Hot stamped article using tailor welded blank method and method of manufacturing the same

Abstract

A hot stamped product using a tailor-welded blank method and a manufacturing method thereof using a tailor-welded blank (TWB) method capable of controlling a microstructure of a welded portion to prevent the welded portion from being fractured. The hot stamped product is manufactured by welding first and second sheet materials made of different kinds of materials to each other by using the TWB method and hot stamping the welded sheet materials. The first and second sheet materials have different upper transformation (A3) temperatures, the first and second sheet materials are welded to each other via a welded portion, each of the first sheet material and the welded portion has a complex structure including ferrite, bainite, and martensite, and the second sheet material has a martensite structure.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a hot stamped article manufactured using a tailor welded blank (TWB) method and a manufacturing method thereof, and more particularly to a hot stamped article manufactured using a TWB method capable of controlling the microstructure of a welded portion to prevent the welded portion from being fractured and a manufacturing method thereof. BACKGROUND

[0002] In general, a sheet manufactured using a tailor welded blank (TWB) method, which is a welded sheet formed by welding different materials having different strengths and thicknesses to each other, is widely used as a vehicle part because weight reduction and a reduction in the number of parts can be achieved.

[0003] Meanwhile, a hot stamping method is a method of heating a sheet to a high temperature, pressing the sheet using a press in which a coolant flows, simultaneously cooling the sheet to shape the sheet, and is a method of shaping various vehicle parts because complex shaping can be performed and high dimensional accuracy and high strength can be ensured.

[0004] Accordingly, in recent years, the TWB method and the hot stamping method have been simultaneously used to manufacture vehicle parts.

[0005] In general, when the TWB method and the hot stamping method are simultaneously used, a welded portion between sheets welded to each other using the TWB method includes a heat affected zone (HAZ) and a weld. When heat treatment of hot stamping is performed in this state, the heat affected zone and the weld are heat treated at an upper austenite transformation temperature A3 or higher, thereby both being transformed into full austenite and finally being transformed into martensite due to cooling at the time of hot stamping. Before heat treatment of hot stamping is performed, the weld already has a fine martensite structure, and then reversely transforms into austenite, and then is heat treated by hot stamping to be transformed into martensite again. As a result, the weld has a finer martensite structure than the martensite structure of the raw material.

[0006] Since the weld has higher hardness than its surroundings, external stress is concentrated on the weld, thereby the welded portion is often fractured.

[0007] The disclosure of this section is to enhance the understanding of the general background of the present disclosure, and should not be taken as an acknowledgement or any form of suggestion that this content constitutes prior art known to those skilled in the art. SUMMARY

[0008] An object of the disclosure is to provide a hot stamped product using a tailor-welded blank (TWB) method capable of controlling a microstructure of a welded portion by adjusting compositions of different kinds of sheet materials welded to each other to prevent the welded portion from being fractured using the TWB method, and a manufacturing method thereof.

[0009] According to an aspect of the disclosure, the above and other objects can be achieved by providing a hot stamped product. The hot stamped product is manufactured by welding first and second sheet materials made of different kinds of materials to each other using a TWB method and hot stamping the welded sheet materials. The first and second sheet materials have different upper transformation (A3) temperatures, the first and second sheet materials are welded to each other via a welded portion, each of the first sheet material and the welded portion has a composite structure including ferrite, bainite, and martensite, and the second sheet material has a martensite structure.

[0010] The first sheet material can include: 0.04 to 0.12 wt% of carbon (C); 0.80 wt% or less of silicon (Si); 1.60 to 2.00 wt% of manganese (Mn); 0.030 wt% or less of phosphorus (P); 0.015 wt% or less of sulfur (S); 0.10 to 0.60 wt% of chromium (Cr); 0.0008 to 0.0050 wt% of boron (B); the remaining wt% of iron (Fe) and other inevitable impurities. The second sheet material can include: 0.27 to 0.33 wt% of C; 0.40 wt% or less of Si; 1.10 to 1.60 wt% of Mn; 0.030 wt% or less of P; 0.015 wt% or less of S; 0.10 to 0.60 wt% of Cr; 0.0008 to 0.0050 wt% of B; the remaining wt% of Fe and other inevitable impurities.

[0011] The first sheet material can have a hardness of 200 to 350 of Vickers pyramid number (Vickers hardness, HV), the welded portion can have a hardness of 350 to 550 HV, and the second sheet material can have a hardness of 550 to 650 HV.

[0012] At the time of hot stamping, the first sheet material, the welded portion, and the second sheet material can be heated in a temperature range between an A3 temperature of the first sheet material and an A3 temperature of the second sheet material.

