Steel material for a welded tailor blank and method for producing a hot-pressed part using the same steel

Abstract

A steel material for a welded tailor blank, comprising 0.04 to 0.06 wt% carbon (C), 1.2 to 1.5 wt% manganese (Mn), 0.01 to 0.10 wt% titanium (Ti), 0.01 to 0.10 wt% niobium (Nb), optionally further comprising more than 0 wt% and not more than 0.03 wt% silicon (Si), more than 0 wt% and not more than 0.018 wt% phosphorus (P) and more than 0 wt% and not more than 0.003 wt% sulfur (S), and the balance is iron (Fe) and unavoidable impurities, and which has a tensile strength (TS) of 550 MPa or more, a yield strength (YS) of 300 MPa or more, and an elongation (EL) of 20% or more, and which has a dual-phase structure of ferrite and martensite.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a steel material for a welded tailor blank and a method of manufacturing a hot forging using the same, and more particularly to a steel material for a welded tailor blank having improved elongation and performance as a shock absorbing material while minimizing variations in properties thereof depending on hot forging process parameters, and a method of manufacturing a hot forging using the same. BACKGROUND TECHNOLOGY

[0002] In recent years, the automotive industry has seen a demand for stricter vehicle crash performance to improve occupant safety. Furthermore, as environmental awareness has increased, fuel economy standards for exhaust emissions regulations have become more stringent, and therefore, the need for weight reduction of vehicle bodies has continuously increased. As part of the effort to simultaneously meet the requirements of improving crash performance and reducing vehicle body weight, the use of high-strength steel plates in vehicle bodies has continuously increased. High-strength parts are used in vehicle body manufacturing to enhance side-impact protection, as they play a very important role in ensuring survival space for the driver in the event of a side-impact collision.High-strength steel material corresponding to a 150K classification, which is primarily used as a vehicle collision energy absorption member, is subject to brittle fracture, compromising driver safety in the event of a side collision. Therefore, another member is welded to the lower end of the high-strength steel material subject to brittle fracture through a tailor blank welding (TBW) process, thereby enhancing the collision energy absorption capacity of the high-strength steel material.

[0003] The related art to which the present disclosure relates is, for example, KR 10 2016 0 061 560 A.

[0004] Furthermore, a method for hot pressing a steel part is known from KR 10 2017 0 035 468 A, KR 10 2016 0 079 467 A or US 2015 / 0 361 532 A1.

[0005] In addition, a method for producing a rolled steel is known from KR 10 2012 0 001 018 A or KR 10 2014 0 141 218 A. SUMMARY OF THE INVENTIONTechnical Problem

[0006] A problem to be solved by the present disclosure is to provide a hot-pressed steel material which can have improved elongation and crash performance while minimizing variations in properties thereof by controlling alloying elements and process conditions, and a method of manufacturing a hot-pressed part using the same. Technical solution

[0007] In accordance with one aspect of the present disclosure, there is provided a steel material for a welded tailor blank comprising 0.04 to 0.06 wt% carbon (C), 1.2 to 1.5 wt% manganese (Mn), 0.01 to 0.10 wt% titanium (Ti), 0.01 to 0.10 wt% niobium (Nb), further optionally comprising more than 0 wt% and not more than 0.03 wt% silicon (Si), more than 0 wt% and not more than 0.018 wt% phosphorus (P), and more than 0 wt% and not more than 0.003 wt% sulfur (S), and the balance being iron (Fe) and unavoidable impurities.

[0008] In the present disclosure, the steel material for a welded tailor blank may be a steel material which has a tensile strength (TS) of 550 MPa or more, a yield strength (YS) of 300 MPa or more, and an elongation (EL) of 20% or more, and which has a dual-phase structure of ferrite and martensite.

[0009] In the present disclosure, the steel material for a welded tailor blank may further include an aluminum (Al)-silicon (Si) coating layer for improving the corrosion resistance of the surface of the steel material.

[0010] In accordance with another aspect of the present disclosure, there is provided a method for manufacturing a hot-pressed part, comprising the steps of: manufacturing a first blank using a steel plate comprising 0.04 to 0.06 wt% of carbon (C), 1.2 to 1.5 wt% of manganese (Mn), 0.01 to 0.10 wt% of titanium (Ti), 0.01 to 0.10 wt% of niobium (Nb), and the remainder being iron (Fe) and unavoidable impurities, and a second blank obtained by cutting a steel plate provided separately from the first blank, forming a composite steel plate by welding the first and second blanks together by tailor blank welding, forming a shaped body by hot-pressing the composite steel material in a die, and forming a hot-pressed part by cooling the shaped body.

[0011] In the present disclosure, the step of producing the first blank may include the steps of: final hot rolling the steel plate at a final exit temperature (FDT) of 860°C to 920°C, cooling the final hot rolled steel plate to a coiling temperature (CT) of 620°C to 660°C, followed by coiling, uncoiling the coiled steel plate, followed by cold rolling, and subjecting the cold rolled steel plate to an annealing heat treatment.

[0012] In the present disclosure, the second blank may be formed as a steel plate having a tensile strength of 1,200 to 1,500 MPa.

[0013] In the present disclosure, the step of forming the molded article may include the steps of heating the composite steel plate at a temperature of 850°C to 950°C and transferring the heated composite steel plate to a die within a transfer time of 9 to 11 seconds.

[0014] In the present disclosure, the cooling of the molded body in the step of forming the hot-pressed part may be carried out at a rate of 30 to 120°C / s.

[0015] In the present disclosure, the first blank may act as a shock absorbing member for an automobile B-pillar and the second blank may act as a collision support member for an automobile B-pillar.

[0016] In the present disclosure, the first blank may have a tensile strength (TS) of 550 MPa or more, a yield strength (YS) of 300 MPa or more, and an elongation (EL) of 20% or more, and may have a dual-phase structure of ferrite and martensite after hot pressing.

[0017] In the present disclosure, the method may further include a step of coating the surface of the steel plate with aluminum (Al) silicon (Si) to improve corrosion resistance. Beneficial effects

[0018] According to the present disclosure, it is possible to minimize variations in properties of the steel material in the hot pressing process, increase the strength and toughness of the steel material by reducing the martensite packing size, and increase the elongation of the steel material.

