Cold-rolled steel sheet and its manufacturing method

Abstract

TECHNICAL FIELD The present invention relates to a cold-rolled steel sheet and a manufacturing method thereof. One aspect of the present invention is to provide a cold-rolled steel sheet having excellent weldability, strength and formability, and a manufacturing method thereof.

Description

Technical Field

[0001] The present invention relates to a cold-rolled steel sheet having excellent weldability, strength and formability, and a method for manufacturing the same.

Background Art

[0002] Recently, in order to reduce the weight and enhance the safety of automobiles, the development of manufacturing technologies for steel sheets with high strength has been promoted, and particularly, the demand for high-strength steel materials with a tensile strength of 980 MPa or higher has been increasing. However, when simply improving the strength, it is a common phenomenon that the ductility and formability decrease. Therefore, high-strength steel sheets for cold forming that overcome this and have formability are highly useful in terms of improving fuel efficiency due to weight reduction, improving component manufacturing / formability productivity, and ensuring the safety of final components.

[0003] In order to improve the formability of steel materials, as a method for increasing the elongation rate, a method of introducing retained austenite and using the TRIP (Transformation Induced Plasticity) phenomenon, as in Patent Document 1, is widely used. However, in the case of such TRIP steel sheets, a large amount of Si and Al needs to be added to introduce retained austenite, which causes LME (Liquid Metal Embrittlement) during spot welding of the steel sheet, restricting the use of electroplated steel sheets and cold-rolled steel sheets welded to electroplated materials. In addition, when the strength of the steel sheet increases, there is a possibility of a problem of hydrogen embrittlement in which the assembled parts suddenly break during use. Therefore, it is necessary to prevent severe corrosion even when the steel sheet is exposed to a corrosive environment.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] One aspect of the present invention is to provide a cold-rolled steel sheet having excellent weldability, strength, and formability, as well as a method for manufacturing the same. [Means for solving the problem]

[0006] One embodiment of the present invention has the following composition in weight percent: C: 0.14~0.16%, Si: 0.3~0.6%, Al: 0.01~0.3%, Mn: 2.6~3.0%, Cr: 0.01~0.25%, Mo: 0.15~0.4%, B: 0.0001~0.005%, Nb: 0.001~0.05%, Ti: 0.001~0.05%, P: 0.04% or less (excluding 0%), S: 0.01% or less (excluding 0%), N: 0.01% or less (0%). The present invention provides a cold-rolled steel sheet with excellent weldability, strength, and formability, comprising the remainder Fe and other unavoidable impurities (excluding %), satisfying the following relational formulas 1 and 2, and having a microstructure consisting of retained austenite: more than 1% but 5% or less, fresh martensite: 10% or more but less than 25%, bainite: less than 20% (excluding 0%), tempered martensite: 55% or more but less than 80%, and ferrite: 5% or less (including 0%). [Relational expression 1]234×[C]-29×[Si]-128×[Al]+29×[Mn]+10×[Cr]-17×[Mo]-37×[Nb]-49×[Ti]+100×[B]≧80 [Relationship 2] 5 × [C] + [Si] + 0.5 × [Al] ≤ 1.5 (However, in the above relational formulas 1 and 2, the content of each element is given in weight percent.)

[0007] Another embodiment of the present invention is characterized by the following composition in weight percent: C: 0.14-0.16%, Si: 0.3-0.6%, Al: 0.01-0.3%, Mn: 2.6-3.0%, Cr: 0.01-0.25%, Mo: 0.15-0.4%, B: 0.0001-0.005%, Nb: 0.001-0.05%, Ti: 0.001-0.05%, P: 0.04% or less (excluding 0%), S: 0.01%. The process involves heating a slab that satisfies the following relationships (excluding 0%), containing N: 0.01% or less (excluding 0%), with the remainder being Fe and other unavoidable impurities; finishing hot-rolling the heated slab at 830-980°C to obtain a hot-rolled steel sheet; winding the hot-rolled steel sheet at 450-700°C; cold-rolling the wound hot-rolled steel sheet to obtain a cold-rolled steel sheet; and the above cold rolled steel plate The present invention provides a method for manufacturing cold-rolled steel sheets having excellent weldability, strength, and formability, comprising the steps of: continuously annealing the steel sheet at a temperature of 800 to 840°C; primary cooling the continuously annealed steel sheet to a primary cooling completion temperature of 550 to 650°C at an average cooling rate of less than 10°C / s; secondary cooling the primary cooled steel sheet to a secondary cooling completion temperature of 320 to 360°C at an average cooling rate of 10°C / s or more; and reheating the secondary cooled steel sheet to a temperature in the range of 380 to 480°C. [Relational expression 1]234×[C]-29×[Si]-128×[Al]+29×[Mn]+10×[Cr]-17×[Mo]-37×[Nb]-49×[Ti]+100×[B]≧80 [Relationship 2] 5 × [C] + [Si] + 0.5 × [Al] ≤ 1.5 (However, in the above relational formulas 1 and 2, the content of each element is given in weight percent.) [Effects of the Invention]

[0008] According to one aspect of the present invention, it is possible to provide a cold-rolled steel sheet having excellent weldability, strength, and formability, as well as a method for manufacturing the same. [Modes for carrying out the invention]

[0009] The following describes a cold-rolled steel sheet having excellent weldability, strength, and formability according to one embodiment of the present invention.