[0013] Each of the first sheet material and the welded portion can have a structure including 30% or less of martensite.

[0014] Each of the first sheet material and the welded portion can have a composite structure including 10 to 50% of ferrite, 10 to 50% of bainite, and 30% or less of martensite.

[0015] According to another aspect of the disclosure, a method of manufacturing a hot stamped product is provided. The method includes a preparation step of preparing a first sheet and a second sheet having different A3 temperatures, a welding step of welding the first sheet and the second sheet using a TWB method to prepare a welded sheet in which the first sheet and the second sheet are welded to each other via a welded portion, and a hot stamping step of hot stamping the welded sheet heated in a temperature range between the A3 temperature of the first sheet and the A3 temperature of the second sheet.

[0016] In the preparation step, one sheet can be prepared as the first sheet and include 0.04 to 0.12 wt% of C, 0.80 wt% or less of Si, 1.60 to 2.00 wt% of Mn, 0.030 wt% or less of P, 0.015 wt% or less of S, 0.10 to 0.60 wt% of Cr, 0.0008 to 0.0050 wt% of B, the remaining wt% of Fe and other inevitable impurities. One sheet can also be prepared as the second sheet and include 0.27 to 0.33 wt% of C, 0.40 wt% or less of Si, 1.10 to 1.60 wt% of Mn, 0.030 wt% or less of P, 0.015 wt% or less of S, 0.10 to 0.60 wt% of Cr, 0.0008 to 0.0050 wt% of B, the remaining wt% of Fe and other inevitable impurities.

[0017] The welded sheet can be heated to 810 to 880℃ in the hot stamping step.

[0018] After the hot stamping step, each of the first sheet and the welded portion of the hot stamped product can have a composite structure including ferrite, bainite, and martensite, and the second sheet of the hot stamped product can have a martensite structure.

[0019] After the hot stamping step, each of the first sheet and the welded portion can have a structure including 30% or less of martensite.

[0020] After the hot stamping step, each of the first sheet and the welded portion can have a composite structure including 10 to 50% of ferrite, 10 to 50% of bainite, and 30% or less of martensite.

[0021] After the hot stamping step, the first sheet of the hot stamped product can have a hardness of 200 to 350 HV, the welded portion of the hot stamped product can have a hardness of 350 to 550 HV, and the second sheet of the hot stamped product can have a hardness of 550 to 650 HV. BRIEF DESCRIPTION OF DRAWINGS

[0022] The above and other objects, features and other advantages of the present disclosure will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

[0023] Figure 1A Observation results of microstructures of the comparative example before hot stamping and hardness values thereof are shown;

[0024] Figure 1B Observation results of microstructures of the comparative example after hot stamping and hardness values thereof are shown;

[0025] Figure 2A Observation results of microstructures of the example of the present disclosure before hot stamping and hardness values thereof are shown;

[0026] Figure 2B Observation results of microstructures of the example of the present disclosure after hot stamping and hardness values thereof are shown;

[0027] Figure 3 Enlarged observation results of microstructures of the example of the present disclosure after hot stamping are shown;

[0028] Figure 4A Formation results of microstructures of the welded portion according to the comparative example based on heat treatment temperature are shown; and

[0029] Figure 4B Formation results of microstructures of the welded portion according to the example of the present disclosure based on heat treatment temperature are shown. DETAILED DESCRIPTION

[0030] Hereinafter, embodiments of the present disclosure are described in detail with reference to the accompanying drawings. However, the present disclosure is not limited to the embodiments disclosed below and can be implemented in various different forms. The embodiments herein are provided to make the disclosure of the present disclosure complete and to fully convey the scope of the present disclosure to those skilled in the art.

[0031] The present disclosure relates to a hot stamped article and a manufacturing method thereof, which is manufactured by welding first and second sheet materials made of different kinds of materials to each other using a tailor welded blank (TWB) method and hot stamping the welded sheet materials. The composition of the first and second sheet materials and a heat treatment temperature for hot stamping are controlled to control a microstructure of a welded portion formed by welding the first and second sheet materials together when welding using the TWB method.

[0032] Preferably, the hot stamped article using a TWB method according to embodiments of the present disclosure is formed by welding first and second sheet materials, which are sheet materials of different kinds, to each other using a TWB method. The first and second sheet materials are welded to each other via a welded portion and are heat treated and hot stamped.

[0033] At this time, the structure of the welded portion is formed as a composite structure including ferrite, bainite, and martensite. As described above, in the case where the structure of the welded portion is formed as a composite structure, external stress is not concentrated locally on the welded portion, but is dispersed throughout the entire plate, thereby preventing the fracture of the welded portion.

