1~1.3%, Al0.01~0.1%, Cr0.01~1.0%, Ti0.01~0.1%, B0.0005~0.08%, Cu0.05~0.4%, Sn0.002~0.1%, P≦0.020%, S≦0.010%, and N≦0.02%, with the remainder of the composition being iron and unavoidable impurities arising from the smelting process and depending on the process route. The press-hardened steel parts have a microstructure containing more than 95% martensite by surface fraction, with the remainder being optional bainite and retained austenite.

Description

[Technical Field]

[0001] This 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 components in automobiles for intrusion prevention or energy absorption functions.

[0003] For such applications, it is desirable to manufacture steel parts that possess high mechanical strength, high impact resistance, and good corrosion resistance. Furthermore, one of the major challenges in the automotive industry is to reduce vehicle weight 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 using steel components having a martensite or bainite / martensite microstructure.

[0005] WO2016163469 relates to a martensitic heat-treated steel sheet component with good scale characteristics, high yield strength, and excellent toughness. This steel component has a toughness of 35 J / cm² as measured by a Charpy impact test at -80°C. 2 (0.35 J / mm 2 Those exceeding 55 J / cm² are considered to have superior toughness. Nevertheless, 55 J / cm² is considered to be the most durable. 2 (0.55 J / mm 2 No steel parts reach a toughness value exceeding ). This means that parts may fracture under high stress. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] International Publication No. 2016 / 163469 [Overview of the Initiative]

[0007] Therefore, the object of the present invention is to solve the above problems and to obtain an average Charpy impact value of 0.90 J / mm², calculated as the average of Charpy impact energy values ​​measured at 20°C, -40°C, -60°C, and -80°C. 2 The above is the objective: to provide press-hardened steel parts with high toughness.

[0008] Preferably, the press-hardened steel part according to the present invention has a Charpy impact energy of 0.75 J / mm² measured at -80°C. 2 That's all.

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

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

[0011] An object of the present invention is achieved by providing the steel plate described in claim 1. Another object is achieved by providing the steel component described in claim 2. The steel component may also have the properties described in any one of claims 3 to 5. Another object is achieved by providing the method according to claim 6. Another object is achieved by providing the method according to claim 7. [Modes for carrying out the invention]

[0012] Herein, the present invention will be described in detail and illustrated by examples without introducing any limitations.

[0013] Next, the composition of the steel sheet according to the present invention will be described, with the content expressed in weight percent (weight%).

[0014] According to the present invention, the carbon content is 0.05% to 0.4% so as to ensure satisfactory strength. If the carbon exceeds 0.4%, the weldability and bendability of the steel may decrease. 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 the 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 the hardening in the solid solution. Silicon is added to limit the formation of carbides. If it exceeds 1.3%, silicon is harmful to toughness. Furthermore, silicon oxide is formed on the surface, which may impair the coating property 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 the steel in the liquid phase during smelting. When the titanium content is not sufficient, aluminum can protect boron. The aluminum content is less than 0.1% to avoid the problems of oxidation during press hardening and the formation of ferrite. 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 involved in the hardenability of the steel sheet and must be higher than 0.01%. The chromium content is less than 1.0% to limit the processing problems and cost.

[0019] According to the present invention, the boron content is 0.0005% to 0.08%. Boron improves the hardenability of steel. To avoid the risk of slab breakage during continuous casting, the boron content is 0.08% or less.

[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, to enhance the toughness of steel parts, the copper content is 0.05 to 0.4%. To limit the risk of hot shortness that may weaken the slab, the copper content is limited to 0.4%. Preferably, the copper content is 0.05 to 0.25%, more preferably 0.07% to 0.25%. More preferably, the copper content is 0.08% to 25%, even more preferably 0.08% to 0.20%. More preferably, the copper content is 0.08% to 0.18%.

[0022] The tin content is 0.002% to 0.1% to improve the hardenability of steel. Above 0.1%, tin may accentuate the risk of hot short circuit 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%, even more preferably 0.1% to 0.3%.

[0024] Several elements can be optionally added.

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

[0026] The nickel content is considered to be up to 0.020% as residual element. Preferably, if added, the nickel content is a maximum of 0.1%, more preferably a maximum of 0.05%.

[0027] The molybdenum content can be optionally added up to 0.40%. Similar to boron, molybdenum improves the hardenability of steel. Molybdenum content is kept below 0.40% to limit costs.

