WO2024136317A1 - Tôle d'acier laminée à froid et son procédé de fabrication - Google Patents

Tôle d'acier laminée à froid et son procédé de fabrication Download PDF

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WO2024136317A1
WO2024136317A1 PCT/KR2023/020693 KR2023020693W WO2024136317A1 WO 2024136317 A1 WO2024136317 A1 WO 2024136317A1 KR 2023020693 W KR2023020693 W KR 2023020693W WO 2024136317 A1 WO2024136317 A1 WO 2024136317A1
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steel sheet
cold
rolled steel
less
hot
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PCT/KR2023/020693
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English (en)
Korean (ko)
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김상현
임영록
서창효
정기택
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주식회사 포스코
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  • the present invention relates to a cold rolled steel sheet and a manufacturing method thereof, and more specifically, to a cold rolled steel sheet that can be preferably applied to collision energy absorbing members such as body-in-white (BIW) structural members, and a manufacturing method thereof. .
  • BIW body-in-white
  • Patent Document 1 shows that steel containing 0.18 to 0.3% C is continuously annealed and then water cooled to room temperature, followed by overaging treatment at a temperature of 120 to 300°C for 1 to 15 minutes, so that the martensite volume ratio is 80 to 80. 97%, and the remainder is ferrite.
  • Ultra-high-strength steel can be manufactured by tempering cold-rolled steel sheets by rapidly cooling them to room temperature after annealing them in two-phase or single-phase zones.
  • the elongation rate In addition, in general, as the strength of the steel sheet increases, the elongation decreases, which causes a problem in that forming processability deteriorates, so its application as a cold stamping material is limited. In order to form steel materials into complex shapes, the elongation rate must be basically high. As a representative method to increase the elongation rate, a method using the TRIP phenomenon by introducing retained austenite as shown in Patent Document 2 is widely used. However, when a large amount of ferrite is introduced to secure additional elongation in addition to retained austenite, as shown in Patent Document 2, yield strength and hole expandability may be inferior.
  • Patent Document 1 Japanese Patent Publication No. 1992-289120
  • Patent Document 2 Japanese Patent Publication No. 2002-382250
  • One aspect of the present invention is to provide a cold rolled steel sheet and a manufacturing method thereof.
  • One preferred aspect of the present invention is to provide a cold-rolled steel sheet with excellent strength, elongation, and hole expansion properties, and a method for manufacturing the same.
  • [C t/5 ] means the average C content of the area from the surface to 1/5 of the predetermined depth (t) in the thickness direction of the steel sheet
  • [C 3t/5 ] means the average C content of the area corresponding to 3/5 of the predetermined depth (t) in the thickness direction of the steel sheet from 1/5 of the predetermined depth (t)
  • [C M ] is the steel sheet Means the average C content of.
  • the fraction of tempered martensite may be 40% or more.
  • the predetermined depth (t) may be 50 to 100 ⁇ m.
  • the cold rolled steel sheet may have one of a hot dip galvanized layer (GI), an alloyed hot dip galvanized layer (GA), and an electro galvanized layer (EG) formed on at least one surface.
  • GI hot dip galvanized layer
  • GA alloyed hot dip galvanized layer
  • EG electro galvanized layer
  • the cold rolled steel sheet may be 30000MPa% ⁇
  • Another embodiment of the present invention is, in weight percent, carbon (C): 0.15 to 0.25%, manganese (Mn): 1.5 to 2.5%, silicon (Si): 1.0 to 2.0%, phosphorus (P): 0.1% or less ( 0% excluded), Sulfur (S): 0.03% or less (0% excluded), Aluminum (Al): 0.01 to 0.1%, Molybdenum (Mo): 0.01% or less (0% excluded), Boron (B) : Heating a slab containing 0.001% or less (excluding 0%), the balance Fe, and other unavoidable impurities at 1100-1300°C; Obtaining a hot rolled steel sheet by finishing hot rolling the heated slab in Ar3 or higher; Winding the hot rolled steel sheet at 700°C or lower; Obtaining a cold-rolled steel sheet by pickling and then cold-rolling the coiled hot-rolled steel sheet; Primary heating the cold-rolled steel sheet at 780°C or more and less than Ac3-10°C for 30 seconds or more under atmospheric conditions
  • the cold rolling can be performed at a cold rolling reduction rate of 30 to 80%.