[0019] Therefore, the hot-pressed part produced by the method according to the present disclosure exhibits a tensile strength (TS) of 559 to 605 MPa, a yield strength (YP) of 360 to 461 MPa, and an elongation (EL) of 28.5 to 32.7%, is easily processed into a complex shape, and is also suitably used for an impact absorbing member for an automobile collision energy absorbing member due to its excellent collision absorption performance. BRIEF DESCRIPTION OF THE DRAWINGS Fig.1 is a process flow diagram illustrating a method of manufacturing a hot-pressed part according to an embodiment of the present disclosure. Fig. 2 is a process flowchart showing a step of preparing a blank for hot pressing in the method of manufacturing a hot-pressed part according to the embodiment of the present disclosure shown in FIG. Fig. 1 is shown. Fig. 3 shows process time-dependent changes in the microstructure of a hot-pressed part of a comparative example, and Fig. 4 shows process time-dependent changes in the microstructure of a hot-pressed part of an example, Fig. 5 to 7 show the surface structures of an example of the present disclosure and of Comparative Example depending on the time of transfer to the hot press mold. Fig.8 shows the surface structure of an example of the present disclosure depending on the cooling rate of the hot press die. Fig. 9 shows the change in the structure of the steel material by adding boron to the steel material according to an embodiment of the present disclosure. Fig. 10 is a graph showing the change in elongation in an example of the present disclosure and a comparative example as a function of the content of manganese. DETAILED DESCRIPTIONRevelation Mode

[0020] Hereinafter, the present disclosure will be described in detail with reference to the accompanying drawings so that those skilled in the art can easily practice the disclosure. The present disclosure may be implemented in various different ways and is not limited to the embodiments described in the present specification. Like reference numerals designate like or similar components throughout this specification. Furthermore, a detailed description of well-known functions and configurations will be omitted, which unnecessarily obscures the subject matter of the present disclosure.

[0021] A B-pillar, which is an important component for an automobile collision energy absorption member, has a structure in which steel materials of different strengths are bonded to an upper collision support member and a lower impact absorption member, respectively, and is formed by welding the two steel materials together, followed by molding. The TWB process, which is mainly used in a manufacturing process, refers to a series of processes of manufacturing a part by cutting two steel plates having different thicknesses, strengths, and properties into a required shape and welding the cut steel plates, followed by press molding. The TWB process is capable of welding two steel materials having different thicknesses, so that each part can have a required property.For example, the shock absorbing member at the upper portion of the B-pillar is made of an ultra-high-strength steel material corresponding to a 120K to 150K classification. A member with good shock absorption performance is bonded to the lower end of the B-pillar, where stress is concentrated, by the TWB method, thereby improving the B-pillar's ability to absorb shock when an automobile collision occurs. The steel material used in the B-pillar shock absorbing member is commonly referred to as a TWB steel.

[0022] Currently, a steel for a TWB is developed as a steel with a 70K-rated tensile strength, along with a final ferrite-martensite dual phase through a hot-pressing process after hot rolling and cold rolling. To form the B-pillar, for example, the 70K-rated steel for the TWB and a 150K-rated steel are welded together using the TWB process and then hot-pressing.

[0023] However, conventional 150K-rated steel does not undergo any changes in its properties during the hot pressing process because it has a 100% martensite structure due to the hot pressing process. However, 70K-rated steel for TWB has a disadvantage in that its properties change rapidly depending on numerous parameters of the hot pressing process, such as the transfer time required to transfer the steel to a hot pressing die after heating, or the cooling rate of the blank or die.Accordingly, when the 70K-rated steel for the TWB is welded to the 150K-rated steel to form a composite steel material, and hot pressing is performed on the composite steel material, it is very difficult to control the process parameters, and therefore, variations in the properties of the hot pressing part occur, making the hot pressing part unsuitable for an automobile collision energy absorption member. To overcome this problem, in the present disclosure, the variations in the properties of the steel material are minimized within the range of the hot pressing process parameters by controlling and eliminating the components of the steel material. Steel material for TWB

[0024] One aspect of the present disclosure relates to a steel material for a TWB subjected to a hot pressing process. In one embodiment, the steel material for a TWB according to one aspect of the present invention comprises 0.04 to 0.06 wt% of carbon (C), more than 0 wt% and not more than 0.03 wt% of silicon (Si), 1.2 to 1.5 wt% of manganese (Mn), more than 0 wt% and not more than 0.018 wt% of phosphorus (P), more than 0 wt% and not more than 0.003 wt% of sulfur (S), 0.01 to 0.10 wt% of titanium (Ti), 0.01 to 0.10 wt% of niobium (Nb), and the remainder is iron (Fe) and unavoidable impurities.

[0025] The steel material for the TWB ultimately has a tensile strength (TS) of 559 to 605 MPa, a yield strength (YS) of 360 to 390 MPa, and an elongation (EL) of 20% or more, and has a dual-phase structure of ferrite and martensite after hot pressing.

[0026] In addition, the steel material for the TWB may further have an aluminum (Al)-silicon (Si) coating layer for improving the corrosion resistance on the surface thereof.

[0027] The steel material for the TWB according to the present disclosure can reliably exert a 55K rated tensile strength after being subjected to a hot pressing process in a state where it is bonded to a 150K rated steel material.

[0028] Therefore, the steel material for the TWB can have a higher shock absorption rate than a conventional 70 K rated steel material in a partial state where it is joined to a 150 K rated steel material.

