Cold-rolled steel sheet and its manufacturing method

Abstract

The present invention relates to a cold-rolled steel sheet and a manufacturing method thereof, and more particularly to a cold-rolled steel sheet suitable for use in collision energy absorbing members such as body-in-white (BIW) structural members, and a manufacturing method thereof. One aspect of the present invention aims to provide a cold-rolled steel sheet excellent in strength, elongation, and hole expandability, and a manufacturing method thereof.

Description

[Technical Field]

[0001] The present invention relates to a cold-rolled steel sheet and a manufacturing method thereof, and more particularly to a cold-rolled steel sheet suitable for use in collision energy absorbing members such as body-in-white (BIW) structural members, and a manufacturing method thereof. [Background technology]

[0002] In recent years, research into reducing vehicle weight has been actively conducted in the automotive industry, primarily in developed countries, particularly in Europe, due to fuel economy regulations and performance improvements. To meet these demands, the steel industry is working to further reduce steel thickness while increasing the strength of comparable materials (such as Mg, Al, and CFRP) at the same grade. Along with weight reduction, stricter CO2 emission regulations and the rapid shift to an electric vehicle era have led to stricter safety regulations for passengers and pedestrians, leading to a trend toward greater stability and strength in auto body materials. In particular, there is growing demand for high-strength steel in the 980-1180 MPa range. For use in crash energy absorption components, 980 MPa steel must not only have high elongation so that it can be formed into complex shapes, but also have excellent hole expandability to prevent fracture during axial deformation.

[0003] Such structural components require high yield strength and hole expandability to effectively absorb impact energy. A typical manufacturing method for increasing yield strength is to utilize water cooling during continuous annealing. Patent Document 1 is a representative example of such a method. Patent Document 1 relates to the production of a steel material having a martensite volume fraction of 80 to 97% and the remainder being ferrite, by continuously annealing a steel material containing 0.18 to 0.3% C, water cooling it to room temperature, and then overaging it at a temperature of 120 to 300°C for 1 to 15 minutes. Ultra-high strength steel can be produced by tempering a cold-rolled steel sheet after annealing in the two-phase region or single-phase region and then quenching it to room temperature. However, although this method provides excellent yield strength and hole expandability, it can cause problems such as deterioration in coil shape quality due to temperature unevenness in the width and length directions, which can lead to problems such as poor material quality and reduced workability in certain locations during roll-forming part processing.

[0004] Furthermore, in general, as the strength of a steel sheet increases, the elongation decreases, resulting in a problem of poor formability, limiting its application to cold stamping materials. To form a steel material into a complex shape, a high elongation is fundamentally required. A widely used representative method for increasing the elongation is to introduce retained austenite and utilize the TRIP phenomenon, as in Patent Document 2. However, when a large amount of ferrite is introduced in addition to the retained austenite to ensure a higher elongation, as in Patent Document 2, the yield strength and hole expandability become inferior.

[0005] Therefore, in order to solve the above-mentioned problems, it is necessary to develop an ultra-high strength steel sheet having excellent elongation and hole expandability and a tensile strength of 980 MPa or more. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 4-289120 [Patent Document 2] Japanese Patent Application Laid-Open No. 2002-382250 Summary of the Invention [Problem to be solved by the invention]

[0007] One aspect of the present invention is to provide a cold-rolled steel sheet and a method for manufacturing the same.

[0008] A preferred aspect of the present invention is to provide a cold-rolled steel sheet excellent in strength, elongation and hole expandability, and a method for producing the same. [Means for solving the problem]

[0009] One embodiment of the present invention is a composition containing, 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 (excluding 0%), sulfur (S): 0.03% or less (excluding 0%), aluminum (Al): 0.01 to 0.1%, molybdenum (Mo): 0.01% or less (excluding 0%), boron (B): 0.001% or less (excluding 0%), and the balance Provided is a cold-rolled steel sheet containing Fe and other unavoidable impurities, and having a microstructure containing, by area%, more than 10% and not more than 45% ferrite, 7 to 15% retained austenite, 10% or less (including 0%) fresh martensite, and 40 to 80% in total of tempered martensite and bainite, and including a soft layer having a predetermined depth (t) from the surface in the thickness direction of the steel sheet, wherein the soft layer satisfies the following relational expressions 1 and 2.