  • the step of forming a hot-dip galvanized layer by immersing the cold-rolled steel sheet in a hot-dip galvanizing bath of 440 to 480° C. may be further included.
  • the step of alloying heat treatment of the cold rolled steel sheet at 450 to 520° C. may be further included.
  • the step of forming an electrogalvanized layer may be further included.
  • a cold rolled steel sheet and a manufacturing method thereof can be provided.
  • a cold rolled steel sheet excellent in strength, elongation, and hole expandability and a method for manufacturing the same can be provided.
  • Figure 1 is a photograph of the microstructure of Inventive Example 1 according to an embodiment of the present invention observed with an SEM microscope.
  • the C is an invasive solid solution element and is the most effective and important element in improving the strength of steel. If the C content is less than 0.15%, it may be difficult to obtain the yield ratio and tensile strength targeted in the present invention. If the C content exceeds 0.25%, the strength may rapidly increase due to excessive formation of martensite during cooling due to an increase in hardenability, resulting in inferior elongation and deteriorated weldability. Therefore, the C content is preferably in the range of 0.15 to 0.25%. The lower limit of the C content is more preferably 0.18%, and even more preferably 0.2%. It is more preferable that the upper limit of the C content is 0.24%.
  • Mn is an element added to ensure strength. If the Mn content is less than 1.5%, it may be difficult to secure the level of strength desired in the present invention. If the Mn content exceeds 2.5%, the Ms temperature decreases during cooling after annealing, making it difficult to smoothly secure the initial martensite phase. This means that it may be difficult to simultaneously secure the strength, elongation, and hole expandability targeted by the present invention due to a decrease in the tempered martensite fraction in the Q&P (Quenching & Partitioning) process. In addition, Mn is segregated in the thickness direction, making it easier to form Mn bands within the slab, which may increase the likelihood of defects occurring during the rolling process along with continuous casting cracks. Therefore, the Mn content is preferably in the range of 1.5 to 2.5%. The lower limit of the Mn content is more preferably 1.8%, and even more preferably 2.0%. It is more preferable that the upper limit of the Mn content is 2.4%.
  • Si is a key element in TRIP (Transformation Induced Plasticity) steel that secures an appropriate level of retained austenite fraction and increases elongation by suppressing the precipitation of cementite. If the Si content is less than 1.0%, the control of cementite precipitation during the reheating and over-aging steps may not be smooth, so the fraction of retained austenite ultimately obtained may be small or stability may be low, resulting in poor elongation. On the other hand, if the Si content exceeds 2.0%, the physical properties of the weld zone deteriorate as LME (Liquid Metal Embrittlement) cracks occur, and the surface properties and plating properties of the steel also deteriorate. Therefore, the Si content is preferably in the range of 1.0 to 2.0%. It is more preferable that the lower limit of the Si content is 1.2%. It is more preferable that the upper limit of the Si content is 1.8%.
  • TRIP Transformation Induced Plasticity
  • P is an impurity element contained in steel, and if its content exceeds 0.1%, weldability may deteriorate and steel may become brittle.
  • S is an impurity that is inevitably included in steel, and if its content exceeds 0.03%, ductility and weldability may decrease.
  • Al is an element added to remove oxygen in molten steel, and like Si, it is effective in stabilizing retained austenite by suppressing precipitation of cementite during the reheating and overaging stages. If the Al content is less than 0.01%, the deoxidation effect cannot be sufficiently obtained, which may impair the cleanliness of the steel material. If the Al content exceeds 0.1%, not only does the castability of the slab deteriorate, but the temperature required for single-phase heating during annealing increases, which may lead to production and equipment problems. Therefore, the Al content is preferably in the range of 0.01 to 0.1%. It is more preferable that the upper limit of the Al content is 0.05%.
  • Mo is a representative element that can improve hardenability, but in the present invention, the balance of strength, elongation, and hole expandability is important, and since it is a steel with a tensile strength of 980 MPa, it is added for the purpose of improving hardenability and facilitating the formation of martensite. There is no need to. When Mo is added, there is a problem of increasing the manufacturing cost, so it is preferable not to intentionally add Mo in the present invention. Therefore, in the present invention, the Mo content can be limited to 0.01% or less. Meanwhile, considering that it is inevitably included during the manufacturing process, the lower limit may be 0.001%.