[0029] The following describes the functions and contents of the components contained in the steel material for the TWB according to the present disclosure. Carbon (C)

[0030] Carbon (C) is a main element that determines the strength and hardness of the steel material and is also included to ensure the tensile strength of the steel material after the hot pressing (hot stamping) process. In one embodiment, carbon (C) is preferably included in an amount of 0.04 to 0.06 wt% based on the total weight of the steel material for the TWB. If carbon (C) is included in an amount of less than 0.04 wt%, it may be difficult to achieve the mechanical strength of the present disclosure, and if carbon (C) is included in an amount of more than 0.06 wt%, the toughness of the steel material may be reduced. Manganese (Mn)

[0031] Manganese (Mn) is included for the purpose of increasing hardenability and strength during heat treatment. Manganese (Mn) is preferably included in an amount of 1.2 to 1.5 wt% based on the total weight of the steel material for the TWB according to the present disclosure. If the content of manganese (Mn) is less than 1.2 wt%, the grain refinement effect is insufficient. On the other hand, if the content of manganese (Mn) is more than 1.5 wt%, a problem arises in that the toughness of the steel is deteriorated due to the occurrence of central segregation, and this content is disadvantageous in terms of production cost. Titanium (Ti)

[0032] Titanium (Ti) is included for the purpose of increasing strength and toughness by reducing the martensite packing size. Furthermore, titanium (Ti) contributes to improving the elongation of the steel by stably ensuring a ferrite region. Titanium (Ti) is preferably included in an amount of 0.01 to 0.10 wt% based on the total weight of the steel material for the TWB according to the present disclosure. If the content of titanium (Ti) is less than 0.01 wt%, the grain refinement effect is insufficient. On the other hand, if the content of titanium (Ti) is greater than 0.10 wt%, it may result in a reduction in toughness. Niobium (Nb)

[0033] Niobium (Nb) is included for the purpose of increasing strength and toughness by reducing the martensite packing size. Furthermore, niobium (Nb) contributes to improving the elongation of the steel material by stably ensuring a ferrite region. In one embodiment, niobium (Nb) is included in an amount of 0.01 to 0.10 wt.% based on the total weight of the steel material for the TWB according to the present disclosure. When niobium (Nb) is included in an amount of less than 0.01 wt.%, the effect of grain refinement of the steel material in hot rolling and cold rolling processes may be insignificant, and when niobium (Nb) is included in an amount of more than 0.10 wt.%, it may generate coarse precipitates in the steelmaking process and may deteriorate the elongation of the steel material and may be disadvantageous in terms of production costs. Silicon (Si)

[0034] Silicon (Si) contributes to improving the strength and elongation of the steel material. However, if silicon (Si) is contained in an amount of more than 0.03 wt% based on the total amount of the steel material for hot pressing according to the present disclosure, it may cause surface defects and deteriorate the coating properties of the steel material. Therefore, in the present disclosure, silicon (Si) is preferably contained in an amount of more than 0 wt% and not more than 0.03 wt% based on the total weight of the steel material for hot pressing. Phosphorus (P)

[0035] Phosphorus (P) is an element that easily segregates and deteriorates the toughness of the steel material. In one embodiment, phosphorus (P) is preferably contained in an amount of more than 0 wt% and not more than 0.018 wt% based on the total weight of the steel material for hot pressing according to the present disclosure. When phosphorus is contained in an amount within the above-described range, toughness can be prevented from deteriorating. When phosphorus (P) is contained in an amount of more than 0.018 wt%, it may generate cracks during processing and may generate a phosphorus iron compound, thereby reducing the toughness of the steel material. Sulfur (S)

[0036] Sulfur (S) is an element that degrades workability and physical properties. In one embodiment, sulfur (S) may be contained in an amount of more than 0 wt% and not more than 0.003 wt% based on the total weight of the steel material for hot rolling according to the present disclosure. If sulfur (S) is contained in an amount of more than 0.003 wt%, it may degrade hot rolling workability and cause surface defects such as cracks by generating macroinclusions.

[0037] A method for manufacturing a hot-pressed part using the steel material for the TWB according to the present disclosure will be described in detail below. Method for producing a hot-pressed part

[0038] Another aspect of the present disclosure relates to a method of manufacturing a hot-pressed part using the steel material for the TWB, which is subjected to the TWB process. Fig. 1 is a process flow diagram showing the method of manufacturing a hot stamped part according to the present disclosure, the Fig. Fig. 2 is a flow chart specifically showing a step of manufacturing a blank for hot pressing used in the Fig. 1 is shown.

[0039] With reference to the Fig.1, the method for manufacturing a hot-pressed part according to an embodiment of the present disclosure includes the steps of: (S110) manufacturing blanks for hot pressing formed from two different types of steel materials, (S120) joining the blanks for hot pressing to each other to form a composite steel material, (S130) performing hot pressing on the composite steel material to form a formed body, and (S140) cooling the formed body to form a hot-pressed part. Step (S110) of producing the blanks for hot pressing

[0040] Step (S110) of preparing the blanks for hot pressing is a step of cutting two different types of steel plates to form a hot pressing part into a desired shape according to the intended use, for example, respectively forming a first blank used as a shock absorbing member and a second blank used as a collision support member to form an automobile B-pillar.

[0041] The first blank is a portion that becomes a shock absorbing member for the B-pillar after hot pressing, and has sufficient strength to protect a driver in an automobile collision, and also preferably has an elongation capable of protecting the driver by absorbing the shock when the automobile collision occurs. According to a preferred embodiment of the present disclosure, the first blank is formed of a steel material that has a tensile strength (TS) of 559 to 605 MPa, a yield strength (YS) of 360 to 390 MPa, and an elongation (EL) of 20% or more, and that has a dual-phase structure of ferrite and martensite after hot pressing.

[0042] The second blank is a portion which becomes a collision support member for the B-pillar after hot pressing, and is formed of an ultra-high-strength steel material having a tensile strength of, for example, 1,200 to 1,500 MPa after hot pressing, to protect a driver by ensuring the survival space for the driver when the automobile collision occurs.

[0043] As it is in the Fig. 2, the process of forming the first blank may include a hot rolling step (S210), a cooling / coiling step (S220), a cold rolling step (S230), and an annealing heat treatment step (S240).

[0044] In the method for manufacturing a hot-pressed part according to the present disclosure, the steel plate in a semi-finished production state, which is formed as the first blank in the process of forming the first blank, comprises 0.04 to 0.06 wt% of carbon (C), more than 0 wt% and not more than 0.03 wt% of silicon (Si), 1.2 to 1.5 wt% of manganese (Mn), more than 0 wt% and not more than 0.018 wt% of phosphorus (P), more than 0 wt% and not more than 0.003 wt% of sulfur (S), 0.01 to 0.10 wt% of titanium (Ti), 0.01 to 0.10 wt% of niobium (Nb), and the balance of iron (Fe) and unavoidable impurities.