[0010] First, the alloy composition of the present invention will be described. The content of the alloy composition mentioned below is in weight percent.

[0011] C: 0.14~0.16% Carbon (C) is an element that ensures the strength of steel materials through solid solution strengthening and precipitation strengthening. If the C content is less than 0.14%, it is difficult to ensure a tensile strength (TS) of 1180 MPa. On the other hand, if the C content exceeds 0.16%, arc weldability and laser weldability deteriorate, and the risk of LME crack formation increases. Therefore, it is preferable that the C content be in the range of 0.14 to 0.16%. It is more preferable that the lower limit of the C content be 0.145%. It is more preferable that the upper limit of the C content be 0.155%.

[0012] Si: 0.3~0.6% Silicon (Si) is a core element of TRIP (Transformation Induced Plasticity) steel, as it inhibits cementite precipitation, thereby increasing the retained austenite fraction and elongation. If the Si content is less than 0.3%, almost no retained austenite remains, resulting in excessively low elongation. On the other hand, if the Si content exceeds 0.6%, it becomes impossible to prevent deterioration of the physical properties of the weld joint due to LME crack formation, leading to poor surface properties and plating properties of the steel. Therefore, the Si content is preferably in the range of 0.3 to 0.6%. The lower limit of the Si content is more preferably 0.35%. The upper limit of the Si content is more preferably 0.55%, even more preferably 0.5%, and even more preferably 0.45%.

[0013] Al: 0.01~0.3% Aluminum (Al) is not only an element included for deoxidation of steel, but is also an effective element for suppressing cementite precipitation and stabilizing retained austenite. If the Al content is less than 0.01%, deoxidation of the steel will not be sufficient, and the cleanliness of the steel will be impaired. On the other hand, if the Al content exceeds 0.3%, the temperature required for single-phase heating during annealing will be higher, and the castability of the steel will be impaired. Therefore, it is preferable that the Al content be in the range of 0.01 to 0.3%. It is more preferable that the lower limit of the Al content be 0.02%. It is more preferable that the upper limit of the Al content be 0.25%, and even more preferable that be 0.2%.

[0014] Mn: 2.6~3.0% Manganese (Mn) is an element added to ensure strength. If the Mn content is less than 2.6%, it becomes difficult to ensure sufficient strength. On the other hand, if the content exceeds 3.0%, the transformation rate of bainite slows down, forming an excessive amount of fresh martensite, making it difficult to obtain high hole-expanding properties. In addition, a band structure is formed due to Mn segregation, impairing the material's uniformity and moldability. Therefore, it is preferable that the Mn content be in the range of 2.6 to 3.0%. It is more preferable that the lower limit of the Mn content be 2.7%. It is more preferable that the upper limit of the Mn content be 2.9%.

[0015] Cr: 0.01~0.25% Chromium (Cr) is an element added to ensure strength and hardening ability. When Mn is added alone, an extremely large amount of Mn must be added, exceeding the Mn content range of the present invention. However, this problem can be solved by adding 0.01% or more of Cr. On the other hand, if the Cr content exceeds 0.25%, local corrosivity deteriorates, hydrogen embrittlement cracks may occur, and oxides form on the surface, impairing phosphate treatment properties. Therefore, it is preferable that the Cr content be in the range of 0.01 to 0.25%. The lower limit of the Cr content is more preferably 0.05%, and even more preferably 0.1%. The upper limit of the Cr content is more preferably 0.2%, and even more preferably 0.15%. moreover preferable.

[0016] Mo: 0.15~0.4% Molybdenum (Mo) is an element added to ensure strength and hardening ability. If the Mo content is less than 0.15%, it is difficult to ensure strength and hardening ability. On the other hand, if the Mo content exceeds 0.4%, phase transformation is suppressed, making it difficult to introduce a bainite structure, and because it is an expensive element, the economic efficiency of steel sheet manufacturing deteriorates. Therefore, it is preferable that the Mo content be in the range of 0.15 to 0.4%. It is more preferable that the lower limit of the Mo content be 0.17%. It is more preferable that the upper limit of the Mo content be 0.3%, and 0.23%. moreover preferable.

[0017] B: 0.0001~0.005% Boron (B) is an element added to ensure hardening ability. When Mn is added alone, an extremely large amount of Mn must be added beyond the range of the Mn content of the present invention. However, by adding 0.0001% or more of the above B, such problems can be solved. However, when the content of the above B exceeds 0.005%, B accumulates excessively on the surface and impairs the plating adhesion of the plating material. Therefore, the content of the above B preferably has a range of 0.0001 to 0.005%. The lower limit of the above B content is more preferably 0.0005%. The upper limit of the above B content is more preferably 0.002%, and even more preferably 0.0015%.

