Cold-rolled steel sheet having excellent weldability, strength and formability, and method for manufacturing the same

Abstract

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

Description

[Technical Field]

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

[0002] Recently, efforts are being made to secure manufacturing technologies for high-strength steel sheets in order to reduce the weight and improve safety of automobiles, and there is a growing demand for high-strength steel materials with tensile strengths of 980 MPa or higher. However, simply improving strength generally results in a decrease in ductility and formability. High-strength steel sheets for cold forming that overcome this issue and combine formability are highly useful in terms of improving fuel efficiency through weight reduction, improving part manufacturing / forming productivity, and ensuring safety in final parts.

[0003] A widely used method for improving the formability of steel materials and increasing elongation is to introduce retained austenite and utilize the TRIPS (transformation induced plasticity) phenomenon. However, in the case of such TRIP steel sheets, the introduction of retained austenite requires the addition of Si and Al, which can cause liquid metal embrittlement (LME) during spot welding of the steel sheets, limiting their use as plated steel sheets and cold-rolled steel sheets to be welded to plated materials. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Korean Patent Publication No. 2017-7015003 Summary of the Invention [Problem 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, and a method for manufacturing the same.

[0006] The object of the present invention is not limited to the above content. Anyone having ordinary skill in the art to which the present invention pertains will have no difficulty in understanding further object of the invention from the entire content of the present specification. [Means for solving the problem]

[0007] One embodiment of the present invention contains, by weight %, C: 0.10 to 0.16%, Si: 0.3 to 0.8%, Al: 0.01 to 0.5%, Mn: 2.0 to 3.0%, Cr: 0.001 to 0.5%, Mo: 0.001 to 0.5%, B: 0.0001 to 0.001%, Nb: 0.001 to 0.05%, Ti: 0.001 to 0.05%, P: 0.04% or less (excluding 0%), S: 0.01% or less (excluding 0%), N: 0.01% or less (excluding 0%), the balance being Fe and other unavoidable impurities, The microstructure contains, by area percentage, ferrite: 10% or less (excluding 0%), retained austenite: more than 1% and 5% or less, martensite: 25% or more and less than 50%, and bainite: 35% or more and less than 70%. The cold-rolled steel sheet has an average size of martensite islands (MA) present inside the bainite of 0.35 to 0.55 μm.

[0008] Furthermore, although not particularly limited, in one embodiment of the present invention, in order to have high local formability, the alloy components of the cold-rolled steel sheet may be controlled so that the value defined by the following Relational Expression 1 satisfies 70 or more.

[0009] [Equation 1] 234×[C]-29×[Si]-128×[Al]+29×[Mn]+10×[Cr]-17×[Mo]-37×[Nb]-49×[Ti]+100×[B] (In the above Relational Formula 1, the above [C], [Si], [Al], [Mn], [Cr], [Mo], [Nb], [Ti], and [B] represent the weight percent content of each element in parentheses.)

[0010] Furthermore, although not particularly limited, in one embodiment of the present invention, in order to have high local formability, the alloy components of the cold-rolled steel sheet may be controlled so that the value defined by the following relational expression 2 satisfies 270 or more and 330 or less.

[0011] [Equation 2] 270×[C]+90×[Mn]+70×[Cr]+80×[Mo] (In the above Relational Formula 2, the above [C], [Mn], [Cr], and [Mo] indicate the weight percent content of each element in parentheses.)

[0012] Furthermore, although not particularly limited thereto, in one embodiment of the present invention, in order to provide resistance to welding LME, the relationship between the contents of C, Si, and Al in the cold-rolled steel sheet may be controlled so that the value defined in the following Relational Expression 3 is 1.8 or less.

[0013] [Equation 3] 5×[C]+[Si]+0.5×[Al] (In the above Relational Formula 3, [C], [Si], and [Al] represent the weight percent content of each element in parentheses.)

[0014] On the other hand, yet another embodiment of the present invention is heating a steel slab containing, by weight, C: 0.10 to 0.16%, Si: 0.3 to 0.8%, Al: 0.01 to 0.5%, Mn: 2.0 to 3.0%, Cr: 0.001 to 0.5%, Mo: 0.001 to 0.5%, B: 0.0001 to 0.001%, Nb: 0.001 to 0.05%, Ti: 0.001 to 0.05%, P: 0.04% or less (excluding 0%), S: 0.01% or less (excluding 0%), N: 0.01% or less (excluding 0%), the balance being Fe and other unavoidable impurities; Finish hot rolling the heated slab at 830 to 980 ° C to obtain a hot-rolled steel sheet; a step of coiling the hot-rolled steel sheet at 450 to 700 ° C; cold rolling the coiled hot-rolled steel sheet; continuous annealing the cold-rolled steel sheet at a temperature of 790 to 830 ° C; 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 end temperature of 450 to 600 ° C.; Secondary cooling the primarily cooled steel sheet to a secondary cooling end temperature of 250 to 350 ° C. at an average cooling rate of 10 ° C. / s or more; Reheating the second-cooled steel sheet to a temperature in the range of 350 to 480°C; Including, The present invention provides a method for producing a cold-rolled steel sheet that satisfies the following relational expression 4.