[0045] In the steel plate reheating step, the steel plate obtained through continuous casting is reheated at a plate reheating temperature (SRT) of 1,200°C to 1,250°C, whereby components segregated during casting are redissolved into a solid solution. When the plate reheating temperature (SRT) is lower than 1,200°C, a problem arises in that components segregated during casting are not sufficiently redissolved into a solid solution, making it difficult to achieve a significant effect of homogenizing alloying elements. For the plate reheating temperature (SRT), it is more preferable to be higher for the homogenization of alloying elements, but when the plate reheating temperature is higher than 1,200°C, the components segregated during casting are not sufficiently dissolved into a solid solution.250°C, the austenite grain size may increase, which makes it difficult to ensure the strength, and also the bake-hardening and anti-aging properties may be deteriorated and the production cost of the steel plate may be increased due to excessive heat process.

[0046] In the hot rolling step (S210), the reheated steel plate is finish rolled at a final exit temperature (FDT) of 860 to 920°C.

[0047] When the final exit temperature (FDT) is excessively low (e.g., lower than 860°C), problems arise in that a mixed grain structure occurs due to dual-phase region rolling, which makes it difficult to ensure the workability of the steel plate, and the workability is reduced due to the non-uniform microstructure. In addition, rapid phase change causes the problem of mass flow during hot rolling. The final exit temperature (FDT) is also preferable for homogenizing the alloying elements, if it is higher than the SRT, and it is determined depending on the SRT and the number of passes. However, if the final exit temperature (FDT) is higher than 920°C, the austenite grains become coarse, resulting in a decrease in bake-hardenability and aging properties.

[0048] In the cooling / coiling step (S220), the hot-rolled steel plate is cooled and coiled at a coiling temperature (CT) of 620 to 660°C. The coiling temperature affects the redistribution of carbon (C). When the coiling temperature is lower than 620°C, strength is advantageously ensured, but a problem arises in that ductility rapidly decreases. On the other hand, when the coiling temperature is higher than 660°C, a problem arises in that deterioration in formability or strength occurs due to abnormal grain growth or excessive grain growth.

[0049] In the cold rolling step (S230), the coiled steel plate is uncoiled, pickled, and then cold rolled. At this time, pickling is performed to remove scale from the coiled plate, i.e., the hot-rolled coil produced by the hot rolling process.

[0050] Cold rolling is preferably performed during cold rolling of the pickled plate at a cold rolling reduction ratio of 60 to 80%. If the cold rolling reduction ratio is less than 60%, the effect of deforming the hot-rolled structure is insignificant. On the other hand, if the cold rolling reduction ratio is more than 80%, problems may arise such as increased cold rolling costs, deteriorated drawability of the steel plate, and cracks appearing at the edge of the steel plate, resulting in breakage of the steel plate.

[0051] The annealing heat treatment step (S240) is a step of subjecting the cold-rolled steel plate material to an annealing heat treatment. In one embodiment, the annealing heat treatment step includes a step of heating the cold-rolled steel plate and cooling the heated cold-rolled steel plate at a cooling rate of 20 to 50°C / s. In one embodiment, the cold-rolled steel plate may be heated to a temperature of 700 to 900°C during the annealing heat treatment. When the cold-rolled steel plate is heated to a temperature within the above-described range, the process efficiency and the strength and formability of the steel material can all be excellent.

[0052] If the cold-rolled steel plate is cooled at a cooling rate below 20°C / s, the productivity of the steel material may be reduced. If the cold-rolled steel plate is cooled at a cooling rate above 50°C / s, it is difficult to ensure the uniform microstructure of the steel material. For example, the cold-rolled steel plate can be cooled at a cooling rate of 30 to 40°C / s.

[0053] Meanwhile, in the hot pressing step (S130) as described below, the Fig.1, the composite steel material, which is the molded article, is softened and press-formed by heating at a high temperature, followed by cooling. Therefore, since the composite steel material is softened by heating at the high temperature, it can be easily press-formed, and the mechanical strength of the steel material is increased by quenching and cooling after forming. However, since the steel material is heated to a high temperature of 800°C or more, iron (Fe) on the surface of the steel material is oxidized to form oxides (scale). For this reason, in one embodiment of the present disclosure, a specific coating may be formed on the cold-rolled steel plate after the annealing heat treatment. Specifically, an aluminum (Al)-based metal coating is formed, which has a higher melting point than an organic or a zinc (Zn)-based metal coating.For example, an aluminum (Al)-silicon (Si)-based coating is formed. The cold-rolled steel plate coated with aluminum (Al)-silicon (Si) is protected from corrosion and prevents the formation of scale on the warm surface of the steel plate during transfer to the die.

[0054] An aluminum-(Al)-silicon-(Si) coating on the steel plate can be carried out by a well-known method. One example is a method of diffusion-coating the steel plate with aluminum-(Al)-silicon-(Si). In this method, the steel plate is placed in a heating furnace that can be heated to a diffusion / coating temperature, and then the surface of the steel plate heated to the diffusion / coating temperature is coated with aluminum-(Al)-silicon-(Si). Another method of coating the steel plate with aluminum-(Al)-silicon-(Si) can be carried out by immersing the steel plate in a coating bath, coating the immersed steel plate in the coating bath with an aluminum-(Al)-silicon-(Si) coating, performing alloying heat treatment on the steel plate, and cooling the alloyed steel plate.

[0055] This coating forms an aluminum (Al)-silicon (Si) coating layer on the surface of the steel plate. This coating layer can serve to prevent a scale layer from forming during a high-temperature heat treatment process described below.

[0056] Meanwhile, the second blank can be formed by performing a hot rolling step, a cooling / coiling step, a cold rolling step, and an annealing heat treatment step.

[0057] In the method for manufacturing a hot-pressed part according to the present disclosure, a steel plate in a semi-finished product state to be formed as the second blank in the process of forming the second blank may comprise, by weight, 0.20 to 0.50% of carbon (C), 0.05 to 1.00% of silicon (Si), 0.10 to 2.50% of manganese (Mn), more than 0% and not more than 0.015% of phosphorus (P), more than 0% and not more than 0.005% of sulfur (S), 0.05 to 1.00% of chromium (Cr), 0.001 to 0.009% of boron (B), 0.01 to 0.09% of titanium (Ti), and the balance of iron (Fe) and unavoidable impurities.