[0018] Nb: 0.001 - 0.05% Niobium (Nb) is an element added to ensure the strength of the steel sheet and refine the microstructure. When less than 0.001% of the above Nb is added, it is difficult to obtain the effects of strength improvement and microstructure refinement. On the other hand, when the content of the above Nb exceeds 0.05%, recrystallization is delayed due to local grain fixing, and the uniformity of the microstructure is impaired. Therefore, the content of the above Nb preferably has a range of 0.001 to 0.05%. The lower limit of the above Nb content is more preferably 0.02%, and even more preferably 0.025%. The upper limit of the above Nb content is more preferably 0.04%, and even more preferably 0.035%.

[0019] Ti: 0.001 - 0.05% Titanium (Ti) is an element added to ensure the strength of the steel plate and refine the microstructure. When the addition of the above Ti is less than 0.001%, it is difficult to obtain the effect of improving strength and refining the microstructure. On the other hand, when the content of the above Ti exceeds 0.05%, the casting property is impaired due to excessive formation of TiN, and recrystallization is delayed due to local grain fixing, resulting in impaired uniformity of the microstructure. Therefore, it is preferable that the content of the above Ti has a range of 0.001 to 0.05%. The lower limit of the above Ti content is more preferably 0.01%, and even more preferably 0.015%. The upper limit of the above Ti content is more preferably 0.03%, and even more preferably 0.02%.

[0020] P: 0.04% or less (excluding 0%) Phosphorus (P) exists as an impurity in the steel, and it is advantageous to control its content as low as possible. However, considering the case where the above P is inevitably contained, 0% is excluded (that is, exceeding 0%). On the other hand, P may be intentionally added to increase the strength of the steel material. However, when the above P is added excessively, the toughness of the steel material deteriorates. Therefore, in the present invention, in order to prevent this, it is preferable to limit its upper limit to 0.04%. The above P content is more preferably 0.02% or less, and even more preferably 0.01% or less.

[0021] S: 0.01% or less (excluding 0%) Sulfur (S) exists as an impurity in the steel as in the case of the above P, and it is advantageous to control its content as low as possible. Also, since the above S deteriorates the ductility and impact characteristics of the steel material, it is preferable to limit its upper limit to 0.01%. The above S content is more preferably 0.003% or less, and even more preferably 0.001% or less. However, considering the case where the above S is inevitably contained, 0% is excluded (that is, exceeding 0%).

[0022] N: 0.01% or less (excluding 0%) Nitrogen (N) is present in steel as an impurity, and if present in large quantities, it may form large amounts of nitrides such as TiN and AlN. Therefore, it is advantageous to control its content to be as low as possible. Accordingly, it is preferable to limit the upper limit of the N content to 0.01%. It is more preferable that the N content be 0.05% or less, and even more preferable that be 0.03% or less. However, considering cases where N is unavoidably present, 0% is excluded (i.e., greater than 0%).

[0023] In addition to the steel composition described above, the remainder may include Fe and unavoidable impurities. Unavoidable impurities are those that can be unintentionally introduced during the normal steel manufacturing process and cannot be completely eliminated; the meaning of this is easily understood by engineers in the field of normal steel manufacturing. Furthermore, the present invention does not completely exclude the addition of compositions other than the steel composition described above.

[0024] On the other hand, the cold-rolled steel sheet of the present invention may selectively further contain one or two of the following: Cu: 0.1% or less (excluding 0%) and Ni: 0.1% or less (excluding 0%).

[0025] Cu: 0.1% or less (excluding 0%) and Ni: 0.1% or less (excluding 0%) The above-mentioned copper (Cu) and nickel (Ni) are elements that increase the strength of steel. While these elements enhance the strength and hardening ability of steel, adding them in excessively large amounts may result in exceeding the target strength grade, and since they are expensive elements, this is economically disadvantageous. Therefore, it is preferable that the upper limit for each is 0.1% or less. On the other hand, in order to obtain better solid solution strengthening, the lower limit for Cu and Ni can be 0.03% each.

[0026] Furthermore, the cold-rolled steel sheet of the present invention may selectively further contain V: 0.05% or less (excluding 0%).

[0027] V: 0.05% or less (excluding 0%) Vanadium (V) can increase the strength of steel even with the addition of trace amounts, but its effect on improving elongation is not significant, so it is preferable to control its content to 0.05% or less.

[0028] In addition, it is preferable that the cold-rolled steel sheet of the present invention satisfies the alloy composition described above, as well as the following relational formulas 1 and 2. However, in relational formulas 1 and 2 below, the content of each element is in weight percent.

[0029] [Relational expression 1]234×[C]-29×[Si]-128×[Al]+29×[Mn]+10×[Cr]-17×[Mo]-37×[Nb]-49×[Ti]+100×[B]≧80

[0030] If the above relational equation 1 is not satisfied, an excessive amount of ferrite fraction intended for use in this invention may be formed during annealing, potentially leading to poor yield strength and hole-expanding properties. In the above relational equation 1, the value of the left-hand side is more preferably 90 or greater, and even more preferably 100 or greater.