  • B is a representative element that can improve hardenability, but in the present invention, the balance of strength, elongation, and hole expandability is important, and since it is a steel with a tensile strength of 980 MPa, it is added for the purpose of improving hardenability and facilitating the formation of martensite. There is no need to do this. Therefore, in the present invention, it is preferable not to add B. Therefore, in the present invention, the content of B can be limited to 0.001% or less. Meanwhile, considering that it is inevitably included during the manufacturing process, the lower limit may be 0.0001%.
  • the remaining ingredient is iron (Fe).
  • Fe iron
  • the microstructure of the cold rolled steel sheet of the present invention is expressed in area%, ferrite: more than 10% and less than 45%, retained austenite: 7 to 15%, fresh martensite: less than 10% (including 0%), tempered martensite and bay. Total of knights: It is desirable to include 40-80%.
  • the ferrite is a structure advantageous for securing elongation. If the ferrite fraction is 10% or less, it may be difficult to secure the elongation desired in the present invention, and if the ferrite fraction exceeds 45%, it may be difficult to secure the strength and hole expandability desired in the present invention. .
  • the retained austenite, along with ferrite formed during the annealing process, is an essential structure for securing elongation.
  • the fraction of retained austenite is less than 7%, it may be difficult to secure the elongation at the level desired in the present invention. If the fraction of retained austenite exceeds 15%, it may be difficult to secure the desired elongation due to insufficient stability of the retained austenite. If the fresh martensite fraction exceeds 10%, it may be difficult to obtain a steel material with excellent strength, elongation, and hole expansion properties.
  • the tempered martensite and bainite are structures necessary to secure strength and hole expansion, and when the phase transformation progresses to include the fraction in the above range, the residual austenite, which is stable at room temperature, is ultimately reduced to a range of 7 to 15%. It can be included.
  • the tempered martensite and bainite fractions are less than 40%, it is difficult to secure the final retained austenite fraction desired in the present invention due to insufficient total transformation amount, and if more than 10% of fresh martensite is secured, it is used for the purpose of the present invention. It may be difficult to secure strength, elongation, and hole expandability. If the fraction of tempered martensite and bainite exceeds 80%, it is possible to secure strength and hole expandability, but it may be difficult to secure the high elongation aimed at in the present invention due to the lack of ferrite and retained austenite fraction. there is. Meanwhile, it is more preferable that the fraction of tempered martensite is 40% or more.
  • the cold-rolled steel sheet of the present invention includes a soft layer having a predetermined depth (t) in the thickness direction of the steel sheet from the surface, and the soft layer preferably satisfies the following equations 1 and 2.
  • the predetermined depth (t) may be 50 to 100 ⁇ m. More specifically, the predetermined depth (t) may be 50 to 80 ⁇ m.
  • the surface refers to the surface of the base steel sheet, excluding the plating layer that may be formed on the surface of the base steel sheet.
  • the soft layer preferably satisfies the following equations 1 and 2.
  • [C t/5 ] means the average C content of the area from the surface to 1/5 of the predetermined depth (t) in the thickness direction of the steel sheet
  • [C 3t/5 ] means the average C content of the area corresponding to 3/5 of the predetermined depth (t) in the thickness direction of the steel sheet from 1/5 of the predetermined depth (t)
  • [C M ] is the steel sheet Means the average C content of.
  • Equations 1 and 2 above If the conditions of Equations 1 and 2 above are not satisfied, it may be difficult to obtain good LME characteristics.
  • the [C 1/5t ] microstructure is expressed in area% and may include 80% or more of ferrite, the balance bainite, and tempered martensite.
  • the cold-rolled steel sheet of the present invention may have one of a hot-dip galvanized layer (GI), an alloyed hot-dip galvanized layer (GA), and an electrogalvanized layer (EG) formed on at least one surface.