[0010] [Equation 1][C t / 5 ] / [C M ]<0.3

[0011] [Equation 2][C 3t / 5 ] / [C M ]<0.6

[0012] (However, in the above relational expressions 1 and 2, [C t / 5 ] means the average C content in the region from the surface to 1 / 5 of the specified depth (t) in the thickness direction of the steel plate, and [C 3t / 5] means the average C content in the region corresponding to the 1 / 5 point of the predetermined depth (t) in the thickness direction of the steel plate to the 3 / 5 point of the predetermined depth (t), and [C M ] means the average C content of the steel sheet.)

[0013] The fraction of the tempered martensite may be 40% or more.

[0014] The predetermined depth (t) may be 50 to 100 μm.

[0015] The cold-rolled steel sheet may have one of a hot-dip galvanized layer (GI), a galvannealed layer (GA) and an electrogalvanized layer (EG) formed on at least one surface thereof.

[0016] The cold-rolled steel sheet may satisfy the condition: 30000 MPa%≦X=yield strength×[total elongation+(2×hole expandability)]≦70000 MPa%.

[0017] Another embodiment of the present invention relates to a steel sheet manufacturing method, comprising the steps of: heating a slab containing, by weight, 0.15 to 0.25% carbon (C), 1.5 to 2.5% manganese (Mn), 1.0 to 2.0% silicon (Si), 0.1% or less (excluding 0%) of phosphorus (P), 0.03% or less (excluding 0%) of sulfur (S), 0.01 to 0.1% of aluminum (Al), 0.01% or less (excluding 0%) of molybdenum (Mo), 0.001% or less (excluding 0%) of boron (B), the balance being Fe and other unavoidable impurities, at 1100 to 1300°C; finish hot rolling the heated slab at Ar3 or more to obtain a hot-rolled steel sheet; and rolling the hot-rolled steel sheet at 700°C. and coiling the hot-rolled steel sheet at a temperature of 00°C or lower; pickling the coiled hot-rolled steel sheet and then cold-rolling it to obtain a cold-rolled steel sheet; primarily heating the cold-rolled steel sheet at a temperature of 780°C or higher and lower than Ac3-10°C for 30 seconds or more under atmospheric conditions with a dew-point temperature of 0 to 30°C; primarily cooling the heated cold-rolled steel sheet to 600 to 750°C at an average cooling rate of 1 to 10°C / s; secondarily cooling the primarily-cooled cold-rolled steel sheet to 150°C to Ms at an average cooling rate of 10 to 45°C / s; and secondarily heating the secondarily-cooled cold-rolled steel sheet to Ms to 480°C, followed by overaging for 1 to 30 minutes.

[0018] The cold rolling may be carried out at a cold reduction rate of 30 to 80%.

[0019] After the second heating and holding, the method may further include a step of immersing the cold-rolled steel sheet in a hot-dip galvanizing bath at 440 to 480° C. to form a hot-dip galvanized layer.

[0020] The method may further include a step of subjecting the cold-rolled steel sheet to an alloying heat treatment at 450 to 520°C after forming the hot-dip galvanized layer.

[0021] After the second heating and holding, the method may further include forming an electrogalvanized layer. [Effects of the Invention]

[0022] According to one aspect of the present invention, a cold-rolled steel sheet and a method for manufacturing the same can be provided.

[0023] According to a preferred embodiment of the present invention, a cold-rolled steel sheet excellent in strength, elongation, and hole expandability and a method for manufacturing the same can be provided. [Brief explanation of the drawings]

[0024] [Figure 1] 1 is a photograph of the microstructure of Example 1 according to one embodiment of the present invention, observed with an SEM microscope. DETAILED DESCRIPTION OF THE INVENTION

[0025] The terminology used herein is merely for the purpose of referring to particular embodiments and is not intended to limit the present invention. As used herein, the singular form includes the plural form unless the phrase clearly dictates otherwise. As used herein, the term "comprises" means to embody a particular property, region, constant, step, operation, element, component, and / or group, and does not exclude the presence or addition of other particular properties, regions, constants, steps, operations, elements, components, and / or groups.

[0026] Unless otherwise defined, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by a person of ordinary skill in the art to which the present invention pertains. Terms defined in commonly used dictionaries are additionally interpreted to have a meaning consistent with the relevant technical literature and the presently disclosed content, and are not interpreted in an ideal or very formal sense unless defined.