[0031] [Relationship 2] 5 × [C] + [Si] + 0.5 × [Al] ≤ 1.5

[0032] On the other hand, according to one embodiment of the present invention, adding a large amount of alloying elements such as C, Si, and Al worsens the spot weldability, and in particular, when spot welding is performed on galvanized steel sheets, LME (Liquid Metal Embrittlement) is induced. Generally, spot welding of steel materials is performed at a current value below the minimum current value at which explosion occurs, and this minimum current value at which explosion occurs can be considered the condition that provides the highest heat input when actually performing spot welding. If the LME resistance is high, LME may not occur even at welding current values ​​above this minimum current value at which explosion occurs, and in this case, the AE value, which is defined as the difference between the minimum current value at which LME occurs and the minimum current value at which explosion occurs, will have a positive value. That is, welding is performed at a current value below the minimum current value at which explosion occurs during actual spot welding, and if LME does not occur at this time, it can be determined that the AE value is 0 or greater. On the other hand, the above AE value has units of kA.

[0033] The above relational equation 2 is a component relational equation derived from the conditions under which LME resistance is excellent, that is, the above AE value is 0 or greater. If the above relational equation 2 is not satisfied, there is a problem of reduced LME resistance. In the above relational equation 2, the value of the left side is more preferably 1.25 or less, and even more preferably 1.05 or less.

[0034] The microstructure of a cold-rolled steel sheet according to one embodiment of the present invention preferably consists of retained austenite: more than 1% but 5% or less, fresh martensite: 10% or more but less than 25% (excluding 0%), bainite: less than 20%, tempered martensite: 55% or more but less than 80%, and ferrite: 5% or less (including 0%). The fractions of the microstructure described below refer to area %.

[0035] Residual austenite: Over 1% to 5% Retained austenite is a microstructure that increases the elongation of steel through the TRIP effect. The higher the percentage of retained austenite, the higher the elongation. In this invention, it is preferable that the percentage of retained austenite exceeds 1% in order to obtain the required level of elongation. However, obtaining more than 5% austenite requires the addition of large amounts of C and Si, which results in poor LME resistance for spot welding.

[0036] Fresh martensite: 10% to less than 25% Fresh martensite is a structural element that is advantageous for ensuring strength. If the proportion of fresh martensite is less than 10%, the tensile strength of the steel will be insufficient, but the yield strength may become too high, which can be a problem. If it is 25% or more, the strength will be too high, resulting in poor hole-expandability.

[0037] Baynight: Less than 20% (excluding 0%) Bainite is less strong than martensite, but it is a favorable structure for stabilizing retained austenite. However, if the bainite fraction exceeds 20%, the martensite fraction will be relatively low, which may lead to a problem of insufficient overall strength.

[0038] Tempered martensite: 55% to less than 80% Tempered martensite is a structure advantageous for ensuring strength and hole expandability. Tempered martensite is formed during the secondary cooling after annealing, when the steel sheet is cooled below the martensite transformation initiation point (Ms) and then reheated, undergoing tempering heat treatment. If the proportion of tempered martensite is less than 55%, an excessive amount of fresh martensite is formed during the final cooling, resulting in excessively high strength and poor hole expandability. Conversely, if the proportion of tempered martensite is 80% or more, the proportion of fresh martensite becomes low, resulting in insufficient strength.

[0039] Ferrite: 5% or less (including 0%) Ferrite is a microstructure that adversely affects yield strength and hole-expanding properties, and theoretically, its fraction is preferably 0%. However, it can be unavoidably formed during the manufacturing process, and if its fraction exceeds 5%, the yield strength may decrease and hole-expanding properties may deteriorate. Therefore, in this invention, the upper limit of the ferrite fraction is limited to 5%.

[0040] As described above, the cold-rolled steel sheet of the present invention has a tensile strength of 1180 to 1350 MPa, a yield strength of 740 to 980 MPa, an elongation of 8% or more, a hole-expandability of 20% or more, and an AE value of 0 kA or more, thereby simultaneously ensuring excellent strength, ductility, hole-expandability, and weldability.

[0041] In addition, the cold-rolled steel sheet of the present invention may have a hot-dip galvanized layer or an alloyed hot-dip galvanized layer formed on at least one surface. The present invention does not particularly limit the composition of the hot-dip galvanized layer, and any hot-dip galvanized layer commonly applied in the art can be suitably applied to the present invention. Furthermore, the hot-dip galvanized layer may be an alloyed hot-dip galvanized layer that is alloyed with some of the alloy components of the steel sheet.

[0042] The following describes a method for manufacturing a cold-rolled steel sheet having excellent weldability, strength, and formability according to one embodiment of the present invention.