  • GI hot-dip galvanized layer
  • GA alloyed hot-dip galvanized layer
  • EG electrogalvanized layer
  • the cold rolled steel sheet of the present invention may have yield strength (YS): 600 MPa or more, tensile strength (TS): 980 MPa or more, total elongation (T-El): 21% or more, and hole expandability (HER): 20 to 40%. . Since the higher the yield strength, tensile strength, total elongation, and uniform elongation, the higher the advantage, the present invention does not specifically limit the upper limits of the yield strength, tensile strength, and total elongation.
  • the cold rolled steel sheet of the present invention may be 30000MPa% ⁇
  • the upper limit of the X value is more preferably 65,000 MPa%, and even more preferably 60,000 MPa%.
  • the slab is heated at 1100 ⁇ 1300°C.
  • the slab heating is performed to smoothly perform the subsequent hot rolling process and obtain the target physical properties of the steel sheet. If the slab heating temperature is less than 1100°C, a problem may occur in which the hot rolling load rapidly increases. If the slab heating temperature exceeds 1300°C, the amount of surface scale may increase and productivity may decrease.
  • the heated slab is subjected to finish hot rolling in Ar3 or higher to obtain a hot rolled steel sheet.
  • the finishing hot rolling temperature is lower than Ar3, rolling of ferrite and austenite in the two-phase region or ferrite phase occurs to create a mixed structure, and malfunction of the equipment may occur due to fluctuations in the hot rolling load.
  • Ar3 can be obtained through Equation 1 below.
  • the hot rolled steel sheet is wound at 700°C or lower. If the coiling temperature exceeds 700°C, an excessive oxide film may be generated on the surface of the steel sheet, causing defects. It is more preferable that the coiling temperature is 650°C or lower. On the other hand, as the coiling temperature is lowered, the strength of the hot-rolled steel sheet increases, which has the disadvantage of increasing the rolling load of cold rolling, which is a post-process. However, since it is not a factor that makes actual production impossible, the present invention does not specifically limit the lower limit. . However, as an example, the lower limit of the coiling temperature may be 300°C.
  • the pickling is a process for removing the oxide layer formed on the surface of the wound hot-rolled steel sheet.
  • the cold rolling can be performed at a cold rolling reduction rate of 30 to 80%. If the cold rolling reduction rate is less than 30%, not only is it difficult to secure the target thickness, but there is a risk that the formation of austenite and securing physical properties during annealing heat treatment may be affected due to the remaining crystal grains formed during hot rolling. If the cold rolling reduction ratio exceeds 80%, material deviation may occur due to uneven rolling reduction in the length and width directions due to work hardening that occurs during cold rolling, and the target thickness may be secured due to the rolling load. can be difficult.
  • the cold rolled steel sheet is first heated for more than 30 seconds at 780°C or more and less than Ac3-10°C under atmospheric conditions with a dew point temperature of 0 to 30°C.
  • the first heating is to form some annealed ferrite in addition to the retained austenite in order to secure an elongation of 21% or more.
  • the dew point temperature is less than 0°C, the soft layer desired in the present invention cannot be sufficiently formed on the surface of the steel sheet. If the dew point temperature exceeds 30°C, there is a problem of reduced equipment life and productivity. It is more preferable that the lower limit of the dew point temperature is 2°C. It is more preferable that the upper limit of the dew point temperature is 25°C.
  • the first heating temperature is less than 780°C, excessive annealed ferrite may be formed, making it difficult to secure strength and hole expandability. If the primary heating temperature is above Ac3-10°C, the fraction of annealed ferrite may become insufficient due to heating at the single-phase level, resulting in poor elongation. It is more preferable that the lower limit of the first heating temperature is 790°C. It is more preferable that the upper limit of the first heating temperature is Ac3-15°C. If the primary heating time is less than 30 seconds, there is a disadvantage in that sufficient annealing effect is not obtained. Meanwhile, the longer the primary heating time, the more advantageous it is, so the present invention does not specifically limit the lower limit. However, as an example, the upper limit of the first heating time may be 500 seconds. Meanwhile, the above-mentioned Ac3 can be obtained through Equation 2 below.
  • the heated cold-rolled steel sheet is first cooled to 600-750°C at an average cooling rate of 1-10°C/s. If the primary cooling end temperature is less than 600°C, there is a risk that a phase such as ferrite or bainite may be formed, resulting in a decrease in strength. If the primary cooling end temperature exceeds 750°C, problems may occur in the actual production line.