[0027] Hereinafter, a cold-rolled steel sheet according to an embodiment of the present invention will be described. First, the alloy composition will be described. The contents of the alloy compositions described below are in weight percent unless otherwise specified.

[0028] Carbon (C): 0.15~0.25% C is an interstitial solid solution element and is the most effective and important element for improving the strength of steel. If the C content is less than 0.15%, it is difficult to achieve the yield ratio and tensile strength targeted in the present invention. If the C content exceeds 0.25%, the hardenability increases, causing excessive martensite formation during cooling, resulting in a rapid increase in strength, poor elongation, and reduced 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%. The upper limit of the C content is more preferably 0.24%.

[0029] Manganese (Mn): 1.5-2.5% Mn is an element added to ensure strength. If the Mn content is less than 1.5%, it is difficult to ensure the strength level 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 obtain the initial martensite phase. That is, due to a decrease in the tempered martensite fraction in the Q&P (Quenching & Partitioning) process, it is difficult to simultaneously obtain the strength, elongation, and hole expandability targeted in the present invention. Furthermore, Mn segregates in the thickness direction, facilitating the formation of Mn bands in the slab, which increases the possibility of defects during the rolling process as well as cracks in continuous casting. 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%. The upper limit of the Mn content is more preferably 2.4%.

[0030] Silicon (Si): 1.0-2.0% Silicon (Si) is a key element in TRIP (Transformation Induced Plasticity) steel, which suppresses cementite precipitation, thereby ensuring an appropriate level of retained austenite fraction and increasing elongation. If the Si content is less than 1.0%, the cementite precipitation is not smoothly controlled during the reheating and overaging stages, resulting in a small fraction or poor stability of the finally obtained retained austenite, resulting in poor elongation. On the other hand, if the Si content exceeds 2.0%, LME (Liquid Metal Embrittlement) cracks are generated, deteriorating the physical properties of the weld and the surface properties and platability of the steel. Therefore, the Si content is preferably in the range of 1.0 to 2.0%. The lower limit of the Si content is more preferably 1.2%. The upper limit of the Si content is more preferably 1.8%.

[0031] Phosphorus (P): 0.1% or less (excluding 0%) P is an impurity element contained in steel, and if its content exceeds 0.1%, it may deteriorate weldability and cause embrittlement of the steel. The lower the P content, the more advantageous it is, but 0% is excluded in consideration of the fact that P is inevitably contained in the manufacturing process. Therefore, the P content is preferably in the range of 0.1% or less (excluding 0%). The P content is more preferably 0.03% or less.

[0032] Sulfur (S): 0.03% or less (excluding 0%) Like P, S is an impurity that is inevitably contained in steel, and if its content exceeds 0.03%, ductility and weldability decrease. The lower the S content, the more advantageous it is, but 0% is excluded in consideration of the fact that S is inevitably contained in the manufacturing process. Therefore, the S content is preferably in the range of 0.03% or less (excluding 0%). The S content is more preferably 0.005% or less.

[0033] Aluminum (Al): 0.01 to 0.1% Al is an element added to remove oxygen from molten steel, and, like Si, is effective in stabilizing retained austenite by suppressing cementite precipitation during reheating and overaging. If the Al content is less than 0.01%, the deoxidizing effect is insufficient, impairing the cleanliness of the steel. If the Al content exceeds 0.1%, not only does the castability of the slab deteriorate, but the temperature required for heating in the single-phase region during annealing becomes high, resulting in problems with production and equipment. Therefore, the Al content is preferably in the range of 0.01 to 0.1%. The upper limit of the Al content is more preferably 0.05%.

[0034] Molybdenum (Mo): 0.01% or less (excluding 0%) Mo is a typical element that improves hardenability. However, in the present invention, the balance between strength, elongation, and hole expandability is important, and since the steel material has a tensile strength of 980 MPa, there is no need to add Mo for the purpose of improving hardenability and facilitating martensite formation. Adding Mo increases 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. On the other hand, considering the case where Mo is inevitably contained during the manufacturing process, the lower limit may be 0.001%.

[0035] Boron (B): 0.001% or less (excluding 0%) B is a typical element that improves hardenability. However, in the present invention, the balance between strength, elongation, and hole expandability is important, and since the steel material has a tensile strength of 980 MPa, there is no need to add B for the purpose of improving hardenability and facilitating martensite formation. Therefore, in the present invention, it is preferable not to add B. Therefore, in the present invention, the B content can be limited to 0.001% or less. On the other hand, considering the case where B is inevitably contained during the manufacturing process, the lower limit may be 0.0001%.