[0043] First, a slab satisfying the alloy composition and relational equations 1 and 2 described above is heated. Although not particularly limited, the heating temperature during the slab heating can be 1150 to 1250°C. If the slab heating temperature is below 1150°C, it may not be possible to perform the next step, hot rolling. On the other hand, if it exceeds 1250°C, a great deal of energy is wasted in order to raise the slab temperature. Therefore, it is preferable that the slab heating temperature be in the range of 1150 to 1250°C. The lower limit of the slab heating temperature is more preferably 1170°C, and even more preferably 1180°C. The upper limit of the slab heating temperature is more preferably 1230°C, and even more preferably 1220°C.

[0044] Subsequently, the heated slab is finished hot-rolled at 830 to 980°C to obtain a hot-rolled steel sheet. If the finish hot-rolling temperature (hereinafter also referred to as "FDT") is below 830°C, the rolling load is large, leading to an increase in shape defects and reduced productivity. On the other hand, if the finish hot-rolling temperature exceeds 980°C, the surface quality deteriorates due to an increase in oxides caused by excessively high-temperature work. Therefore, it is preferable that the finish hot-rolling temperature be in the range of 830 to 980°C. The lower limit of the finish hot-rolling temperature is more preferably 850°C, and even more preferably 880°C. The upper limit of the finish hot-rolling temperature is more preferably 950°C, and even more preferably 930°C.

[0045] Subsequently, the hot-rolled steel sheet is wound at 450 to 700°C. If the winding temperature (hereinafter also referred to as "CT") exceeds 700°C, coarse internal oxidation of the hot-rolled steel occurs, resulting in the disadvantage of poor surface properties. On the other hand, if the winding temperature is below 450°C, it falls into the transition boiling region, resulting in poor controllability of the winding temperature and the disadvantage of deterioration of the steel sheet shape. Therefore, it is preferable that the winding temperature be in the range of 450 to 700°C. It is more preferable that the lower limit of the winding temperature be 500°C. It is more preferable that the upper limit of the winding temperature be 650°C, and even more preferable that it be 620°C.

[0046] On the other hand, although not particularly limited, after the above-mentioned finish hot rolling, the sheet can be cooled to the coiling temperature at an average cooling rate of 10 to 100°C / s. If the average cooling rate is less than 10°C / s, the productivity of hot rolling decreases, and there is a disadvantage that a cooling medium that reduces cooling capacity during actual production must be deliberately adopted. If it exceeds 100°C / s, there is a disadvantage that the temperature deviation inside the steel sheet becomes uneven, resulting in a poor shape and excessively high strength of the steel sheet.

[0047] Subsequently, the wound hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet. During the cold-rolling process, the cold reduction ratio can be 30-60%. If the cold reduction ratio is less than 30%, it becomes difficult to achieve the target thickness accuracy, and it may also become difficult to correct the shape of the steel sheet. On the other hand, if the cold reduction ratio exceeds 60%, there is a higher possibility of cracks occurring at the edges of the steel sheet, and the cold-rolling load may become excessively large.

[0048] Subsequently, the above cold rolled steel plate The material is continuously annealed at a temperature of 800-840°C. The purpose of this continuous annealing step is to heat the steel sheet to the austenite single-phase region to form nearly 100% austenite, which will be used for subsequent phase transformations. If the continuous annealing temperature (hereinafter also referred to as "SS") is below 800°C, sufficient recrystallization and austenite transformation will not occur, and there is a possibility that ferrite exceeding 5 area percent will be formed. On the other hand, if the continuous annealing temperature exceeds 840°C, productivity will decrease, coarse austenite may be formed and the material may deteriorate, and surface quality will worsen, such as peeling of the plating material. The continuous annealing can be performed in a continuous alloying hot-dip galvanizing furnace.

[0049] On the other hand, although not particularly limited, during the above continuous annealing process, the atmosphere inside the continuous annealing furnace can be controlled with a gas consisting of 95% or more nitrogen and the remainder being hydrogen by volume. If the nitrogen fraction is less than 95%, and the proportion of hydrogen does not increase accordingly, an oxidizing atmosphere will form inside the furnace, causing oxides to form on the surface of the steel sheet and degrading the surface quality. If the proportion of hydrogen is high, process difficulties such as explosion prevention will be exacerbated.

[0050] Subsequently, the continuously annealed steel sheet is subjected to primary cooling at an average cooling rate of less than 10°C / s to a primary cooling completion temperature of 550-650°C (hereinafter also referred to as "SCS"). The primary cooling completion temperature can be defined as the point at which the rapid cooling equipment not applied in the primary cooling is further applied and secondary cooling (rapid cooling) begins. When the above cooling process is carried out in stages, divided into primary and secondary cooling, the temperature distribution of the steel sheet can be made uniform in the slow cooling stage, reducing the final temperature and material variation and allowing the desired phase configuration to be obtained. If the primary cooling completion temperature exceeds 650°C, the amount of cooling required to reach the secondary cooling completion temperature will be large, potentially resulting in a poor steel sheet shape. If it is below 550°C, the load on the slow cooling process will be large. If the primary cooling rate is 10°C / s or higher, the amount of cooling in the secondary cooling will be large, increasing the final temperature and material variation. On the other hand, if the primary cooling rate is less than 1°C / s, a large amount of ferrite phase may be formed during cooling, making it difficult to obtain the target microstructure and material. The lower limit of the primary cooling stop temperature is more preferably 570°C. The upper limit of the primary cooling stop temperature is more preferably 630°C. The primary cooling rate is more preferably in the range of 1°C / s or more and less than 10°C / s.