[0058] In one embodiment, the hot rolling step may include the steps of reheating the steel plate to a temperature of 1,200°C to 1,250°C, finish rolling the reheated plate at a temperature of 900°C to 950°C, and cooling the hot-rolled steel plate to a temperature of 680°C to 800°C, followed by coiling. Then, the cold rolling step may include a step of pickling the coiled steel plate, followed by cold rolling. Next, the annealing heat treatment step may include a step of annealing the cold-rolled steel plate at a temperature of 740°C to 820°C. The plate material subjected to the annealing heat treatment may be cooled to room temperature, for example, at a cooling rate of 5 to 50°C / s. Step (S120) of forming the composite steel material

[0059] The different types of first and second blanks are respectively bonded together by the TWB method to form a composite steel material. In one embodiment, the first and second blanks may be arranged in such a manner that the first blank becomes a shock absorbing member at a lower end of the B-pillar, and the second blank becomes a collision support member at the upper portion thereof. Then, the first and second blanks may be welded together, for example, by a butt welding method using a laser. Hot pressing step (S130)

[0060] The composite steel material is heated in a heating furnace to a temperature of approximately 850 to 950°C. For example, heating may be carried out at a temperature of 930°C for approximately 5 minutes. Next, the composite steel material is transferred into a die. At this time, the transfer time may be 9 to 11 seconds. After the composite steel material is formed into the final part shape in the die for hot pressing, the die is rapidly cooled at a cooling rate of approximately 30 to 120°C / s to form a final product.

[0061] Although not shown in the drawings, the die may have a cooling channel therein through which a coolant circulates. The circulation of the coolant supplied through the cooling channel can quench the heated blank. At this time, to maintain the desired shape while preventing the composite steel material from springing back, pressure quenching may be performed in a state where the die is closed.

[0062] According to the hot-pressed part manufactured according to the processes described above (S110 to S130), it is possible to compensate for the strength and ensure elongation as a result of maximizing precipitation rather than reducing the proportion of martensite by limiting the carbon (C) content contained in the steel material portion corresponding to the first blank used as the shock absorbing member to 0.04 to 0.06 wt%. Furthermore, as a result of limiting the contents of titanium (Ti) and niobium (Nb), it is possible to reduce the martensite packing size, thereby increasing the strength and toughness of the hot-pressed part. Furthermore, by stably ensuring the ferrite region, it is possible to increase the elongation and increase the variation of the properties of the hot-pressed part, which depend on the hot-pressing process parameters.Furthermore, even if the expensive hardening elements such as molybdenum (Mo) are not added, a superior 55K classified hot forging can be produced, which can exert shock absorption performance due to the large elongation thereof.

[0063] Therefore, the hot-pressed part manufactured by the method of the present disclosure, as a shock absorbing member, comprises the steel material having the dual-phase structure of ferrite and martensite while satisfying a tensile strength (TS) of 550 MPa or more, a yield strength (YP) of 300 MPa or more, and an elongation (EL) of 20% or more. Therefore, it is easily manufactured in a complex shape and is also suitable for use in an automobile collision energy absorbing member due to its excellent collision absorption performance. Furthermore, the collision support member of the hot-pressed part can maintain a high tensile strength (TS) of 1,200 to 1,500 MPa.

[0064] Hereinafter, the configuration and effects of the present disclosure will be described in detail with reference to the preferred embodiments. However, these embodiments are presented as preferred examples of the present disclosure and are not to be construed as limiting the present disclosure in any way. Moreover, the contents not disclosed herein are sufficiently understood and technically understood by those skilled in the art, and therefore, a description thereof will be omitted. First embodiment 1. Preparation of samples

[0065] A steel plate containing the components shown in Table 1 below, the remainder being iron (Fe) and unavoidable impurities, was reheated to a plate reheating temperature of 1200°C, finish-rolled to a final exit temperature of 900°C, and then cooled to a coiling temperature of 640°C and coiled, thereby producing a hot-rolled coil. The hot-rolled coil was uncoiled and then cold-rolled to produce a cold-rolled steel plate. The cold-rolled steel plate was subjected to annealing heat treatment by heating to a temperature of 810°C and then cooled at a cooling rate of 33°C / s, thereby producing a steel material of Example 1.

[0066] Furthermore, the steel materials of Comparative Examples 1 and 2 were manufactured in the same manner as the above Example 1, except that steel plates containing the components shown in Table 1 and the remainder consisting of iron (Fe) and unavoidable impurities were used. In the case of the steel material sample of Comparative Example 2, titanium (Ti) and molybdenum (Mo) were added in amounts different from those of the steel material sample of Comparative Example 1. [Table 1] classification C (wt%) Mn (wt%) Nb (wt%) Ti (wt%) Mo (wt%) Target tensile strength after hot pressing Example 1 0,05 1,40 0,06 0,07 - 550 MPa or more Comparison example 1 0,08 1,60 0,05 0,07 - 500 to 700 MPa Comparison example 2 0,08 1,80 0,05 0,065 0,20 650 to 750 MPa

[0067] Next, first blanks of the steel material of Example 1 and Comparative Examples 1 and 2 were prepared. In addition, a second blank was separately prepared, which has a tensile strength of 1,500 MPa. The second blank may contain, by weight %, 0.20 to 0.50% carbon, 0.05 to 1.00% silicon (Si), 0.10 to 2.50% manganese (Mn), more than 0% and not more than 0.015% phosphorus (P), more than 0% and not more than 0.005% sulfur (S), 0.05 to 1.00% chromium (Cr), 0.001 to 0.009% boron (B), 0.01 to 0.09% titanium (Ti), and the balance iron (Fe) and unavoidable impurities.

[0068] Each of the first blanks was laser welded to the second blank, thereby producing composite steel materials according to Example 1 and Comparative Examples 1 and 2. Each of the composite steel materials was heated at a temperature of 930°C for 5 minutes, and then each of the composite steel materials was transferred to the hot press die within a transfer time of approximately 10 seconds to produce the molded parts. Each of the molded parts was cooled at a cooling rate of 100°C / s, thereby producing the final hot press parts. 2. Evaluation of mechanical properties

[0069] Of the manufactured molded bodies, the tensile strength (MPa), yield strength (MPa), and elongation (%) were measured for the steel material section corresponding to each of Example 1 and Comparative Examples 1 and 2. The results are shown in Table 2 below. [Table 2] classification Tensile strength (TS) (MPa) Yield strength (YS) (MPa) Elongation (EL) (%) Target value 550 MPa or more 300 MPa or more 20% or more Example 1 1 580 370 26,0 Comparison example 1 570 390 13,2 Comparison example 2 730 530 12,3

[0070] Referring to the results in Table 2 above, the steel material sample of Example 1 exhibits better elongation than the steel materials of Comparative Examples 1 and 2 within the ranges of tensile strength and yield strength required for a collision absorbing material. Particularly, in the case of elongation, the steel material of the example of the present disclosure exhibits elongation of 20% or more, indicating that it has excellent ability to absorb shock when an automobile collision occurs, and therefore has excellent performance as a shock absorbing material.