[0015] [Equation 4] V1 / V2×t>0.5 (In the above relational expression 4, V1 represents the average cooling rate during primary cooling, V2 represents the average cooling rate during secondary cooling, and t represents the thickness of the cold-rolled steel sheet.)

[0016] Although not particularly limited, in one embodiment of the present invention, in order to have high local formability, the alloy composition of the steel slab can be controlled so that the value defined by the following relational expression 1 satisfies 70 or more.

[0017] [Equation 1] 234×[C]-29×[Si]-128×[Al]+29×[Mn]+10×[Cr]-17×[Mo]-37×[Nb]-49×[Ti]+100×[B] (In the above Relational Formula 1, the above [C], [Si], [Al], [Mn], [Cr], [Mo], [Nb], [Ti], and [B] represent the weight percent content of each element in parentheses.)

[0018] Furthermore, although not particularly limited, in one embodiment of the present invention, in order to have high local formability, the alloy components of the steel slab can be controlled so that the value defined by the following relational expression 2 satisfies 270 or more and 330 or less.

[0019] [Equation 2] 270×[C]+90×[Mn]+70×[Cr]+80×[Mo] (In the above Relational Formula 2, the above [C], [Mn], [Cr], and [Mo] indicate the weight percent content of each element in parentheses.)

[0020] Furthermore, although not particularly limited thereto, in one embodiment of the present invention, in order to provide resistance to welding LME, the relationship between the contents of C, Si, and Al in the steel slab may be controlled so that the value defined in the following Relational Formula 3 is 1.8 or less.

[0021] [Equation 3] 5×[C]+[Si]+0.5×[Al] (In the above Relational Formula 3, [C], [Si], and [Al] represent the weight percent content of each element in parentheses.)

[0022] Furthermore, an embodiment of the present invention may further include, if necessary, a step of plating the reheated steel sheet in a galvanizing bath at 450 to 470°C after the reheating step.

[0023] Furthermore, one embodiment of the present invention may further include a step of subjecting the plated steel sheet to an alloying heat treatment at a temperature in the range of 470 to 550°C, if necessary. [Effects of the Invention]

[0024] According to one aspect of the present invention, it is possible to provide a cold-rolled steel sheet having excellent weldability, strength, and formability, and a method for manufacturing the same.

[0025] The various beneficial advantages and effects of the present invention are not limited to the above-mentioned contents, but can be more easily understood in the course of describing specific embodiments of the present invention. [Brief explanation of the drawings]

[0026] [Figure 1] 1 shows a photograph of a cross section in the thickness direction of the cold-rolled steel sheet obtained in Example 1 of the present application, observed at 5,000 magnifications using a scanning electron microscope (SEM) to observe island martensite (MA) present inside bainite. DETAILED DESCRIPTION OF THE INVENTION

[0027] Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention can be modified into various other forms, and the scope of the present invention is not limited to the embodiments described below. Furthermore, the embodiments of the present invention are provided to more completely explain the present invention to those having average knowledge in the art.

[0028] It should be noted that the terms used in this specification are intended to describe specific embodiments and are not intended to limit the present invention. For example, the singular forms used in this specification include the plural forms unless the related definition clearly indicates otherwise. Furthermore, the meaning of "comprises" used in this specification embodies a configuration and does not exclude the presence or addition of other configurations.

[0029] In conventional technology, trip steel sheets were developed that incorporate retained austenite to ensure high strength of tensile strength of 980 MPa or more and improve formability, but this required the addition of Si and Al, which posed the problem of LME (Liguid Metal Embrittlement) occurring during spot welding.

[0030] For this reason, research is underway to maximize elongation while limiting the amounts of C, Si, and Al added to steel sheets, and to compensate for insufficient formability by improving local formability. Reducing the hardness variation between the microstructures that make up the steel is an effective way to improve local formability. Hole expansion ratio (HER) measurement is a widely used test for evaluating industrial local formability. HER is measured by drilling a 10 mm diameter hole in a test specimen with a punch, fixing it to a die, and expanding the hole by pushing it upward with a conical punch. The diameter of the expanded hole is measured at the point where a crack penetrating the entire thickness occurs, resulting in a value calculated using the following relational expression A. Detailed evaluation criteria for hole expandability are in accordance with ISO 16630.