[0036] The remaining component is iron (Fe). However, since unintentional impurities may be inevitably mixed in from raw materials or the surrounding environment during normal manufacturing processes, it is not possible to exclude them. Since these impurities would be known to anyone skilled in normal manufacturing processes, the contents of all of them will not be specifically mentioned in this specification.

[0037] The microstructure of the cold-rolled steel sheet of the present invention preferably contains, in area percentages, ferrite: more than 10% to 45% or less, retained austenite: 7 to 15%, fresh martensite: 10% or less (including 0%), and a total of tempered martensite and bainite: 40 to 80%. The ferrite is an advantageous structure for ensuring elongation. If the ferrite fraction is 10% or less, it is difficult to ensure the elongation targeted in the present invention. If the ferrite fraction exceeds 45%, it is difficult to ensure the strength and hole expandability targeted in the present invention. The retained austenite, together with the ferrite formed during the annealing process, is an essential structure for ensuring elongation. If the retained austenite fraction is less than 7%, it is difficult to ensure the elongation level targeted in the present invention. If the retained austenite fraction exceeds 15%, the stability of the retained austenite is insufficient, making it difficult to ensure the targeted elongation. If the fraction of fresh martensite exceeds 10%, it is difficult to obtain a steel material excellent in all of strength, elongation, and hole expandability. The tempered martensite and bainite are structures necessary for ensuring strength and hole expandability. When the phase transformation progresses to include fractions within the above ranges, the steel can ultimately contain 7 to 15% of retained austenite, which is stable at room temperature. If the fraction of tempered martensite and bainite is less than 40%, the total amount of transformation is insufficient, making it difficult to ensure the final fraction of retained austenite targeted by the present invention. However, if 10% or more of fresh martensite is secured, it is difficult to ensure the strength, elongation, and hole expandability targeted by the present invention. If the fraction of tempered martensite and bainite exceeds 80%, it is possible to ensure strength and hole expandability, but the fractions of ferrite and retained austenite are insufficient, making it difficult to ensure the high elongation targeted by the present invention. On the other hand, it is more preferable that the fraction of tempered martensite is 40% or more.

[0038] The cold-rolled steel sheet of the present invention preferably includes a soft layer having a predetermined depth (t) from the surface in the thickness direction of the steel sheet, and the soft layer preferably satisfies the following relations 1 and 2. By forming the soft layer, it is possible to prevent LME (Liquid Metal Embrittlement) cracks. The predetermined depth (t) may be 50 to 100 μm. More specifically, the predetermined depth (t) may be 50 to 80 μm. Here, the surface means the surface of the base steel sheet, and does not include a coating layer formed on the surface of the base steel sheet.

[0039] The soft layer preferably satisfies the following relations 1 and 2.

[0040] [Equation 1][C t / 5 ] / [C M ]<0.3

[0041] [Equation 2][C 3t / 5 ] / [C M ]<0.6

[0042] (However, in the above relational expressions 1 and 2, [C t / 5 ] means the average C content in the region from the surface to 1 / 5 of the specified depth (t) in the thickness direction of the steel plate, and [C 3t / 5 ] means the average C content in the region corresponding to the 1 / 5 point of the predetermined depth (t) in the thickness direction of the steel plate to the 3 / 5 point of the predetermined depth (t), and [C M ] means the average C content of the steel sheet.)

[0043] If the conditions of the above relations 1 and 2 are not satisfied, it is difficult to obtain good LME characteristics.

[0044] On the other hand, the above [C 1 / 5t ] may have a microstructure containing, by area %, 80% or more of ferrite, the remainder being bainite and tempered martensite.

[0045] The cold-rolled steel sheet of the present invention may have one of a hot-dip galvanized layer (GI), a galvannealed layer (GA), and an electrogalvanized layer (EG) formed on at least one surface thereof. In the present invention, the specific conditions of the hot-dip galvanized layer (GI), the galvannealed layer (GA), and the electrogalvanized layer (EG) are not particularly limited, and any type commonly used in the technical field can be used.

[0046] The cold-rolled steel sheet of the present invention may have a yield strength (YS): 600 MPa or more, a tensile strength (TS): 980 MPa or more, a total elongation (T-El): 21% or more, and a hole expandability (HER): 20 to 40%. Since the higher the yield strength, tensile strength, total elongation, and uniform elongation, the more advantageous it is, the present invention does not particularly limit the upper limits of the yield strength, tensile strength, and total elongation.