[0071] Fig. 3 is a graph showing process time-dependent changes in the microstructure of the molded bodies corresponding to the steel materials of Comparative Examples 1 and 2, and Fig.Figure 4 is a graph showing process time-dependent changes in the microstructure of the molded body corresponding to the steel material of Example 1.

[0072] During the production of the molded articles of Example 1 and Comparative Examples 1 and 2, measurements were taken at transfer times of 7 s, 9 s, 11 s, and 13 s to investigate the transfer time-dependent changes in the microstructures of the steel materials when the composite steel materials were transferred to the hot press. Table 3 below shows the distribution ranges of the tensile strength, yield strength, and elongation within the transfer time range. [Table 3] classification Tensile strength (TS) (MPa) Yield strength (YS) (MPa) Elongation (EL) (%) Example 1 559 to 605 360 to 390 26.0 to 32.7 Comparison example 1 529 to 619 354 to 435 7.0 to 19.4 Comparison example 2 685 to 773 466 to 601 8.5 to 16.1

[0073] Referring to Table 3 above, it can be seen that variations of tensile strength, yield strength and elongation within the transfer time range in the steel materials of Comparative Examples 1 and 2 were larger than in Example 1.

[0074] With reference to the Fig. 3, it can be seen that the steel materials of Comparative Examples 1 and 2 are subject to a rapid change in the proportions of martensite and ferrite depending on the transfer time or the blank or die cooling rate after heating for hot pressing. That is, as shown in the Fig.As shown in Figure 3, it can be seen that the time axis division (reference symbol "A") between a ferrite transformation curve 320 and a bainite transformation curve 320 of the steel material is narrow, indicating that the properties of the steel material change rapidly when the temperature curve 330 of the steel material is moved left or right on the time axis depending on the process parameters. Since it is not easy to control the process conditions of the hot pressing process, it can be seen that if the proportion distribution in the microstructure phases between different portions of the molded article occurs as described above, the molded article is not suitable for use as an automobile collision energy absorption member.

[0075] In contrast, with reference to the Fig. 4 it can be seen that the ferrite transformation curve 410 of the steel material compared to that shown in the Fig.3 is significantly shifted to the left on the time axis. Therefore, the distance (reference symbol "B") along the time axis between the ferrite transformation curve 410 and a bainite transformation curve 420 is wide, and thus, even if a time-dependent temperature curve 430 of the steel material is shifted to the left or right due to the process conditions, it is shifted between the two transformation curves 410 and 420, and therefore, the variation characteristics of the hot forging can be minimized. That is, the structure of the steel material of Example 1 after hot forging can be a dual-phase structure of ferrite and martensite. This is because the content of carbon (C) is limited and the contents of niobium (Nb) and titanium (Ti) are controlled, which makes it possible to stably ensure the desired properties of the hot forging.

[0076] Fig.5 shows the surface structure of the molded body corresponding to the steel material of Example 1 of the present disclosure, depending on the transfer time to the hot press mold, Fig. Figure 6 shows the surface structure of the molded body corresponding to the steel material of Comparative Example 1, depending on the transfer time to the hot press mold, and Fig. Figure 7 shows the surface structure of the molded body corresponding to the steel material of Comparative Example 2 depending on the transfer time to the hot press mold.

[0077] With reference to the Fig. 6 and Fig.7, the steel material of Example 1 of the present disclosure had the ferrite and martensite microstructure, and, as shown in Table 3, had a relatively small variation in the tensile strength, yield strength, and elongation properties within the mold transfer time range. However, it was found that within the mold transfer time range, Comparative Example 1 did not ensure the relatively stable ferrite and martensite microstructures, and Comparative Example 2 did not ensure the relatively stable ferrite and martensite microstructures. The low-temperature phase was found to be a microstructure of martensite and bainite.Furthermore, it could be seen that Comparative Examples 1 and 2 showed a large variation in tensile strength, yield strength, and elongation properties depending on the time range of transfer to the mold compared with the steel material of Example 1, as shown in Table 3 above.

[0078] Fig. Figure 8 shows the surface structure of the molded body corresponding to the steel material of Example 1 of the present disclosure, depending on the cooling rate of the hot press mold. In particular, Fig. 8 the surface structures at cooling rates of 30°C / s, 60°C / s and 120°C / s, respectively. As shown in the Fig. As shown in Figure 8, the proportions of the microstructure were almost constant depending on the cooling rate, and the variations of the tensile strength, yield strength and elongation depending on the cooling rate were also not large.

[0079] Taking the above results together, it can be seen that when titanium (Ti) and niobium (Nb) are added to ensure the ferrite region and enhance the hardenability to prevent the variations in the properties of the molded body that occur depending on the process parameters such as the transfer time to the mold and the cooling rate, which are difficult to control, and when the content of martensite is reduced by reducing the amount of added carbon (C), the steel material according to the example of the present disclosure can stably ensure microstructures within the range of the hot pressing parameters (the transfer time to the hot pressing die and the cooling rate), and therefore, the variation properties between different portions of the molded body can be minimized.Furthermore, it can be seen that when expensive molybdenum (Mo) is added, the steel material of the example of the present disclosure has better toughness than the steel materials of the comparative examples and therefore has excellent economic efficiency. Second embodiment 1. Preparation of the samples

[0080] A steel material of an example having the alloy composition of Example 1 of the first embodiment was manufactured by sequentially performing the hot rolling, cold rolling, and annealing heat treatment processes as described for the first embodiment. Furthermore, steel materials of the majority of comparative examples having an alloy composition containing more than 0 wt% and not more than 0.0020 wt% of boron were added to the alloy composition of Example 1 as shown in Table 1 of the first embodiment, manufactured by performing the processes described in the first embodiment under the same process conditions.