[0031] [Relationship A] λ(HER)=(df-do) / do (In the above relational expression A, do represents the initial hole diameter, and df represents the hole diameter at thickness break.)

[0032] Therefore, the present inventors have conducted extensive research to provide a cold-rolled steel sheet that can suppress the problem of LME while ensuring high strength of tensile strength of 980 MPa class or more and excellent formability and hole expandability. As a result, they have found that this problem can be solved by precisely controlling the alloy composition and manufacturing conditions, and have thus completed the present invention.

[0033] Hereinafter, a cold-rolled steel sheet having excellent weldability, strength, and formability and a method for manufacturing the same according to one embodiment of the present invention will be described.

[0034] First, the alloy composition of the cold-rolled steel sheet according to one embodiment of the present invention will be described. The content of the alloy composition mentioned below means wt%.

[0035] C: 0.10 to 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 0.10%, it is difficult to ensure a tensile strength (TS) of 980 MPa. On the other hand, if the C content exceeds 0.16%, arc weldability and laser weldability deteriorate, increasing the risk of LME cracking. Therefore, the C content is preferably in the range of 0.10% to 0.16%. On the other hand, the lower limit of the C content is more preferably 0.137%. Furthermore, the upper limit of the C content is more preferably 0.151%.

[0036] Si: 0.3 to 0.8% Silicon (Si) is a core element of TRIP (Transformation Induced Plasticity) steel, which inhibits cementite precipitation and thereby increases the fraction of retained austenite 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 0.8%, it becomes impossible to prevent deterioration of the physical properties of the weld due to the formation of LME cracks, and the surface properties and platability of the steel material deteriorate. Therefore, the Si content is preferably in the range of 0.3 to 0.8%. On the other hand, the lower limit of the Si content is more preferably 0.49%. Furthermore, the upper limit of the Si content is more preferably 0.70%.

[0037] Al: 0.01 to 0.5% Aluminum (Al) is an element contained not only for deoxidizing steel but also for stabilizing retained austenite by suppressing cementite precipitation. If the Al content is less than 0.01%, the steel is not sufficiently deoxidized, impairing the cleanliness of the steel. On the other hand, if the Al content exceeds 0.5%, the castability of the steel is impaired. Therefore, the Al content is preferably in the range of 0.01 to 0.5%. On the other hand, the lower limit of the Al content is more preferably 0.027%. Furthermore, the upper limit of the Al content is more preferably 0.085%.

[0038] Mn: 2.0 to 3.0% Manganese (Mn) is an element added to ensure strength. If the Mn content is less than 2.0%, it becomes difficult to ensure strength. On the other hand, if the Mn content exceeds 3.0%, the transformation rate of bainite slows down, excessive fresh martensite is formed, and it becomes difficult to obtain high hole expandability. In addition, band structures are formed due to Mn segregation, which impairs the material uniformity and formability of the material. Therefore, the Mn content is preferably in the range of 2.0 to 3.0%. The lower limit of the Mn content is more preferably 2.2%, and even more preferably 2.3%. The upper limit of the Mn content is more preferably 2.8%, and even more preferably 2.7%.

[0039] Cr: 0.001 to 0.5% Chromium (Cr) is an element added to ensure strength and hardenability. When Mn is added alone, a very large amount of Mn must be added, exceeding the Mn content range of the present invention. However, adding 0.001% or more of Cr can solve this problem. On the other hand, if the Cr content exceeds 0.5%, local corrosion resistance deteriorates and oxides form on the surface, impairing phosphate treatability. Therefore, the Cr content is preferably in the range of 0.001 to 0.5%. Meanwhile, the lower limit of the Cr content is more preferably 0.002%, and the upper limit of the Cr content is more preferably 0.38%.

[0040] Mo: 0.001 to 0.5% Molybdenum (Mo) is an element added to ensure strength and hardenability. When Mn is added alone, a very large amount of Mn must be added, exceeding the Mn content range of the present invention. However, adding Mo at 0.001% or more can solve this problem. If the Mo content exceeds 0.5%, phase transformation is suppressed, making it difficult to introduce a bainite structure, and Mo, being an expensive element, reduces the economic viability of the steel sheet. Therefore, the Mo content is preferably in the range of 0.001 to 0.5%. Meanwhile, the lower limit of the Mo content is more preferably 0.07%. Furthermore, the upper limit of the Mo content is more preferably 0.3%, and most preferably 0.21%.