[0047] The cold-rolled steel sheet of the present invention may satisfy the following condition: 30,000 MPa%≦X=yield strength×[total elongation+(2×hole expandability)]≦70,000 MPa%. The X value is controlled to ensure a yield strength of 600 MPa or more, which is the target of the present invention, as well as excellent elongation and hole expandability. If the X value is less than 30,000 MPa% or more than 70,000 MPa%, one or more of the properties of strength, elongation, and hole expandability that are the objective of the present invention will be inferior, making it difficult to use the steel sheet for collision energy absorption components. The lower limit of the X value is more preferably 35,000 MPa%. The upper limit of the X value is more preferably 65,000 MPa%, and even more preferably 60,000 MPa%.

[0048] Hereinafter, a method for manufacturing a cold-rolled steel sheet according to one embodiment of the present invention will be described.

[0049] First, the slab is heated to 1100 to 1300°C. The slab is heated to ensure smooth operation of the subsequent hot rolling process and to obtain the desired physical properties of the steel sheet. If the slab is heated to a temperature lower than 1100°C, a problem occurs in that the hot rolling load increases rapidly. If the slab is heated to a temperature higher than 1300°C, the amount of surface scale increases, resulting in reduced productivity.

[0050] Next, the heated slab is finish hot-rolled at Ar3 or higher to obtain a hot-rolled steel sheet. If the finish hot-rolling temperature is lower than Ar3, rolling occurs in the ferrite-austenite two-phase region or the ferrite region, forming a duplex structure, which may cause equipment malfunction due to fluctuations in the hot-rolling load. Meanwhile, Ar3 can be calculated using the following equation 1:

[0051] [Formula 1] Ar3(℃)=910-203√[C]+44.7[Si]+31.5[Mo]-30[Mn]-11[Cr]+700[P]+400[Al]+400[Ti]

[0052] Next, the hot-rolled steel sheet is coiled at 700°C or less. If the coiling temperature exceeds 700°C, an excessive oxide film is formed on the surface of the steel sheet, resulting in defects. The coiling temperature is more preferably 650°C or less. On the other hand, as the coiling temperature decreases, the strength of the hot-rolled steel sheet increases, which has the disadvantage of increasing the rolling load in the subsequent cold rolling process. However, this does not actually make production impossible, so the present invention does not particularly limit the lower limit of the coiling temperature. However, as an example, the lower limit of the coiling temperature may be 300°C.

[0053] Next, the coiled hot-rolled steel sheet is pickled and then cold-rolled to obtain a cold-rolled steel sheet. The pickling is a process for removing an oxide layer formed on the surface of the coiled hot-rolled steel sheet. The cold rolling can be performed at a cold reduction of 30 to 80%. If the cold reduction is less than 30%, not only is it difficult to ensure the target thickness, but the remaining crystal grains formed during hot rolling may affect the formation of austenite and the securing of physical properties during annealing. If the cold reduction is more than 80%, work hardening occurs during cold rolling, resulting in uneven reduction in the length and width directions, resulting in variations in material quality, and the rolling load makes it difficult to ensure the target thickness.

[0054] Next, the cold-rolled steel sheet is subjected to primary heating at 780°C or higher and lower than Ac3-10°C for 30 seconds or longer in an atmosphere having a dew-point temperature of 0 to 30°C. The primary heating is performed to partially form annealed ferrite in addition to retained austenite in order to ensure an elongation of 21% or higher. If the dew-point temperature is lower than 0°C, the soft layer intended in the present invention is not sufficiently formed on the surface of the steel sheet. If the dew-point temperature exceeds 30°C, problems arise in terms of reduced equipment life and productivity. The lower limit of the dew-point temperature is more preferably 2°C. The upper limit of the dew-point temperature is more preferably 25°C. If the primary heating temperature is lower than 780°C, excessive annealed ferrite is formed, making it difficult to ensure strength and hole expandability. If the primary heating temperature is higher than Ac3-10°C, the fraction of annealed ferrite becomes insufficient due to heating at a single-phase region level, resulting in poor elongation. The lower limit of the primary heating temperature is more preferably 790°C. The upper limit of the temperature of the primary heating is more preferably Ac3-15°C. If the time of the primary heating is less than 30 seconds, there is a drawback in that a sufficient annealing effect cannot be obtained. On the other hand, since the longer the time of the primary heating, the more advantageous it is, the lower limit is not particularly limited in the present invention. However, as an example, the upper limit of the time of the primary heating may be 500 seconds. Meanwhile, the aforementioned Ac3 can be calculated by the following formula 2.