[0028] Niobium can be optionally added to improve the ductility of the steel, up to a maximum of 0.08%. Adding more than 0.08% increases the risk of NbC or Nb(C,N) carbide formation, impairing bendability. Preferably, the niobium content is 0.05% or less.

[0029] Calcium may also be added as an optional element in amounts up to 0.1%, preferably as little as 0.0001%. The addition of Ca at the liquid stage allows for the formation of fine oxides, which promotes castability in continuous casting. Furthermore, calcium can encourage the formation of harmful MnS by promoting the formation of CaO-CaS.

[0030] The remainder of the steel composition consists of iron and unavoidable impurities arising from the smelting process, depending on the process route. In the case of a manufacturing route using a blast furnace, the level of unavoidable impurities is very low. In the case of a manufacturing route using an electric arc furnace loaded with scrap, the steel sheet 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 process route. Their content is 0.010% or less for S, 0.020% or less for P, and 0.02% or less for N.

[0032] In certain embodiments, the steel plate is expressed as follows, in weight percentage: C: 0.062~0.095% Mn: 1.4~1.9% Si: 0.2~0.5% Al: 0.020~0.070% Cr: 0.02~0.1% It has a chemical composition containing the element, 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% Sn: 0.002~0.1% S ≤ 0.003% P ≤ 0.020% N ≤ 0.009% And, at your discretion, in weight percentage, the following elements, 0.0001 ≤ Ca ≤ 0.003% Includes one or more of the following: The remainder of the composition consists of iron and unavoidable impurities resulting from the smelting process, depending on the process route.

[0033] In another specific embodiment, the steel plate is expressed as follows, in weight percentage: C: 0.15~0.4% Mn: 0.5~3% Si: 0.1~0.5% Cr: 0.01~1% Ti 0.01~0.1% Al 0.01~0.1% B: 0.0005~0.08% Cu: 0.05~0.4% Sn: 0.002~0.1% S ≤ 0.010% P ≤ 0.020% N ≤ 0.02% It has a chemical composition that includes the element, The remainder of the composition consists of iron and unavoidable impurities resulting from the smelting process, depending on the process route.

[0034] In another specific embodiment, the steel plate is expressed as follows, in weight percentage: C: 0.15~0.25% Mn: 0.5~1.8% Si: 0.1~1.25% Cr 0.1~1% Ti 0.01~0.1% Al 0.01~0.1% B: 0.001~0.004% Cu: 0.05~0.4% Sn: 0.002~0.1% S ≤ 0.010% P ≤ 0.020% N ≤ 0.02% It has a chemical composition containing the following elements, and optionally, in weight percentage, the following elements: Mo≦0.40% Nb ≤ 0.08% Ca ≤ 0.1% Includes one or more of the following: The remainder of the composition consists of iron and unavoidable impurities resulting from the smelting process, depending on the process route.

[0035] In another specific embodiment, the steel plate is expressed as follows, in weight percentage: C: 0.24~0.4% Mn: 0.5~3% Si: 0.10~1.3% 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% Sn: 0.002~0.1% 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 that includes the element, Optionally, in weight percentage, the following elements: Mo: 0.05% and 0.40% Ca 0.0005 and 0.005% Includes one or more of the following: The remainder of the composition consists of iron and unavoidable impurities resulting from the smelting process, depending on the process route.

[0036] In another specific embodiment, the steel plate is expressed as follows, in weight percentage: C: 0.3~0.4% Mn: 0.5~1.0% Si: 0.4~0.8% Cr: 0.1~1.0% Mo: 0.1~0.4% Nb: 0.01~0.08% Al: 0.01~0.1% Cu: 0.05~0.4% Sn: 0.002~0.1% Ti: 0.01~0.03% B: 0.0005~0.003% P ≤ 0.020% Ca ≤ 0.0010% S ≤ 0.010% N ≤ 0.02% It has a chemical composition containing the element, and optionally, Ni < 0.4% It contains [the specified ingredient], and the remainder of the composition consists of iron and unavoidable impurities resulting from the smelting process, depending on the process route.