[0041] B: 0.0001 to 0.001% Boron (B) is an element added to ensure hardenability. When Mn is added alone, a very large amount of Mn must be added, exceeding the Mn content range of the present invention. However, adding 0.0001% or more of B can solve this problem. However, if the B content exceeds 0.0001%, B will accumulate excessively on the surface, impairing the plating adhesion of the plated material. Therefore, the B content is preferably in the range of 0.0001 to 0.001%. On the other hand, the lower limit of the B content is more preferably 0.00010%, and the upper limit of the B content is more preferably 0.0005%.

[0042] Nb: 0.001 to 0.05% Niobium (Nb) is an element added to ensure the strength of steel sheets and refine the structure. If the Nb content is less than 0.001%, it is difficult to obtain the effects of improving strength and refining the structure. If the Nb content exceeds 0.05%, localized fixation of crystal grains delays recrystallization, impairing the uniformity of the structure. Therefore, the Nb content is preferably in the range of 0.001 to 0.05%. On the other hand, the lower limit of the Nb content is more preferably 0.015%. Furthermore, the upper limit of the Nb content is more preferably 0.031%.

[0043] Ti: 0.001 to 0.05% Titanium (Ti) is an element added to ensure the strength of steel sheets and refine the structure. If Ti is added in an amount of less than 0.001%, it is difficult to obtain the effects of improving strength and refining the structure. On the other hand, if the Ti content exceeds 0.05%, castability is impaired due to excessive formation of TiN, and recrystallization is delayed due to localized grain fixation, resulting in a loss of structural uniformity. Therefore, the Ti content is preferably in the range of 0.001 to 0.05%. On the other hand, the lower limit of the Ti content is more preferably 0.015%, and the upper limit of the Ti content is more preferably 0.03%.

[0044] P: 0.04% or less (excluding 0%) Phosphorus (P) exists as an impurity in steel, and it is advantageous to control its content as low as possible. Therefore, the lower limit of the P content is set to a value other than 0% (i.e., greater than 0%), taking into account cases where P is unavoidably contained. However, P may be intentionally added to increase the strength of steel. However, if P is added in excess, the toughness of the steel deteriorates. Therefore, in the present invention, to prevent this, it is preferable to limit the upper limit to 0.04%. On the other hand, the lower limit of the P content is more preferably 0.002%, or the upper limit of the P content is more preferably 0.0173%.

[0045] S: 0.01% or less (excluding 0%) Like P, sulfur (S) exists as an impurity in steel, and it is advantageous to control its content as low as possible. Therefore, the lower limit of the S content is set to a value other than 0% (i.e., greater than 0%), taking into account cases where S is unavoidably contained. However, because S deteriorates the ductility and impact properties of steel, it is preferable to set the upper limit to 0.01%. On the other hand, the lower limit of the S content is more preferably 0.0009%, or the upper limit of the S content is more preferably 0.0021%.

[0046] N: 0.01% or less (excluding 0%) In the present invention, nitrogen (N) is contained in the steel as an impurity, and it is advantageous to control its content as low as possible. Therefore, the lower limit of the N content excludes 0% (i.e., exceeds 0%), taking into consideration cases where N is unavoidably contained. However, it is preferable to limit the upper limit of the N content to 0.01%. The lower limit of the N content is more preferably 0.0005%. Furthermore, the upper limit of the N content is even more preferably 0.007%, more preferably 0.006%, and most preferably 0.0052%.

[0047] In addition to the above-mentioned steel composition, the remainder may include Fe and inevitable impurities. Unavoidable impurities are impurities that may be unintentionally mixed in during a typical steel manufacturing process, and they cannot be completely eliminated. This meaning is easily understood by engineers in the field of typical steel manufacturing. Furthermore, the present invention does not completely exclude the addition of other components than the above-mentioned steel composition.

[0048] According to one embodiment of the present invention, although not particularly limited, the cold-rolled steel sheet may further include one or more selected from the group consisting of Cu: 0.1% or less (excluding 0%) and Ni: 0.1% or less (excluding 0%).

[0049] Cu: 0.1% or less (excluding 0%), Ni: 0.1% or less (excluding 0%) Copper (Cu) and nickel (Ni) are elements that increase the strength of steel. Although these elements increase the strength and hardenability of steel, adding excessive amounts of these elements may result in the steel exceeding the target strength grade. Because these elements are expensive, it is preferable to limit their upper limits to 0.1% or less from an economical standpoint. Meanwhile, because Cu and Ni act as solid-solution strengthening elements, adding at least one of Cu and Ni at less than 0.03% may result in little solid-solution strengthening effect. Therefore, it is preferable to add at least 0.03% of each.

[0050] According to one embodiment of the present invention, the cold rolled steel sheet may further include, but is not limited to, 0.05% or less of V (excluding 0%).