[0055] [Formula 2] Ac3(℃)=910-203√[C]-15.2[Ni]+44.7[Si]+104[V]+31.5[Mo]+13.1[W]

[0056] Next, the heated cold-rolled steel sheet is subjected to primary cooling to 600 to 750°C at an average cooling rate of 1 to 10°C / s. If the end temperature of the primary cooling is less than 600°C, phases such as ferrite and bainite may be formed, resulting in a decrease in strength. If the end temperature of the primary cooling exceeds 750°C, problems may arise in actual production lines. The lower limit of the end temperature of the primary cooling is more preferably 610°C, and even more preferably 630°C. The upper limit of the end temperature of the primary cooling is more preferably 740°C, and even more preferably 730°C. If the primary average cooling rate is less than 1°C / s, ferrite is formed during cooling, making it difficult to ensure the target strength. If the primary average cooling rate exceeds 10°C / s, the average cooling rate is reduced during secondary cooling, making it difficult to ensure sufficient martensite. This immediately leads to a decrease in the fraction of tempered martensite, making it difficult to simultaneously ensure both strength and hole expandability. The upper limit of the primary average cooling rate is more preferably 6°C / s.

[0057] Next, the cold-rolled steel sheet after the primary cooling is subjected to secondary cooling at an average cooling rate of 10 to 45°C / s to 150°C to Ms. In order to ensure a tempered martensite structure of 40% or more as required by the present invention, it is necessary to cool the steel sheet between the martensite transformation start temperature (Ms) and finish temperature (Mf) during the secondary cooling. That is, for this purpose, the end temperature of the secondary cooling is preferably in the range of 150°C to Ms. If the end temperature of the secondary cooling is less than 150°C, the fraction of tempered martensite becomes excessively high, the fraction of retained austenite decreases, and elongation becomes poor. If the end temperature of the secondary cooling exceeds Ms, it becomes difficult to form a tempered martensite structure, resulting in poor strength and hole expandability. The lower limit of the end temperature of the secondary cooling is more preferably 180°C. If the second average cooling rate is less than 10°C / s, a bainite structure is partially formed during the secondary cooling from the primary cooling section. If the secondary average cooling rate exceeds 45°C / s, the rapid martensite transformation rate at the time of secondary cooling will cause the surface shape of the steel sheet to deteriorate, resulting in problems with material variations in the width direction. The lower limit of the secondary average cooling rate is more preferably 12°C / s. The upper limit of the secondary average cooling rate is more preferably 42°C / s. Meanwhile, the aforementioned Ms can be calculated using the following formula 3.

[0058] [Formula 3] Ms(℃)=539-423[C]-30.4[Mn]-7.5[Si]+30[Al]-12.1[Cr]-17.7[Ni]-7.5[Mo]

[0059] Next, the second-cooled cold-rolled steel sheet is secondarily heated at Ms to 480°C and then overaged for 1 to 30 minutes. The second heating and overaging treatment aim to improve toughness by converting the hard martensite with a high dislocation density formed during the second cooling into tempered martensite. Furthermore, by ensuring sufficient amounts of tempered martensite and bainite during the second heating and overaging treatment, carbon is enriched in the austenite remaining from the annealing (partitioning). During this process, the martensitic transformation start temperature (Ms) of the C-enriched austenite is lowered to below room temperature, ultimately forming a large amount of retained austenite, thereby ensuring the physical properties targeted in the present invention. If the second heating temperature is lower than Ms or exceeds 480°C, it is difficult to ensure the microstructure fraction targeted in the present invention. The lower limit of the second heating temperature is more preferably 360°C. The upper limit of the second heating temperature is more preferably 460°C. If the overaging treatment time is less than 1 minute, the transformation does not proceed sufficiently, making it difficult to obtain the partitioning effect.If the overaging treatment time is more than 30 minutes, the secondary heating and overaging treatment intervals must be very long, reducing productivity and making it difficult to apply to an actual production line.