[0081] Each of the blanks formed from the steel materials of the example and the majority of comparative examples was laser welded to a blank formed from a 150K-rated steel material, thereby producing composite steel materials according to the example and the majority of comparative examples, respectively. The hot pressing process of the first embodiment was performed on the composite steel materials, thereby producing the finished hot-pressed parts. 2. Evaluation of mechanical properties

[0082] Of the prepared molded bodies, the proportion of bainite produced in the steel material by adding boron was measured for the steel material section corresponding to each of the example and the majority of comparative examples.

[0083] Fig.9 is a graph showing the change in structure by adding boron to the steel material according to an embodiment of the present disclosure. Referring to the Fig. 9, in the steel material of the example to which no boron was added, no bainite was detected. However, in the steel materials of the majority of the comparative examples, the proportion of bainite tends to be increased as the boron content is increased. That is, as shown in the Fig. 3, it can be expected that when boron is added to the steel material, a portion in which the temperature curve 330 of the steel material passes through the bainite transformation curve 320 will increase in the cooling process. However, as shown in the Fig.As shown in Figure 4, in the case of the steel material of the example, the temperature curve 330 of the steel material does not meet the bainite transformation curve 320 in the cooling process. Therefore, the portion corresponding to the bainite structure produced in the steel material of the plurality of comparative examples can be included in the portion of the ferrite structure of the steel material of Example 1. Therefore, the steel material of the example can have better ductility than that of the steel materials of the plurality of comparative examples. Third embodiment 1. Preparation of samples

[0084] A plurality of steel plates, each containing the components shown in Table 4 below and the balance being iron (Fe) and other unavoidable impurities, were reheated at a plate reheating temperature of 1,200°C, hot-rolled to a final exit temperature of 900°C, and then cooled and coiled at a temperature of 640°C, thereby producing hot-rolled coils. The hot-rolled coils were uncoiled and then cold-rolled, thereby producing cold-rolled steel plate materials. Next, the cold-rolled plate materials were subjected to annealing heat treatment by heating to a temperature of 810°C and then cooling at a cooling rate of 33°C / s, thereby producing the steel materials of Examples 2 to 5 and Comparative Examples 3 to 7. [Table 4] classification C (wt%) Mn (wt%) Ti (wt%) Nb (wt%) Example 2 0,05 1,2 0,07 0,06 Example 3 1,3 Example 4 1,4 Example 5 1,5 Comparison example 3 1,6 Comparison example 4 1,7 Comparison example 5 1,8 Comparison example 6 1,9 Comparison example 7 2,0

[0085] Then, each of the blanks formed from the steel materials of Examples 2 to 5 and Comparative Examples 3 to 7 was laser welded to a blank formed from a 150K-rated steel material, thereby producing the composite steel materials according to Examples 2 to 5 and Comparative Examples 3 to 7, respectively. Each of the composite steel materials was heated to a temperature of 930°C for 5 minutes. Then, each of the heated composite steel materials was transferred to a hot press mold within a transfer time of approximately 10 seconds and then hot press-formed, thereby producing molded articles. The molded articles were cooled at a cooling rate of 100°C / s, thereby producing the final hot press parts. 2. Evaluation of mechanical properties

[0086] Of the prepared molded bodies, the elongation depending on the amount of manganese added was measured for the steel material section corresponding to each of Examples 2 to 5 and Comparative Examples 3 to 7. To measure the elongation, ten samples were prepared for each of Examples 2 to 5 and Comparative Examples 3 to 7, and then the elongation was measured by a room temperature tensile test.

[0087] The Fig. Figure 10 is a graph showing the changes in elongation of one of the examples of the present disclosure and the comparative example as a function of manganese content. Table 5 below shows the average elongation (%) and standard deviation of the ten samples prepared for each of Examples 2 to 5 and Comparative Examples 3 to 7. [Table 5] classification Average elongation (%) Standard deviation Example 2 28 1,2 Example 3 27 1,6 Example 4 26 1,3 Example 5 23 2,5 Comparison example 3 19 2,7 Comparison example 4 18 3,1 Comparison example 5 17 3,4 Comparison example 6 16 3,6 Comparison example 7 15 3,9

[0088] With reference to Table 5 above and the Fig.10, the steel materials of Examples 2 to 5 had better average elongation than the steel materials of Comparative Examples 3 to 7. Moreover, the standard deviation of the elongation of the steel materials of Examples 2 to 5 of the present disclosure was measured to be smaller than that of the steel materials of Comparative Examples 3 to 7. That is, in the case of Comparative Examples 3 to 7, in which the content of manganese in the steel material was 1.6 wt% or more, the strength is easy to ensure due to the increase in solid solution improvement caused by the manganese, but there may be a risk that the elongation decreases and the standard deviation of the elongation increases. In contrast, in the case of Examples 2 to 5, after hot pressing, the elongation increases while the standard deviation of the elongation decreases relatively, and therefore, the performance of the part can be stabilized. Fourth embodiment 1. Preparation of samples

[0089] A plurality of steel plates having the alloy compositions of Examples 2 to 5 in Table 4 above, and the balance being iron (Fe) and unavoidable impurities, were reheated at a plate reheating temperature of 1,200°C, hot-rolled to a final discharge temperature of 900°C, and then cooled and coiled at a coiling temperature of 640°C, thereby producing hot-rolled coils. The hot-rolled coils were uncoiled and then cold-rolled, thereby producing cold-rolled steel plates. The cold-rolled steel plate materials were subjected to annealing heat treatment by heating to a temperature of 810°C and then cooling at a cooling rate of 33°C / s, thereby producing the steel materials of Examples 2 to 5.