[0051] V: 0.05% or less (excluding 0%) Although vanadium (V) can increase the strength of steel materials even with a small amount of addition, its effect on improving elongation is not significant, so its content is preferably controlled to 0.05% or less, more preferably 0.04% or less, and even more preferably 0.03% or less.

[0052] The microstructure of the cold-rolled steel sheet according to one embodiment of the present invention preferably contains, in area percentages, ferrite: 10% or less (excluding 0%), retained austenite: more than 1% and 5% or less, martensite: 25% or more and less than 50%, and bainite: 35% or more and less than 70%.

[0053] Although not particularly limited, according to one embodiment of the present invention, the cold-rolled steel sheet has an objective of ensuring excellent formability even at a tensile strength (TS) of 980 MPa or higher. In particular, to obtain high local formability, it is necessary to reduce the hardness difference between the microstructural phases constituting the steel sheet. In the present invention, it was confirmed that, under normal annealing conditions, when the alloy composition described above is satisfied and the components are controlled so that the value defined by the following Relational Formula 1 is 70 or higher, a single austenite phase can be obtained and the ferrite fraction can be maintained at a low level of 10 area % or less. If the ferrite fraction exceeds 10 area %, the yield strength may be reduced and the hole expandability may be impaired. On the other hand, from the viewpoint of ensuring the high yield strength and excellent hole expandability described above, the lower limit of the ferrite fraction may more preferably be 2 area %, and the upper limit of the ferrite fraction may be 7 area %.

[0054] [Equation 1] 234×[C]-29×[Si]-128×[Al]+29×[Mn]+10×[Cr]-17×[Mo]-37×[Nb]-49×[Ti]+100×[B] (In the above Relational Formula 1, the above [C], [Si], [Al], [Mn], [Cr], [Mo], [Nb], [Ti], and [B] represent the weight percent content of each element in parentheses.)

[0055] When the value defined in the above relational expression 1 satisfies 70 or more, a soft ferrite phase can be avoided, but if the bainite phase, which is the second softest phase after ferrite, is not sufficiently introduced, it may be difficult to ensure the ductility of the steel material. From the viewpoint of further improving the above-mentioned effect, more preferably, the lower limit of the value defined in the above relational expression 1 may be 75.7, or the upper limit of the value defined in the above relational expression 1 may be 90.

[0056] Therefore, although not particularly limited, according to one embodiment of the present invention, even under normal continuous plating annealing conditions, the value defined by the following relational expression 2 can be controlled to satisfy 270 or more and 330 or less, as a condition for sufficiently introducing the bainite phase.

[0057] [Equation 2] 270×[C]+90×[Mn]+70×[Cr]+80×[Mo] (In the above Relational Formula 2, the above [C], [Mn], [Cr], and [Mo] indicate the weight percent content of each element in parentheses.)

[0058] The cold-rolled steel sheet according to the present invention is mainly composed of martensite and bainite, and if the hardness difference between these main phases is large, local formability will be poor. Since bainite generally has lower strength than martensite, a method for improving the strength of the bainite structure is required to reduce the hardness difference.

[0059] As described above, the inventors have conducted extensive research to reduce the hardness deviation between martensite and bainite and improve the properties, and have discovered that by controlling the average size of martensite islands (MA) present inside bainite within an appropriate range, it is possible to dramatically reduce the hardness difference between the two main phases.

[0060] Specifically, in the cold-rolled steel sheet according to the present invention, the average size of the island martensite (MA) present inside the bainite may be in the range of 0.35 to 0.55 μm.

[0061] If the average size of the island martensite (MA) present inside the bainite is less than 0.35 μm, the strength of the bainite will be reduced, increasing the difference in hardness with the martensite phase, which may cause a problem of difficulty in ensuring high hole expandability.On the other hand, if the average size of the island martensite (MA) present inside the bainite is more than 0.55 μm, the effect of the hard island martensite will be increased, inducing brittleness and possibly causing a problem of reduced hole expandability.

[0062] On the other hand, according to one embodiment of the present invention, from the viewpoint of improving the above-mentioned effect, more preferably, the lower limit of the average size of the martensite islands (MA) present inside the bainite may be 0.4 μm, or the upper limit of the average size of the martensite islands (MA) present inside the bainite may be 0.5 μm.

[0063] In this specification, the average size of island martensite (MA) present inside bainite is a value measured based on the average size of island martensite (MA) completely contained inside all bainite, based on a cross section cut in the thickness direction of the steel sheet. Also, the average size of island martensite (MA) refers to the average value of the maximum length penetrating inside the island martensite (MA).