[0060] Meanwhile, after the secondary heating and holding, the cold-rolled steel sheet can be immersed in a hot-dip galvanizing bath at 440 to 480°C to form a hot-dip galvanized layer. If the temperature of the hot-dip galvanizing bath is less than 440°C, it is difficult to control the hot-dip galvanizing bath, and if the temperature of the hot-dip galvanizing bath is more than 480°C, the final elongation decreases.

[0061] Furthermore, after forming the hot-dip galvanized layer, the cold-rolled steel sheet on which the hot-dip galvanized layer has been formed may be subjected to an alloying heat treatment at 450 to 520° C. If the alloying heat treatment temperature is less than 450° C., it is difficult to form a sufficient Fe-Zn alloyed plating layer, and if the alloying heat treatment temperature is more than 520° C., the retained austenite formed in the previous step is decomposed, resulting in a poor final elongation.

[0062] On the other hand, after the second heating and holding, an electrogalvanized layer can be formed. [Example]

[0063] The present invention will be described in more detail below with reference to examples. However, it should be noted that the following examples are merely for the purpose of illustrating and explaining the present invention in more detail, and are not intended to limit the scope of the present invention. The scope of the present invention is defined by the matters described in the claims and matters that can be reasonably inferred therefrom.

[0064] (Example) A slab having the alloy composition shown in Table 1 below was heated at 1100 to 1300°C and then finish hot-rolled at 900 to 1000°C to produce a hot-rolled steel sheet. The hot-rolled steel sheet was coiled at 350 to 650°C, pickled, and then cold-rolled at a cold reduction of 45 to 65%, and then a cold-rolled steel sheet was produced under the conditions shown in Table 2 below. The conditions shown in Table 2 below were based on the surface temperature of the steel sheet. The produced cold-rolled steel sheet was then subjected to hot-dip galvanizing or hot-dip galvanizing and alloying heat treatment under the conditions shown in Table 2 below.

[0065] The microstructure and mechanical properties of the cold-rolled steel sheets thus produced were measured, and the results are shown in Tables 3 and 4 below.

[0066] The phase fractions in the microstructure were measured using XRD and EBSD for a quarter thickness (t: thickness of the steel) of the cold-rolled steel sheet.

[0067] The presence or absence of the formation of a soft layer having a predetermined depth (t) was measured using GDS. The average C content [C t / 5 ] and the average C content [C 3t / 5 ] was calculated as the average value of the C content measured using GDS, and [CM ] was measured using the results of OES and ICP analysis of the C component of the base material.

[0068] Yield strength (YS), tensile strength (TS), total elongation (T-El), and uniform elongation (U-El) were measured by processing the cold-rolled steel sheets into specimens conforming to the JIS standard (gauge length: width x length: 25 x 50 mm, total length of specimen: 200-260 mm) and then conducting tensile tests at a test speed of 28 mm / min.

[0069] The hole expandability (HER) was measured according to the ISO 16330 standard, where the holes were sheared with a 10 mm diameter punch at a clearance of 12%.

[0070] LME was evaluated according to the ISO 18278-2 standard, and if CType cracks occurred, they were marked with ○, and if they did not occur, they were marked with ×.

[0071] [Table 1]

[0072] [Table 2]

[0073] [Table 3]

[0074] [Table 4]

[0075] As can be seen from Tables 1 to 4 above, in the case of Examples 1 to 9, which satisfy the alloy composition and manufacturing conditions proposed in the present invention, excellent physical properties are ensured by ensuring the microstructure and soft layer targeted by the present invention.

[0076] In the cases of Comparative Examples 1 to 4, although the strength, elongation, and hole expandability required by the present invention are satisfied, the dew point temperature, which is one of the manufacturing conditions, is not satisfied, and therefore the soft layer cannot be secured, and the LME characteristics are poor.

[0077] In the case of Comparative Example 5, although the strength, elongation, and hole expandability required by the present invention are satisfied, the dew point temperature, which is one of the manufacturing conditions, is not satisfied, and therefore decarburization occurs weakly, Relational Formula 1 is not satisfied, and it can be seen that the LME characteristics are poor.

[0078] In the case of Comparative Examples 6 to 16, the manufacturing conditions of the present invention were not satisfied, and therefore the microstructure targeted by the present invention could not be obtained, and one or more of the strength, elongation, and hole expandability required by the present invention were not satisfied.