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

[0038] The semi-finished product that can be further hot-rolled is provided with the steel composition described above. 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, for example, by a steelmaking process using a basic oxygen furnace (BOF) route. In the BOF route, hot metal or pig iron obtained, for example, in a blast furnace or a smelting furnace is decarburized to become 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 this 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 iron scrap to directly produce liquid steel. DRI may also be charged together with iron scrap in the EAF.

[0041] The semi-finished product is heated to a temperature of 1100 °C to 1300 °C. Then, the steel plate is hot-rolled at a finishing hot rolling temperature (FRT) of 830 °C to 950 °C. Preferably, the FRT is composed of 850 °C to 950 °C. Then, the hot-rolled steel is cooled and wound up 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 plate is then cooled to room temperature.

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

[0044] In another preferred embodiment of the present invention, the hot-rolled steel plate is cold-rolled and annealed at an annealing temperature T of 700 °C to 850 °C A and maintained at the annealing temperature T for a holding time t of 10 seconds to 1200 seconds A AIt is maintained at room temperature and optionally coated with an aluminum coating, an aluminum alloy coating, a zinc coating, or a zinc alloy coating, and then cooled to room temperature.

[0045] In another preferred embodiment of the present invention, the hot-rolled steel sheet is cold-rolled and annealed at a temperature of 500°C to 750°C T A The steel is annealed to a certain extent, and a holding time of 300 seconds to 80 hours is required to obtain an annealed steel sheet. A Over the annealing temperature T A It is maintained.

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

[0047] The steel components according to the present invention can be manufactured by any suitable manufacturing method, which can be determined by those skilled in the art. However, it is preferable to use the method according to the present invention, which includes the following steps.

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

[0049] Next, the steel blank is heated to a temperature T1 of 800°C to 980°C and maintained at the T1 temperature for a residence time t1 of 10 to 900 seconds to obtain a heated steel blank. Then, the heated steel blank is 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 component according to the present invention will be described.

[0051] During heating of the steel blank cut from the steel sheet, all microstructural elements are transformed into 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 is transformed into more than 95% martensite, with the remainder being optional bainite and retained austenite. Preferably, the microstructure contained more 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 / mm², calculated as the average of Charpy impact energy values ​​measured at 20°C, -40°C, -60°C, and -80°C. 2 The average Charpy impact energy value is as described above. Preferably, this average value is 0.95 J / mm². 2 That's all.

[0053] Toughness is measured by Charpy impact energy at 20°C and at 40°C, 60°C, and 80°C, in accordance with 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 / mm² at -80°C. 2 It has a higher Charpy impact energy.

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

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

[0057] In a preferred embodiment of the present invention, a martensitic steel sheet can be manufactured by a method comprising the following steps: preparing a hot-rolled steel sheet having the above chemical composition; optionally annealing it to a temperature T of 500°C to 750°C; maintaining it at the 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 period of t1 of 10 seconds to 900 seconds and cooled to less than Ms. The steel sheet is optionally reheated to a temperature of 150°C to 270°C, maintained at the 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 containing more than 95% martensite, with the remainder being optionally bainite and retained austenite.

[0058] Preferably, this martensitic steel sheet has an average Charpy impact energy value of 0.90 J / mm², calculated as the average of the Charpy impact energy values ​​measured at 20°C, -40°C, -60°C, and -80°C. 2 That concludes the explanation. Preferably, this average value is 0.95 J / mm². 2 That's all.

[0059] Preferably, the martensitic steel sheet has a humidity of 0.75 J / mm² at -80°C. 2 It has a higher Charpy impact energy.

[0060] Preferably, the martensitic steel sheet has a ductility reduction 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 higher. More preferably, the martensitic steel sheet has a TS of 1350 MPa or higher.

[0062] The present invention will be described below with reference to the following examples, but these are by no means limiting. [Examples]

[0063] The five grades whose compositions are summarized in Table 1 were cast into semi-finished products, and then processed into steel plates and then steel parts according to the processing parameters summarized in Table 3.

[0064] Table 1 - Composition The tested compositions are summarized in the table below, with elemental content expressed in weight percentage (W%).

[0065] [Table 1] Steels A to B are according to the present invention, and C to E are for reference only. Underlined values: Not applicable to this invention

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

[0067] [Table 2]

[0068] The surface fraction is determined by the following method: the specimen is cut from a steel plate, polished, and etched with a reagent known to the extent of the microstructure. Then, the section is examined optically.