[0051] Subsequently, the steel sheet that has been cooled in the primary stage is secondary-cooled to a secondary cooling completion temperature of 320-360°C (hereinafter also referred to as "RCS") at an average cooling rate of 10°C / s or more. The secondary cooling completion temperature is set to be below the Ms temperature of the steel sheet so that martensitic transformation occurs during cooling. This martensite then undergoes a subsequent reheating process to ultimately become tempered martensite. Since the Ms temperature of 1180 MPa class high-stretch steel sheets is almost always below 400°C, the present invention controls the secondary cooling completion temperature to be within the range of 320-360°C. If the secondary cooling completion temperature is below 320°C, the initial amount of martensitic transformation becomes too large, resulting in high yield strength and poor formability. On the other hand, if the secondary cooling completion temperature exceeds 360°C, tempered martensite is not generated, the proportion of brittle fresh martensite increases, and yield strength and hole-expandability deteriorate. If the secondary cooling rate is less than 10°C / s, even if the target secondary cooling termination temperature is reached, a high-temperature phase transformation will occur during cooling, and the target martensite fraction and high strength will not be obtained. The lower limit of the secondary cooling termination temperature is more preferably 330°C. The upper limit of the secondary cooling termination temperature is more preferably 350°C.

[0052] On the other hand, as mentioned above, the secondary cooling can be further modified by applying rapid cooling equipment that was not used in the primary cooling. While the present invention does not particularly limit the type of rapid cooling equipment, a hydrogen rapid cooling system can be used as a preferred example. More specifically, the hydrogen rapid cooling system can use a gas consisting of 5-80% hydrogen and the remainder nitrogen by volume. If the hydrogen fraction exceeds 80%, it becomes difficult to manage the equipment, such as controlling explosions. If it is less than 5%, it becomes difficult to utilize the efficient heat transfer characteristics of hydrogen, which is a light element.

[0053] Subsequently, the steel sheet that has been secondarily cooled is reheated to a temperature in the range of 380 to 480°C. Through the above process, interphase carbon distribution and additional bainite phase transformation necessary for stabilizing retained austenite are obtained. In this invention, the end temperature of the above heating section is conveniently referred to as the reheating temperature (hereinafter also called "RHS"). If the reheating temperature is less than 380°C, the strength becomes too high and the elongation rate deteriorates. On the other hand, if the reheating temperature exceeds 480°C, the austenite phase remains without transformation, and the fraction of fresh martensite that transforms during final cooling increases, impairing hole expansion properties and elongation rate. On the other hand, the temperature at which bainite transformation is most active, the so-called nose temperature, is at a level of approximately 400 to 420°C, and considering this, it is more preferable to maintain the reheating temperature in the range of 400 to 420°C.

[0054] On the other hand, although not particularly limited, the average heating rate during the reheating process can be 0.5 to 2.5°C / s. If the average heating rate is less than 0.5°C / s, the overall process time may become too long, potentially leading to excessive heat treatment. If it exceeds 2.5°C / s, it becomes difficult to achieve the physical properties desired in this invention.

[0055] According to one embodiment of the present invention, after the reheating step, the cold-rolled steel sheet may be further subjected to hot-dip galvanizing, alloyed hot-dip galvanizing, or temper rolling, if necessary.

[0056] Specifically, the process may further include a step of hot-dip galvanizing the reheated cold-rolled steel sheet in a plating bath at 450-470°C. Furthermore, if necessary, the process may further include a step of alloying heat treatment of the hot-dip galvanized cold-rolled steel sheet at 470-550°C. This alloying heat treatment is for obtaining an appropriate level of alloying, and its temperature is determined by the surface condition of the steel sheet. By controlling the surface condition of the steel material, the alloying heat treatment temperature can be kept below 550°C, thereby preventing excessive tempering, softening of the steel sheet, and loss of retained austenite. On the other hand, to accelerate alloying, the alloying heat treatment temperature is preferably higher than the hot-dip galvanizing temperature, and its lower limit can be controlled to 470°C. Additionally, after the alloying heat treatment, the process may further include a step of cooling the alloyed cold-rolled steel sheet to room temperature to correct its shape and adjust its yield strength, followed by temper rolling at a reduction ratio of less than 1%. [Examples]

[0057] The present invention will be described in more detail below through examples. However, it should be noted that the following examples are for illustrative purposes and to illustrate the present invention, and are not intended to limit the scope of the rights of the present invention. This is because the scope of the rights of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred therefrom.