[0034] Next, a method of manufacturing a hot stamped product having the above-described structure is described.

[0035] The method of manufacturing a hot stamped product according to an embodiment of the disclosure includes a preparation step of preparing a first plate and a second plate having different upper transformation (A3) temperatures, a welding step of welding the first plate and the second plate using a TWB method to prepare a welded plate obtained by welding the first plate and the second plate to each other via a welded portion, and a hot stamping step of hot stamping the welded plate heated in a temperature range between the A3 temperature of the first plate and the A3 temperature of the second plate.

[0036] The preparation step is a step of preparing a first plate and a second plate having different A3 temperatures. For example, the composition of the first plate is adjusted so that the A3 temperature of the first plate is 880°C, and the composition of the second plate is adjusted so that the A3 temperature of the first plate is 810°C.

[0037] In other words, one plate is used as the first plate, which includes: 0.04 to 0.12 wt% of C; 0.80 wt% or less of Si; 1.60 to 2.00 wt% of Mn; 0.030 wt% or less of P; 0.015 wt% or less of S; 0.10 to 0.60 wt% of Cr; 0.0008 to 0.0050 wt% of B; the remaining wt% of Fe and other inevitable impurities.

[0038] In addition, one plate is used as the second plate, which includes: 0.27 to 0.33 wt% of C; 0.40 wt% or less of Si; 1.10 to 1.60 wt% of Mn; 0.030 wt% or less of P; 0.015 wt% or less of S; 0.10 to 0.60 wt% of Cr; 0.0008 to 0.0050 wt% of B; the remaining wt% of Fe and other inevitable impurities.

[0039] The welding step is a step of welding the prepared first plate and the second plate to each other using a TWB method. The first plate and the second plate are welded to prepare a welded plate obtained by welding the first plate and the second plate to each other via a welded portion.

[0040] When the first sheet and the second sheet, each of which has the composition adjusted using the TWB method, are welded to each other, the compositions of the first sheet and the second sheet are mixed with each other in the welded portion. The A3 temperature of the welded portion is adjusted to a range of 810 to 880℃.

[0041] The hot stamping step is a step of hot stamping the heated welded sheet.

[0042] In one example, when the heat treatment is performed before the hot stamping, the heat treatment temperature is maintained in a range of 810 to 880℃ to form a composite structure in the welded portion at the time of the hot stamping.

[0043] As a result, the second sheet is heat-treated at the A3 temperature or higher, whereby the second sheet is transformed into full austenite. The second sheet is cooled at the time of the hot stamping, whereby the second sheet is transformed into full martensite.

[0044] However, the first sheet is heat-treated at the A3 temperature or lower, whereby the first sheet is not transformed into full austenite. Also, the welded portion in which the compositions of the first sheet and the second sheet are mixed with each other is not transformed into full austenite. As a result, each of the first sheet and the welded portion has a composite structure including ferrite, bainite, and martensite due to rapid cooling at the time of the hot stamping.

[0045] Meanwhile, in the case where the first sheet and the second sheet having the above-described compositions are used and the heat treatment temperature is maintained at 810 to 880℃ in the hot stamping step, the second sheet has a full martensite structure, and each of the first sheet and the welded portion has a composite structure including 10 to 50% of ferrite, 10 to 50% of bainite, and 30% or less of martensite.

[0046] As a result of the above-described structure formation, the first sheet has a hardness of 200 to 350 Vickers Pyramid Number (HV), the welded portion has a hardness of 350 to 550 HV, and the second sheet has a hardness of 550 to 650 HV.

[0047] Next, the present disclosure is described based on a comparative example and an example according to the present disclosure (i.e., Example).

[0048] In the comparative example, a first comparative sheet having a tensile strength value of 100K grade and a second comparative sheet having a tensile strength value of 150K grade are prepared using a conventional general steel type. These comparative sheets are welded to each other using the TWB method.

[0049] Table 1 shows the compositions of the first comparative sheet and the second comparative sheet.

[0050] [Table 1]

[0051]

[0052] The welded plate obtained as a result of welding the first and second comparative plate materials to each other was heat treated at 930°C, and then hot stamped.

[0053] At this time, the microstructure of the welded portion and its surroundings before and after hot stamping was observed, and the hardness of each region was measured. The results are shown in Figure 1A and Figure 1B

[0054] Figure 1A Observation results of the microstructure before hot stamping and its hardness values are shown. Figure 1B Observation results of the microstructure after hot stamping and its hardness values are shown.