[0090] Then, each of the blanks formed from the steel materials of Examples 2 to 5 was laser welded to a blank formed from a steel material having a tensile strength of 1500 MPa, thereby producing the composite steel materials according to Examples 2 to 5. Each of the composite steel materials was heated at 930°C for 5 minutes, and then each of the heated composite steel materials was transferred to a hot press mold within a transfer time of approximately 10 seconds and hot-pressed, thereby producing molded articles. The molded articles were cooled at a cooling rate of 75°C / s, thereby producing finished hot-pressed parts. 2. Consideration of the microstructure

[0091] For the molded bodies, the microstructure fractions in the steel material samples of Examples 2 to 5 were measured. The measurement was performed using a well-known ASTM E562-11 "systematic manual point count method." The results of the area fraction measurements are shown in Table 6 below. [Table 6] Manganese content (wt%) Ferrite area fraction (%) Martensite area fraction (%) Example 2 1,2 88 to 98 2 to 12 Example 3 1,3 87 to 97 3 to 13 Example 4 1,4 88 to 97 3 to 12 Example 5 1,5 87 to 98 2 to 12

[0092] Referring to Table 6 above, it was confirmed that the steel material samples of Examples 2 to 5 did not exhibit a large variation depending on the manganese content. Within 1.2 to 1.5 wt%, which is the manganese content range of the present disclosure, the steel material samples of Examples 2 to 5 were considered to be composed of a microstructure including ferrite having an area ratio of 87 to 98% and martensite having an area ratio of 2 to 13%. Fifth Embodiment 1. Preparation of Samples

[0093] A plurality of steel plates, each containing 0.05 wt% carbon, 1.4 wt% manganese, 0.07 wt% titanium, 0.06 wt% niobium, and the balance iron (Fe) and other unavoidable impurities, were reheated to a plate reheating temperature of 1,200°C, hot-rolled to a final exit temperature of 900°C, and then cooled and coiled at a coiling temperature of 640°C, thereby producing hot-rolled coils. The hot-rolled coils were uncoiled and then cold-rolled, thereby producing cold-rolled steel plates. The cold-rolled steel plates were then subjected to annealing heat treatment at a temperature of 810°C and then cooled at a cooling rate of 33°C / s, thereby producing steel materials.

[0094] Each of the blanks formed from the steel materials subjected to the annealing heat treatment was laser welded to a blank formed from a steel material having a tensile strength of 1500 MPa, thereby producing composite steel materials. The composite steel materials were heated at a temperature of 930°C for 5 minutes, and then each of the composite steel materials was transferred to a hot press mold within a transfer time of approximately 10 seconds and then hot-pressed, thereby producing molded articles. The molded articles were then cooled at a cooling rate of 34°C / s, 63°C / s, 94°C / s, and 115°C / s, respectively, thereby producing hot presses comprising the steel materials of Examples 6 to 9. 2. Consideration of the microstructure

[0095] For the hot-pressed parts produced, the microstructure fractions in the steel material sections of Examples 6 to 9 were measured. The measurement was performed using a well-known ASTM E562-11 "systematic manual point count method." The results of the area fraction measurements are shown in Table 7 below. [Table 7] Cooling rate (°C / s) Ferrite area fraction (%) Martensite area fraction (%) Example 6 34 90 to 98 2 to 10 Example 7 63 88 to 97 3 to 12 Example 8 94 88 to 96 4 to 12 Example 9 115 83 to 95 5 to 17

[0096] Referring to Table 7 above, it can be confirmed that the steel material samples of Examples 6 to 9 do not exhibit a large variation depending on the cooling rate. Within the cooling rate range of 34 to 115°C / s, the steel material samples of Examples 6 to 9 were observed to be composed of a microstructure with ferrite having an area ratio of 83 to 98% and martensite having an area ratio of 2 to 17%.

Claims

[1] A steel material for a welded tailor blank, comprising 0.04 to 0.06 wt% carbon (C), 1.2 to 1.5 wt% manganese (Mn), 0.01 to 0.10 wt% titanium (Ti), 0.01 to 0.10 wt% niobium (Nb), optionally further comprising more than 0 wt% and not more than 0.03 wt% silicon (Si), more than 0 wt% and not more than 0.018 wt% phosphorus (P), and more than 0 wt% and not more than 0.003 wt% sulfur (S), and the balance is iron (Fe) and unavoidable impurities, and which has a tensile strength (TS) of 550 MPa or more, a yield strength (YS) of 300 MPa or more, and an elongation (EL) of 20% or more, and which has a dual-phase structure of ferrite and martensite. [2] The steel material according to claim 1, further comprising an aluminum (Al)-silicon (Si) coating layer for improving corrosion resistance on a surface of the steel material. [3] A method for producing a hot-pressed part, comprising the steps of: (a) Producing a first blank using a steel plate comprising 0.04 to 0.06 wt% carbon (C), 1.2 to 1.5 wt% manganese (Mn), 0.01 to 0.10 wt% titanium (Ti), 0.01 to 0.10 wt% niobium (Nb), and the remainder being iron (Fe) and unavoidable impurities, and a second blank obtained by cutting a steel plate provided separately from the first blank, (b) welding the first and second blanks together by a tailor blank welding process, thereby forming a composite steel material, (c) hot pressing the composite steel material in a die to form a shaped body, and (d) cooling the molded body, thereby producing a hot-pressed part. [4] The method according to claim 3, wherein step (a) comprises: (a-1) Final hot rolling of the steel plate of the first blank at a final exit temperature (FDT) of 860°C to 920°C, (a-2) Cooling the final hot-rolled steel plate at a coiling temperature (CT) of 620°C to 660°C, followed by coiling, (a-3) Uncoiling the coiled steel plate, followed by cold rolling and (a-4) Subjecting the cold-rolled steel plate to annealing heat treatment. [5] The method according to claim 3, wherein the second blank has a tensile strength of 1,200 to 1,500 MPa after hot pressing. [6] The method according to claim 3, wherein step (c) comprises: (c1) heating the composite steel plate at a temperature of 850°C to 950°C and (c2) Transferring the heated composite steel plate to a press mold within a transfer time of 9 to 11 seconds. [7] The method according to claim 6, wherein the cooling of the molded body in step (d) is carried out at a rate of 30 to 120°C / s. [8] The method according to claim 3, wherein the first blank acts as a shock absorbing member for an automobile B-pillar and the second blank acts as a crash support member for an automobile B-pillar. [9] The method according to claim 8, wherein the first blank has a tensile strength (TS) of 550 MPa or more, a yield strength (YS) of 300 MPa or more, and an elongation (EL) of 20% or more, and has a dual-phase structure of ferrite and martensite after hot pressing. [10] The method according to claim 4, further comprising, after the step (a-4), a step of coating a surface of the steel plate with aluminum (Al)-silicon (Al) to improve corrosion resistance.

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