  • the lower limit of the primary cooling end temperature is more preferably 610°C, and even more preferably 630°C.
  • the upper limit of the primary cooling end temperature is more preferably 740°C, and even more preferably 730°C. If the first average cooling rate is less than 1°C/s, ferrite is formed during cooling, making it difficult to secure the target strength.
  • the primary average cooling rate exceeds 10°C/s, the average cooling rate decreases during secondary cooling, making it difficult to secure sufficient martensite, which leads to a decrease in the fraction of tempered martensite, thereby reducing strength and hole expandability. It may be difficult to secure both at the same time. It is more preferable that the upper limit of the primary average cooling rate is 6°C/s.
  • the primary cooled cold rolled steel sheet is secondarily cooled to 150°C to Ms at an average cooling rate of 10 to 45°C/s.
  • the secondary cooling end temperature is preferably in the range of 150°C to Ms. If the secondary cooling end temperature is less than 150°C, the tempered martensite fraction may become excessively high and the retained austenite fraction may decrease, resulting in poor elongation.
  • the secondary cooling end temperature exceeds Ms, the formation of a tempered martensite structure may become difficult, resulting in poor strength and hole expandability. It is more preferable that the lower limit of the secondary cooling end temperature is 180°C. If the secondary average cooling rate is less than 10°C/s, some bainite structure may be formed during secondary cooling from the primary cooling section. If the secondary average cooling rate exceeds 45°C/s, the surface shape of the steel sheet may become inferior due to a rapid martensite transformation rate at the time of secondary cooling, and a problem of material deviation in the width direction may occur. It is more preferable that the lower limit of the secondary average cooling rate is 12°C/s. It is more preferable that the upper limit of the secondary average cooling rate is 42°C/s. Meanwhile, the above-mentioned Ms can be obtained through Equation 3 below.
  • the secondarily cooled cold rolled steel sheet is secondarily heated at Ms ⁇ 480°C and then subjected to overaging treatment for 1 to 30 minutes.
  • the secondary heating and over-aging treatment is intended to improve toughness by changing the high dislocation density and hard martensite formed during secondary cooling into tempered martensite.
  • C is enriched with austenite remaining from annealing by securing a sufficient amount of tempered martensite and bainite (partitioning).
  • the martensite transformation initiation temperature (Ms) of C-enriched austenite is lowered to below room temperature, and a large amount of retained austenite is ultimately formed, thereby securing the physical properties targeted by the present invention. .
  • the secondary heating temperature is less than Ms or exceeds 480°C, it is difficult to secure the fraction of microstructure targeted by the present invention. It is more preferable that the lower limit of the secondary heating temperature is 360°C. It is more preferable that the upper limit of the secondary heating temperature is 460°C. If the over-aging treatment time is less than 1 minute, sufficient transformation does not proceed, making it difficult to obtain the partitioning effect. If the over-aging treatment time exceeds 30 minutes, the secondary heating and over-aging treatment section must be very long, and productivity is reduced, so it may be difficult to apply it to an actual production line.
  • the cold-rolled steel sheet may be immersed in a hot-dip galvanizing bath at 440 to 480° C. to form a hot-dip galvanizing layer. If the hot dip galvanizing bath temperature is less than 440°C, management of the hot dip galvanizing bath may be difficult, and if the hot dip galvanizing bath temperature exceeds 480°C, the final elongation may decrease.
  • the cold-rolled steel sheet on which the hot-dip galvanized layer is formed may be subjected to alloying heat treatment at 450 to 520°C. If the alloying heat treatment temperature is less than 450°C, it may be difficult to form a sufficient Fe-Zn alloy plating layer, and if the alloying heat treatment temperature exceeds 520°C, the residual austenite formed in the previous step is decomposed, resulting in the final elongation. It can be inferior.
  • an electric zinc plating layer can be formed.
  • a slab having an alloy composition shown in Table 1 below was heated at 1100 to 1300°C and then subjected to final hot rolling at 900 to 1000°C to produce a hot rolled steel sheet.