[0064] Furthermore, although not particularly limited, according to one embodiment of the present invention, while satisfying the above-mentioned alloy composition, the value defined by the above Relational Formula 2 can be controlled to be 270 or more and 330 or less. As a result, even under normal annealing conditions, bainite having an MA phase (martensite-austenite aggregate) as a second phase is formed in an area percentage of 35% or more and less than 70%, thereby further improving hole expandability. The reason why the strength of the bainite phase is ensured in proximity to martensite is believed to be because the MA phase, a relatively hard second phase, is contained within the bainite phase through carbon partitioning.

[0065] If the value defined by the above relational expression 2 exceeds 330, it becomes difficult to ensure a sufficient bainite fraction of 35 area % or more, resulting in an excessively high strength, which may result in poor elongation and HER value. Conversely, if the value defined by the above relational expression 2 is less than 270, the steel sheet may be too soft, although the ductility may be sufficient, making it difficult to obtain a tensile strength of 980 MPa or more. In this case, from the viewpoint of further improving the above-mentioned effects, it is more preferable that the lower limit of the value defined by the above relational expression 2 may be 286, or the upper limit of the value defined by the above relational expression 2 may be 311.

[0066] Meanwhile, in the present invention, retained austenite is a structure that increases the elongation of a steel material due to the TRIP effect. The higher the retained austenite fraction, the higher the elongation. To obtain the required level of elongation, the retained austenite fraction is preferably greater than 1 area %. However, to obtain austenite greater than 5 area %, large amounts of C and Si must be added, which deteriorates spot welding LME resistance. Therefore, the retained austenite fraction in the present invention can be controlled to 5 area % or less. In this case, from the viewpoint of further improving the above-mentioned effects, the lower limit of the retained austenite fraction may be more preferably 2 area %, or the upper limit of the retained austenite fraction may be 4 area %.

[0067] In the present invention, the martensite fraction may be 25 area% or more and less than 50 area%. If the martensite fraction is less than 25 area%, the overall tensile strength of the steel material may be insufficient. If the martensite fraction is 50 area% or more, the strength may be too high, resulting in poor hole expandability. In this case, from the viewpoint of further improving the above-mentioned effects, the lower limit of the martensite fraction may more preferably be 29 area%, or the upper limit of the martensite fraction may be 49 area%.

[0068] Furthermore, in the present invention, the bainite fraction may be 35 area% or more and less than 70 area%. If the bainite fraction is less than 35 area%, the martensite or ferrite fraction will be relatively high, which may result in a problem of reduced hole expandability, while if the bainite fraction is 70 area% or more, the martensite fraction will be low, which may result in a problem of insufficient overall strength. In this case, from the viewpoint of further improving the above-mentioned effects, more preferably, the lower limit of the bainite fraction may be 45 area%, or the upper limit of the bainite fraction may be 63%.

[0069] Meanwhile, according to one aspect of the present invention, the cold-rolled steel sheet may further include other phases in addition to the above-described microstructure, such as island martensite (MA), for example, island martensite (MA) present within bainite.

[0070] Meanwhile, according to one embodiment of the present invention, the addition of large amounts of alloying elements such as C, Si, and Al deteriorates spot weldability, particularly when spot welding galvanized steel sheets, which can induce liquid metal embrittlement (LME). Generally, spot welding of steel materials is performed at a current below the minimum value at which explosion occurs, and the minimum current at which explosion occurs can be considered the condition under which the highest heat input can be provided during actual spot welding. When LME resistance is high, LME may not occur even at welding currents above the minimum current at which explosion occurs. In this case, the AE value, defined as the difference between the minimum current at which LME occurs and the minimum current at which explosion occurs, has a positive value. That is, if spot welding is actually performed below the minimum current at which explosion occurs and LME does not occur, the AE value can be determined to be 0 or greater. The AE value is expressed in kA.

[0071] Although not particularly limited, according to one embodiment of the present invention, the spot weldability of steel sheets with a tensile strength of 980 MPa and various alloy components was evaluated, and the conditions for alloy components that provide excellent LME resistance, i.e., the AE value is 0 or more, were derived. As a result, it was recognized that the relationship between the C, Si, and Al contents must be controlled so that the value defined by the following relational expression 3 is 1.8 or less.

[0072] [Equation 3] 5×[C]+[Si]+0.5×[Al] (In the above Relational Formula 3, [C], [Si], and [Al] represent the weight percent content of each element in parentheses.)

[0073] According to one embodiment of the present invention, the cold-rolled steel sheet has a tensile strength (TS) of 980 MPa or more (preferably 980 to 1150 MPa, more preferably 980 to 1075 MPa), a yield strength (YS) of 740 to 950 MPa (more preferably 790 to 920 MPa), a hole expandability (HER) of 45% or more (more preferably 50 to 65%), and an elongation (El) of 12% or more (more preferably 12 to 20%), thereby ensuring excellent strength, ductility, and hole expandability simultaneously.