[0079] In the case of Comparative Examples 17 and 18, the alloy composition or alloy composition and manufacturing conditions of the present invention were not satisfied, and therefore the microstructure targeted by the present invention could not be secured, and one or more of the strength, elongation, and hole expandability required by the present invention were not satisfied.

[0080] Figure 1 is a photograph of the microstructure of Example 1 observed with an SEM microscope. As can be seen from Figure 1, Example 1 has a microstructure with an appropriate fraction that is sought to be obtained by the present invention.

Claims

1. 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 (excluding 0%), sulfur (S): 0.03% or less (excluding 0%), aluminum (Al): 0.01 to 0.1%, molybdenum (Mo): 0.01% or less (excluding 0%), boron (B): 0.001% or less (excluding 0%), the balance being Fe and other unavoidable impurities, The microstructure contains, in area %, ferrite: more than 10% and not more than 45%, retained austenite: 7 to 15%, fresh martensite: not more than 10% (including 0%), and the total of tempered martensite and bainite: 40 to 80%; The soft layer has a predetermined depth (t) from the surface in the thickness direction of the steel plate, The soft layer is a cold-rolled steel sheet that satisfies the following relations 1 and 2. [Relationship 1] [C ｔ／５ ] / [C Ｍ ]<0.3 [Relationship 2] [C ３ｔ／５ ] / [C Ｍ ]<0.6 (However, in the above relational expressions 1 and 2, [C ｔ／５ ] means the average C content in the region from the surface to 1 / 5 of the predetermined depth (t) in the thickness direction of the steel plate, ３ｔ／５ ] means the average C content in the region corresponding to the point from 1 / 5 of the predetermined depth (t) to 3 / 5 of the predetermined depth (t) in the thickness direction of the steel plate, and [C Ｍ ] means the average C content of the steel sheet.)

2. The cold-rolled steel sheet according to claim 1, wherein a fraction of the tempered martensite is 40% or more.

3. The cold-rolled steel sheet according to claim 1, wherein the predetermined depth (t) is 50 to 100 μm.

4. The cold-rolled steel sheet according to claim 1, wherein the cold-rolled steel sheet has one of a hot-dip galvanized layer (GI), a galvannealed layer (GA), and an electrogalvanized layer (EG) formed on at least one surface thereof.

5. The cold-rolled steel sheet according to claim 1, wherein the cold-rolled steel sheet satisfies 30000 MPa% ≦ X = yield strength × [total elongation + (2 × hole expandability)] ≦ 70000 MPa%.

6. heating a slab containing, by weight, 0.15-0.25% carbon (C), 1.5-2.5% manganese (Mn), 1.0-2.0% silicon (Si), 0.1% or less (excluding 0%) phosphorus (P), 0.03% or less (excluding 0%) sulfur (S), 0.01-0.1% aluminum (Al), 0.01% or less (excluding 0%) molybdenum (Mo), 0.001% or less (excluding 0%) boron (B), the balance being Fe and other unavoidable impurities, at 1100-1300°C; Finish hot rolling the heated slab at Ar3 or higher to obtain a hot-rolled steel sheet; coiling the hot-rolled steel sheet at 700°C or less; pickling the coiled hot-rolled steel sheet and then cold-rolling it to obtain a cold-rolled steel sheet; a step of primarily 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 having a dew point temperature of 0 to 30°C; performing primary cooling of the heated cold-rolled steel sheet to 600 to 750°C at an average cooling rate of 1 to 10°C / s; Secondarily cooling the primarily cooled cold-rolled steel sheet to 150°C to Ms at an average cooling rate of 10 to 45°C / s; and secondarily heating the second-cooled cold-rolled steel sheet at Ms to 480°C, and then overaging the second-cooled cold-rolled steel sheet for 1 to 30 minutes.

7. The method for producing a cold-rolled steel sheet according to claim 6, wherein the cold rolling is performed at a cold reduction rate of 30 to 80%.

8. 7. The method of claim 6, further comprising the step of immersing the cold-rolled steel sheet in a hot-dip galvanizing bath at 440 to 480°C to form a hot-dip galvanized layer after the secondary heating and holding.

9. The method for manufacturing a cold-rolled steel sheet according to claim 8, further comprising the step of subjecting the cold-rolled steel sheet to an alloying heat treatment at 450 to 520°C after forming the hot-dip galvanized layer.

10. The method for manufacturing a cold-rolled steel sheet according to claim 6, further comprising forming an electrogalvanized layer after the secondary heating and holding.

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