  • the hot-rolled steel sheet was wound at 350-650°C, pickled, and cold-rolled at a cold rolling reduction rate of 45-65%, and then the cold-rolled steel sheet was manufactured by applying the conditions shown in Table 2 below. Meanwhile, the conditions listed in Table 2 below were based on the surface temperature of the steel plate. Thereafter, hot-dip galvanizing or hot-dip galvanizing and alloying heat treatment were performed on the manufactured cold-rolled steel sheet under the conditions shown in Table 2 below.
  • phase fraction of the microstructure was measured using XRD and EBSD for t/4 (t: steel thickness) of the cold rolled steel sheet.
  • the formation of a soft layer with a predetermined depth (t) was measured using GDS.
  • the average C content [C 3t/5 ] of the area corresponding to 3/5 of the predetermined depth (t) was calculated as the average value of the C content measured through GDS, and [C M ] is the OES and It was measured using the ICP C component analysis results.
  • Yield strength (YS), tensile strength (TS), total elongation (T-El), and uniform elongation (U-El) were determined for cold-rolled steel sheets according to JIS standards (gauge length: 25 ⁇ 50 mm, total specimen length: 200 ⁇ After processing into a specimen measuring 260 mm), a tensile test was performed and measured at a test speed of 28 mm/min.
  • Hole expandability was measured according to the ISO 16330 standard, and the hole was sheared with a clearance of 12% using a 10 mm diameter punch.
  • LME was evaluated based on ISO 18278-2, and was marked as ⁇ if a C Type crack occurred, and as ⁇ if it did not occur.
  • Figure 1 is a photograph of the microstructure of Invention Example 1 observed with an SEM microscope. As can be seen from Figure 1, in the case of Inventive Example 1, it can be seen that the microstructure of the appropriate fraction desired by the present invention is secured.

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Abstract

La présente invention concerne une tôle d'acier laminée à froid et son procédé de fabrication et, plus particulièrement, une tôle d'acier laminée à froid qui est de préférence applicable à des éléments d'absorption d'énergie de collision tels qu'un élément structural de caisse en blanc (BIW), et un procédé de fabrication de la tôle d'acier laminée à froid. Un aspect de la présente invention est de fournir une tôle d'acier laminée à froid ayant une excellente résistance, un excellent allongement et d'excellentes propriétés d'expansion de trou, et son procédé de fabrication.
PCT/KR2023/020693 2022-12-21 2023-12-14 Tôle d'acier laminée à froid et son procédé de fabrication WO2024136317A1 (fr)

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Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2010222696A (ja) * 2009-03-25 2010-10-07 Nisshin Steel Co Ltd 調質熱処理済み鋼帯およびその製造方法
US20180237877A1 (en) * 2017-02-17 2018-08-23 GM Global Technology Operations LLC Mitigating liquid metal embrittlement in zinc-coated press hardened steels
KR20200062926A (ko) * 2018-11-27 2020-06-04 주식회사 포스코 수소취성 저항성이 우수한 초고강도 냉연강판 및 그 제조 방법
KR20220013405A (ko) * 2019-07-30 2022-02-04 제이에프이 스틸 가부시키가이샤 고강도 강판 및 그 제조 방법
KR20220092974A (ko) * 2020-03-16 2022-07-04 닛폰세이테츠 가부시키가이샤 강판

Family Cites Families (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2528387B2 (ja) 1990-12-29 1996-08-28 日本鋼管株式会社 成形性及びストリップ形状の良好な超高強度冷延鋼板の製造法

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2010222696A (ja) * 2009-03-25 2010-10-07 Nisshin Steel Co Ltd 調質熱処理済み鋼帯およびその製造方法
US20180237877A1 (en) * 2017-02-17 2018-08-23 GM Global Technology Operations LLC Mitigating liquid metal embrittlement in zinc-coated press hardened steels
KR20200062926A (ko) * 2018-11-27 2020-06-04 주식회사 포스코 수소취성 저항성이 우수한 초고강도 냉연강판 및 그 제조 방법
KR20220013405A (ko) * 2019-07-30 2022-02-04 제이에프이 스틸 가부시키가이샤 고강도 강판 및 그 제조 방법
KR20220092974A (ko) * 2020-03-16 2022-07-04 닛폰세이테츠 가부시키가이샤 강판

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