[0074] On the other hand, the cold-rolled steel sheet of the present invention has a hot-dip galvanized layer formed on at least one surface. In the present invention, the configuration of the hot-dip galvanized layer is not particularly limited, and any hot-dip galvanized layer commonly used in the technical field can be preferably applied to the present invention. In addition, the hot-dip galvanized layer may be an alloyed hot-dip galvanized layer alloyed with some alloy components of the steel sheet.

[0075] Hereinafter, a method for producing a cold-rolled steel sheet having excellent weldability, strength, and formability according to one embodiment of the present invention will be described. However, this does not necessarily mean that the cold-rolled steel sheet of the present invention should be produced only by the following production method.

[0076] First, a slab having the aforementioned alloy composition is heated. The heating temperature during the slab heating is preferably 1150 to 1250°C. If the slab heating temperature is less than 1150°C, it may be impossible to perform the next step, hot rolling. On the other hand, if the slab heating temperature exceeds 1250°C, a large amount of energy is unnecessarily consumed to increase the slab temperature. Therefore, the slab heating temperature is preferably 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.

[0077] The heated slab is then finish 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 less than 830°C, the rolling load is large, resulting in 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, the finish hot rolling temperature is preferably in the range of 830 to 980°C. The lower limit of the finish hot rolling temperature is more preferably 880°C. The upper limit of the finish hot rolling temperature is more preferably 950°C, and even more preferably 930°C.

[0078] Next, the hot-rolled steel sheet is coiled at 450 to 700°C. If the coiling temperature (hereinafter also referred to as "CT") exceeds 700°C, there is a drawback in that coarse internal oxidation occurs during hot rolling, resulting in poor surface properties. On the other hand, if the coiling temperature is less than 450°C, there is a drawback in that the coiling temperature falls within the transition boiling region, making it difficult to control the coiling temperature and resulting in poor steel sheet shape. The lower limit of the coiling temperature is more preferably 480°C, and even more preferably 500°C. The upper limit of the coiling temperature is more preferably 670°C, and even more preferably 640°C.

[0079] After the finish hot rolling, the steel sheet is preferably 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 the hot rolling decreases, and a cooling medium with poor cooling capacity must be selected during actual production. If the average cooling rate exceeds 100°C / s, the temperature deviation within the steel sheet becomes uneven, resulting in poor shape and excessive strength of the steel sheet. Therefore, the average cooling rate is preferably in the range of 10 to 100°C / s.

[0080] The coiled hot-rolled steel sheet is then cold-rolled. During the cold rolling, the cold reduction may be 30 to 60%. If the cold reduction is less than 30%, not only is it difficult to ensure the target thickness accuracy, but it may also be difficult to correct the shape of the steel sheet. On the other hand, if the cold reduction is more than 60%, cracks are more likely to occur at the edges of the steel sheet, and the cold rolling load may become excessively large. Therefore, it is preferable that the cold reduction is in the range of 30 to 60%.

[0081] Next, the cold-rolled steel sheet is subjected to continuous annealing in the range of 790°C to 830°C. The purpose of the continuous annealing step is to heat the steel sheet to the austenite single-phase region to form nearly 100% austenite for use in subsequent phase transformation. If the continuous annealing temperature (hereinafter also referred to as "SS") is less than 790°C, sufficient recrystallization and austenite transformation do not occur, and the desired martensite and bainite fractions cannot be secured after annealing. On the other hand, if the continuous annealing temperature exceeds 830°C, productivity decreases, coarse austenite is formed, which may deteriorate the material quality, and the surface quality may deteriorate, such as peeling of the plated material. In addition, the continuous annealing can be performed in a continuous alloying hot-dip galvanizing furnace.

[0082] During the continuous annealing, it is preferable to control the atmosphere in the continuous annealing furnace with a gas consisting of, by volume, 95% or more nitrogen and the remainder hydrogen. If the nitrogen fraction is less than 95%, an oxidizing atmosphere will be formed in the furnace unless the hydrogen fraction is increased accordingly, resulting in oxides being formed on the steel sheet surface, deteriorating the surface quality, and if the hydrogen fraction is increased, process difficulties such as preventing explosions will increase.