[0058] (Examples) After preparing slabs having the alloy compositions listed in Table 1 below, cold-rolled steel sheets were manufactured by reheating them at 1180-1220°C and then performing hot rolling, coiling, annealing, primary cooling, secondary cooling, and reheating processes under the conditions listed in Table 2 below. At this time, the cooling rate after finish hot rolling was 30-50°C / s, the cold reduction ratio was 33-55%, the gas used during continuous annealing was 95 vol%N-5 vol%H, and the gas used during secondary cooling was 75 vol%H-25 vol%N.

[0059] The microstructure and mechanical properties of the cold-rolled steel sheets manufactured in this manner were measured, and the results are shown in Table 3 below.

[0060] The microstructure was measured using the point counting method from scanning electron microscope (SEM) images, and the fraction of retained austenite was measured by XRD.

[0061] Among the mechanical properties, yield strength (YS), tensile strength (TS), and elongation (EL) were measured by tensile testing in the direction perpendicular to the rolling direction. The test specimen specifications used were a gauge length of 50 mm and a tensile test specimen width of 25 mm.

[0062] Among the mechanical properties, hole expandability (HER) was measured according to the ISO 16330 standard, and the holes were sheared with a 12% clearance using a 10 mm diameter punch.

[0063] On the other hand, the above cold-rolled steel sheets were subjected to hot-dip galvanizing (GI) under the conditions described in Table 2 below, and some steel types were subjected to alloying heat treatment (GA) before spot welding. The AE value was then measured, and the results are shown in Table 3 below. The above AE value represents the value obtained by subtracting the minimum current value at which spatter occurs from the minimum current value at which LME occurs. In the above spot welding test, the current was increased in increments of 0.5 kA starting from a low current value, and a cooling time was given between each current value to prevent excessive heat input to the material. The minimum current value at which the weld nugget exploded was measured as the current value was increased in this way, and at the same time, the minimum current value at which LME occurred was measured from observation of the weld surface and cross-section, and the results are shown in Table 3 below. The presence or absence of LME was evaluated as a pass if no cracks due to LME were observed when the surface of the weld was observed at 10x magnification and the cross-section at 100x magnification, and a fail if cracks were observed.

[0064] Furthermore, a 10 mm diameter hole was punched into the cold-rolled steel sheet with a clearance of 12% using a punch, and the sheet was immersed in a hydrochloric acid solution of 0.1 normal concentration for 100 hours to corrode it. After that, the presence or absence of hydrogen embrittlement cracks in the punched area was evaluated.

[0065] [Table 1]

[0066] [Table 2]

[0067] [Table 3]

[0068] As shown in Tables 1 to 3 above, in the case of Invention Examples 1 to 4, which satisfy the alloy composition, relational formulas 1 and 2, and manufacturing conditions proposed by the present invention, it can be seen that the alloy not only has a tensile strength of 1180 to 1350 MPa, a yield strength of 740 to 980 MPa, an elongation of 8% or more, and a hole-expanding ability of 20% or more, but also excellent LME properties.

[0069] In contrast, in the case of Comparative Examples 1 and 2, although the alloy composition of the present invention satisfies relational formulas 1 and 2, the secondary cooling completion temperature falls outside the range of the present invention, making it impossible to secure the microstructure that the present invention aims to obtain. As a result, the yield strength is too high, the hole expansion ability is low (less than 20%), and the formability is poor.

[0070] In the case of Comparative Examples 3 and 4, although the manufacturing conditions of the present invention are met, the alloy composition of the present invention does not satisfy formula 1, and therefore the microstructure to be obtained by the present invention cannot be secured, resulting in inferior mechanical properties.

[0071] In the case of Comparative Examples 5 and 6, although the manufacturing conditions of the present invention are met, it can be seen that the LME properties are inferior because the alloy composition of the present invention does not satisfy relational formula 2.

[0072] In particular, in Comparative Examples 4, 5, and 6, it can be seen that hydrogen embrittlement cracks occurred due to the addition of excessive Cr.

Claims

1. In weight percent, it contains C: 0.14-0.16%, Si: 0.3-0.6%, Al: 0.01-0.3%, Mn: 2.6-3.0%, Cr: 0.01-0.25%, Mo: 0.15-0.4%, B: 0.0001-0.005%, Nb: 0.001-0.05%, Ti: 0.001-0.05%, P: 0.04% or less (excluding 0%), S: 0.01% or less (excluding 0%), and N: 0.01% or less (excluding 0%), with the remainder being Fe and other unavoidable impurities. The following relational equations 1 and 2 are satisfied, The microstructure of the cold-rolled steel sheet consists of retained austenite: more than 1% but 5% or less, fresh martensite: 10% or more but less than 25%, bainite: less than 20% (excluding 0%), tempered martensite: 55% or more but less than 80%, and ferrite: 5% or less (including 0%). [Relational expression 1] 234×[C]-29×[Si]-128×[Al]+29×[Mn]+10×[Cr]-17×[Mo]-37×[Nb]-49×[Ti]+100×[B]≧80 [Relationship 2] 5 × [C] + [Si] + 0.5 × [Al] ≤ 1.5 (However, in the above relational formulas 1 and 2, the content of each element is in weight percent.)