[0069] Table 3 - Processing Parameters Next, the steel plate was cut to obtain a steel blank, which was heated to a temperature T1 and maintained at that temperature for a residence time t1, and then hot-formed. The following specific conditions were applied:

[0070] [Table 3] Underlined values: Not applicable to this invention

[0071] The steel components were analyzed, and the corresponding microstructures were summarized in Table 4. The mechanical properties were 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: the specimen is cut from a press-hardened steel part, polished, and etched with a reagent known to the extent of the microstructure. The section is then examined with an optical microscope or a scanning electron microscope, for example, a scanning electron microscope equipped with a field emission electron gun ("FEG-SEM") with a magnification of more than 5000x 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 Charpy impact tests were performed at four temperatures: 20°C, -40°C, -60°C, and -80°C, and the results are summarized in the table below. The average Charpy impact energy value is calculated by averaging the four toughness values. The ductility reduction 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 the target value

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

[0078] The steel component in Test 3 has a similar chemical composition to the steel component in Test 1, except for a lower level of copper. At the same Charpy impact 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 demonstrated by the average Charpy impact energy value, which is calculated by averaging the four toughness values. A higher average value indicates higher toughness.

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

[0080] Tests 4 and 5 relate to steel parts with low levels of copper. The average Charpy impact energy value was 0.90 J / mm². 2 A value less than 600 indicates that the steel component has low toughness and high DBTT.

Claims

1. A steel plate made of steel, by weight percentage, C: 0.05-0.4% Mn: 0.5–4% 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: 0.05-0.4% Sn: 0.002-0.1% P ≤ 0.020% S ≤ 0.010% N ≤ 0.02% Including, optionally, in weight percentage, the following elements: Ni ≤ 0.4% Mo ≤ 0.40% Nb ≤ 0.08% Ca ≤ 0.1% A steel sheet having a composition containing one or more of the following, the remainder of the composition being iron and unavoidable impurities resulting from refining, the steel sheet having a microstructure containing 50% or more ferrite by surface fraction, and the remainder being pearlite or cementite.

2. A press-hardened steel part made of the steel described in claim 1, wherein the steel part has a microstructure containing more than 95% martensite by surface fraction, with the remainder being optionally selected bainite and retained austenite.

3. The average Charpy impact energy measured at 20°C, -40°C, -60°C, and -80°C was calculated as 0.90 J / mm². ２ The press-hardened steel part according to claim 2, having the above average Charpy impact energy value.

4. 0.75 J / mm ２ A press-hardened steel part according to claim 2 or 3, having a higher Charpy impact energy measured at -80°C.

5. A press-hardened steel part according to any one of claims 2 to 4, having a ductility reduction Δ of less than 25% between the Charpy impact energy measured at 20°C and the Charpy impact energy measured at -80°C.

6. A method for manufacturing press-hardened steel parts, comprising the following series of steps: The steps of preparing the steel plate described in claim 1, To obtain a steel blank, the steps include cutting the steel plate into a predetermined shape, The steel blank is heated to a temperature T of 800°C to 980°C. １ Heat to a certain temperature and leave for a residence time of 10 to 900 seconds. １ During the period, the aforementioned T １ The steps include maintaining the temperature to obtain a heated steel blank, The steps include transferring the heated steel blank to a forming press, The steps include obtaining a molded part by hot forming the heated steel blank in the forming press, A method comprising the step of die-quenching the molded part.

7. A method for manufacturing a martensitic steel sheet, comprising the following series of steps: The steps of preparing the steel plate described in claim 1, The process optionally involves annealing the steel plate to an annealing temperature T of 500°C to 750°C and maintaining it at the annealing temperature for a holding time t of 300 seconds to 80 hours. Optionally, the step of cold-rolling the steel sheet, The steel plate is heated to a temperature T of 800°C to 980°C. １ Annealed to the T １ Temperature-dependent holding time of 10 seconds to 900 seconds １ The steps to maintain during this time, The steps include cooling the steel plate to less than Ms, Optionally, the steel plate is reheated to a temperature of 150°C to 270°C and maintained at that temperature for a holding time of 1 second to 600 seconds. The steps include: cooling the steel plate to room temperature and A method for obtaining a martensitic steel sheet having a microstructure in which more than 95% of the surface fraction of martensite is included, and the remainder is optionally selected bainite and retained austenite.

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