[0055] As can be seen from Figure 1A and 1B , the comparative examples were heat treated at a temperature higher than the A3 temperature of the first and second comparative plate materials. Thereby, each of the first comparative plate material, the second comparative plate material, and the welded portion was transformed into full austenite. Upon rapid cooling due to hot stamping, the second comparative plate material had a full martensite structure, and the first comparative plate material had a martensite structure and a partial bainite structure. The welded portion had a full martensite structure.

[0056] As a result, as shown in Figure 1B , it can be seen that the hardness of the welded portion is higher than the hardness of the first comparative plate material and the hardness of the second comparative plate material. This result can indicate that external stress is concentrated on the welded portion, whereby the possibility of the welded portion breaking is high.

[0057] On the other hand, in the example, a first plate material identical to the first comparative plate material having a tensile strength value of 100 K class was prepared, wherein the first comparative plate material is a conventional general-purpose steel grade. A second plate material was also prepared, which has a higher content of carbon (C) and manganese (Mn) than the second comparative plate material, thereby having an A3 temperature of 810°C. These plate materials were welded to each other using the TWB method.

[0058] Table 2 shows the composition of the first and second plate materials.

[0059]

Table 2

[0060]

[0061] The welded plate obtained as a result of welding the first and second plate materials to each other was heat treated at 810 to 880°C, and then hot stamped.

[0062] At this time, the microstructure of the welded portion and its surroundings before and after hot stamping was observed, and the hardness of each region was measured. The results are shown in​Figure 2A and Figure 2B are shown.

[0063] Figure 2A Observation results of the microstructure before hot stamping and its hardness value are shown. Figure 2B Observation results of the microstructure after hot stamping and its hardness value are shown.

[0064] It is observed that the microstructure of the welded portion after hot stamping is in a magnified state. As a result, in Figure 3 are shown.

[0065] From Figure 2A and Figure 2B it can be seen that the example is heat treated at a temperature between the A3 temperature of the first sheet and the A3 temperature of the second sheet, whereby the second sheet is transformed into full austenite. Upon rapid cooling due to hot stamping, the second sheet has a full martensite structure. However, upon heat treatment, each of the first sheet and the welded portion is not transformed into full austenite. Thus, upon rapid cooling due to hot stamping, each of the first sheet and the welded portion has a complex structure including ferrite, bainite, and martensite, although the second sheet has a full martensite structure. These results can be confirmed from Figure 3 .

[0066] As a result, as shown in Figure 2B , it can be seen that the hardness of the welded portion is between the hardness of the first sheet and the hardness of the second sheet. This result can indicate that the external stress is prevented from being concentrated on the welded portion, thereby reducing the likelihood of the welded portion breaking.

[0067] Next, experiments are conducted to observe the change in the A3 temperature of the welded portion based on the carbon (C) content of the first sheet and the change in its microstructure caused thereby.

[0068] In the comparative example, the first sheet and the second sheet of Table 2 are used, in which the carbon (C) content of the first sheet is adjusted to 0.2 wt%. In the comparative example, the first sheet and the second sheet of Table 2 are used, in which the carbon (C) content of the first sheet is adjusted to 0.05 wt%.

[0069] The formation of the microstructure of the welded portion is simulated under the condition that the first sheet and the second sheet are welded to each other using the TWB method to prepare a welded sheet and the welded sheet is heat treated at 810 to 880℃.

[0070] The simulation results are shown in Figure 4A and Figure 4B .

[0071] Figure 4AThe formation results of the microstructure of the welded portion according to the comparative example based on the heat treatment temperature are shown. Figure 4B The formation results of the microstructure of the welded portion according to the example based on the heat treatment temperature are shown.

[0072] As can be seen from Figure 4A In the case where the carbon (C) content of the first sheet exceeds the range proposed in the present disclosure, the A3 temperature of the welded portion is lower than the range of 810 to 880℃, which is the heat treatment temperature range. As a result, during heat treatment, the welded portion is transformed into full austenite, and a full martensite structure is formed at the time of rapid cooling due to hot stamping.

[0073] On the other hand, as can be seen from Figure 4B In the case where the carbon (C) content of the first sheet is within the range proposed in the present disclosure, the A3 temperature of the welded portion is adjusted to the range of 810 to 880℃, which is the heat treatment temperature range. As a result, during heat treatment, the welded portion is not transformed into full austenite, is partially transformed into ferrite, and a complex structure containing ferrite, bainite, and martensite is formed at the time of rapid cooling due to hot stamping.