2. The cold-rolled steel sheet according to claim 1, further comprising one or two of the following: Cu: 0.1% or less (excluding 0%) and Ni: 0.1% or less (excluding 0%).

3. The cold-rolled steel sheet according to claim 1, further comprising V: 0.05% or less (excluding 0%).

4. The cold-rolled steel sheet according to claim 1, wherein the cold-rolled steel sheet has a tensile strength of 1180 to 1350 MPa, a yield strength of 740 to 980 MPa, an elongation of 8% or more, a hole-expanding property of 20% or more, and an AE value of 0 kA or more. (However, the AE value mentioned above represents the difference between the minimum current value at which LME occurs and the minimum current value at which scattering occurs.)

5. The cold-rolled steel sheet according to claim 1, wherein the cold-rolled steel sheet has a hot-dip galvanized layer or an alloyed hot-dip galvanized layer formed on at least one surface.

6. A step of heating a slab that, in weight percent, contains C: 0.14-0.16%, Si: 0.3-0.6%, Al: 0.01-0.3%, Mn: 2.6-3.0%, Cr: 0.01-0.25%, Mo: 0.15-0.4%, B: 0.0001-0.005%, Nb: 0.001-0.05%, Ti: 0.001-0.05%, P: 0.04% or less (excluding 0%), S: 0.01% or less (excluding 0%), and N: 0.01% or less (excluding 0%), with the remainder being Fe and other unavoidable impurities, and that satisfies the following relational formulas 1 and 2; The heated slab is then finished hot-rolled at 830 to 980°C to obtain a hot-rolled steel sheet; The step of winding the hot-rolled steel sheet at 450 to 700°C; The step of cold-rolling the wound hot-rolled steel sheet to obtain a cold-rolled steel sheet; The step of continuously annealing the cold-rolled steel sheet at a temperature of 800 to 840°C; The step of first cooling the continuously annealed steel sheet to a primary cooling completion temperature of 550 to 650°C at an average cooling rate of 1°C / s or more and less than 10°C / s; The steps include: a step of secondarily cooling the first cooled steel plate to a secondary cooling completion temperature of 320 to 360°C at an average cooling rate of 10°C / s or more; and A method for manufacturing a cold-rolled steel sheet according to claim 1, comprising the step of reheating the secondary-cooled steel sheet to a range of 380 to 480°C. [Relational expression 1] 234×[C]-29×[Si]-128×[Al]+29×[Mn]+10×[Cr]-17×[Mo]-37×[Nb]-49×[Ti]+100×[B]≧80 [Relationship 2] 5 × [C] + [Si] + 0.5 × [Al] ≤ 1.5 (However, in the above relational formulas 1 and 2, the content of each element is in weight percent.)

7. The method for manufacturing a cold-rolled steel sheet according to claim 6, wherein the slab further comprises one or two of the following: Cu: 0.1% or less (excluding 0%) and Ni: 0.1% or less (excluding 0%).

8. The method for manufacturing a cold-rolled steel sheet according to claim 6, wherein the slab further comprises V: 0.05% or less (excluding 0%).

9. The method for manufacturing a cold-rolled steel sheet according to claim 6, wherein the heating temperature during slab heating is 1150 to 1250°C.

10. The method for manufacturing a cold-rolled steel sheet according to claim 6, wherein, after the finish hot rolling, the sheet is cooled to the winding temperature at an average cooling rate of 10 to 100°C / s.

11. The method for manufacturing a cold-rolled steel sheet according to claim 6, wherein the cold reduction ratio during the cold rolling process is 30 to 60%.

12. The method for manufacturing a cold-rolled steel sheet according to claim 6, wherein during the continuous annealing, the atmosphere inside the continuous annealing furnace is controlled with a gas consisting of 95% or more nitrogen and the remainder hydrogen by volume.

13. The method for manufacturing a cold-rolled steel sheet according to claim 6, wherein during the secondary cooling, a gas consisting of 5 to 80% hydrogen and the remainder nitrogen by volume is used.

14. The method for manufacturing a cold-rolled steel sheet according to claim 6, wherein the average heating rate during the reheating is 0.5 to 2.5°C / s.

15. The method for manufacturing a cold-rolled steel sheet according to claim 6, further comprising the step of hot-dip galvanizing the cold-rolled steel sheet in a plating bath at 450 to 470°C after the reheating step.

16. The method for manufacturing a cold-rolled steel sheet according to claim 15, further comprising the step of alloying the cold-rolled steel sheet at 470 to 550°C after the step of hot-dip galvanizing.

17. The method for manufacturing a cold-rolled steel sheet according to claim 16, further comprising the step of cooling the cold-rolled steel sheet to room temperature after the alloying heat treatment, and then temper-rolling it with a reduction ratio of less than 1%.

Images