[0074] As is apparent from the above description, according to the embodiments of the present disclosure, the components of the different kinds of sheet materials welded to each other using the TWB method are adjusted to adjust the A3 temperature of the welded portion in which the components of the different kinds of sheet materials are mixed with each other. The temperature is also controlled at the time of heat treatment for hot stamping. Therefore, a complex structure containing ferrite, bainite, and martensite can be formed in the welded portion after hot stamping.

[0075] Therefore, external stress is prevented from being concentrated on the welded portion, thereby preventing the fracture of the welded portion.

[0076] Although the disclosed embodiments of the present disclosure have been described above with reference to the accompanying drawings, it will be understood by those of ordinary skill in the art that the present disclosure can be implemented in various other embodiments without changing the technical idea thereof or its features.

Claims

1. A hot-stamped article, said hot-stamped article being manufactured by welding a first sheet and a second sheet made of different kinds of materials together and then hot-stamping the welded sheets using a sheet welding method, said hot-stamped article comprising: The first plate and the second plate have different upper transition temperatures; The first plate and the second plate are welded together via a welding section; Each of the first plate and the welded portion has a composite structure comprising ferrite, bainite, and martensite; and The second plate has a martensitic structure. The first plate comprises: 0.04 to 0.12 wt% carbon; 0.80 wt% or less silicon; 1.60 to 2.00 wt% manganese; 0.030 wt% or less phosphorus; 0.015 wt% or less sulfur; 0.10 to 0.60 wt% chromium; 0.0008 to 0.0050 wt% boron; and the remaining wt% iron and other unavoidable impurities. The second plate contains: 0.27 to 0.33 wt% carbon; 0.40 wt% or less silicon; 1.10 to 1.60 wt% manganese; 0.030 wt% or less phosphorus; 0.015 wt% or less sulfur; 0.10 to 0.60 wt% chromium; 0.0008 to 0.0050 wt% boron; and the remaining wt% iron and other unavoidable impurities. Wherein, the first plate has a hardness of 200 to 350 HV, the welded portion has a hardness of 350 to 550 HV, and the second plate has a hardness of 550 to 650 HV. Each of the first plate and the welded portion has a composite structure comprising 10 to 50% ferrite, 10 to 50% bainite and 30% or less martensite.

2. The hot-stamped product according to claim 1, wherein, During hot stamping, the first sheet, the welded portion, and the second sheet are heated within a temperature range between the upper transition temperature of the first sheet and the upper transition temperature of the second sheet.

3. The hot-stamped product according to claim 1, wherein, Each of the first plate and the welded portion has a structure containing 30% or less martensite.

4. A method for manufacturing hot-stamped articles, the method comprising: Preparation steps: Prepare a first plate and a second plate with different up-transformation temperatures; Welding steps: The first plate and the second plate are welded using a welded plate method to prepare a welded plate obtained by welding the first plate and the second plate together via welded portions; and Hot stamping step: The welded sheet, heated within a temperature range between the upper transition temperature of the first sheet and the upper transition temperature of the second sheet, is hot stamped. In the preparation step, The first plate is prepared from a plate containing 0.04 to 0.12 wt% carbon, 0.80 wt% or less silicon, 1.60 to 2.00 wt% manganese, 0.030 wt% or less phosphorus, 0.015 wt% or less sulfur, 0.10 to 0.60 wt% chromium, 0.0008 to 0.0050 wt% boron, and the remaining wt% iron and other unavoidable impurities. The second plate is prepared from a plate containing 0.27 to 0.33 wt% carbon, 0.40 wt% or less silicon, 1.10 to 1.60 wt% manganese, 0.030 wt% or less phosphorus, 0.015 wt% or less sulfur, 0.10 to 0.60 wt% chromium, 0.0008 to 0.0050 wt% boron, and the remaining wt% Fe and other unavoidable impurities. Wherein, after the hot stamping step, each of the first sheet metal and the welded portion has a composite structure comprising 10 to 50% ferrite, 10 to 50% bainite, and 30% or less martensite; and Wherein, after the hot stamping step, the first sheet of the hot-stamped product has a hardness of 200 to 350 HV, the welded portion of the hot-stamped product has a hardness of 350 to 550 HV, and the second sheet of the hot-stamped product has a hardness of 550 to 650 HV.

5. The method according to claim 4, wherein, In the hot stamping step, the welded sheet is heated to 810 to 880°C.

6. The method according to claim 4, wherein, After the hot stamping step Each of the first sheet metal and the welded portion of the hot-stamped product has a composite structure comprising ferrite, bainite, and martensite. The second sheet material of the hot-stamped product has a martensitic structure.

7. The method according to claim 6, wherein, After the hot stamping step, each of the first sheet metal and the welded portion has a structure containing 30% or less martensite.

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