[0083] The continuously annealed steel sheet is then subjected to primary cooling at an average cooling rate of less than 10°C / s (more preferably, 1°C / s or more but less than 10°C / s) to a primary cooling end temperature (hereinafter also referred to as "SCS") of 450 to 600°C. The primary cooling end temperature can be defined as the point at which secondary cooling (quenching) is initiated by applying a quenching device that was not applied in the primary cooling. When the cooling process is performed stepwise, dividing it into primary and secondary cooling, the temperature distribution of the steel sheet can be uniform in the slow cooling stage, reducing the final temperature and material deviation and achieving the desired phase structure. In particular, the bainite structure of the present invention can only be actively formed from the primary cooling stage to achieve the desired elongation. If the primary cooling end temperature is less than 450°C, the bainite fraction becomes excessively high, and due to the length of the actual equipment, it is difficult to cool the steel sheet to 450°C or below at a cooling rate of less than 10°C / s. If the primary cooling end temperature exceeds 600°C, the cooling amount up to the secondary cooling end temperature will be large, resulting in a poor shape of the steel sheet and a lower bainite fraction than the target level. On the other hand, if the primary cooling rate is less than 1°C / s, the amount of ferrite phase precipitation during cooling will increase, making it difficult to obtain high-strength steel. If the primary cooling rate exceeds 10°C / s, the cooling amount in secondary cooling will increase, resulting in increased final temperature deviation and material deviation. From the viewpoint of improving the above-mentioned effects, more preferably, the lower limit of the primary cooling rate may be 3°C / s and the upper limit of the primary cooling rate may be 8°C / s.

[0084] The steel sheet that has undergone primary cooling is then subjected to secondary cooling at an average cooling rate of 10°C / s or more to a secondary cooling finish temperature (hereinafter also referred to as "RCS") of 250 to 350°C. By setting the secondary cooling finish temperature below the Ms temperature of the steel sheet, martensitic transformation occurs during cooling, and this martensite eventually transforms into a tempered martensite phase after a subsequent reheating step. Since the Ms temperature of most 980 MPa-class highly elongated steel sheets is below 400°C, in the present invention, the secondary cooling finish temperature is controlled to a range of 250 to 350°C. If the secondary cooling finish temperature is below 250°C, the amount of transformation of initial martensite is too large, resulting in a high yield strength and poor formability. On the other hand, if the secondary cooling finish temperature exceeds 350°C, martensite is not formed during cooling, making it difficult to obtain high yield strength and hole expandability. If the secondary cooling rate is less than 10°C / s, even if the target secondary cooling end temperature is reached, a high-temperature phase transformation occurs during cooling, making it impossible to obtain the target martensite fraction and high strength. From the viewpoint of improving the above-mentioned effects, more preferably, the lower limit of the secondary cooling rate may be 11°C / s, and the upper limit of the secondary cooling rate may be 30°C / s.

[0085] As mentioned above, the secondary cooling can be performed using a quenching equipment that was not used in the primary cooling. The present invention does not particularly limit the type of quenching equipment, but a preferred example is a hydrogen quenching equipment. More specifically, the hydrogen quenching equipment can use a gas consisting of 5 to 80% hydrogen by volume and the remainder nitrogen. If the hydrogen fraction exceeds 80%, it may be difficult to manage the equipment, such as by preventing explosions. If it is less than 5%, it may be difficult to utilize the efficient heat transfer properties of hydrogen, a light element.

[0086] The secondarily cooled steel sheet is then reheated to 350 to 480°C. This process achieves interphase carbon partitioning and further bainite phase transformation, which are necessary for stabilizing the retained austenite. In the present invention, the end temperature of the heating section is referred to as the reheating temperature (hereinafter also referred to as "RHS") for convenience. If the reheating temperature is less than 350°C, the strength becomes too high, resulting in poor elongation. On the other hand, if the reheating temperature exceeds 480°C, austenite phase transformation does not occur, and the austenite remains and then transforms into fresh martensite during final cooling, resulting in poor hole expandability and elongation. Meanwhile, the so-called nose temperature, where bainite transformation is most active, is approximately 400 to 420°C. In consideration of this, the lower limit of the reheating temperature is more preferably 411°C, and the upper limit of the reheating temperature is more preferably 440°C.

[0087] Meanwhile, although not particularly limited, according to one embodiment of the present invention, the average heating rate during the reheating may 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, resulting in a problem of excessive heat treatment, while if it exceeds 2.5°C / s, it may be difficult to ensure the physical properties desired in the present invention.

[0088] Furthermore, the inventors have conducted extensive research and have found that by precisely controlling the conditions for the above-mentioned primary cooling and secondary cooling so as to satisfy the following relational expression 4, a sufficient bainite structure can be obtained in the primary cooling and secondary cooling sections, thereby reducing the hardness difference between the phases and improving the hole expandability.

[0089] [Equation 4] V1 / V2×t>0.5 (In the above relational expression 4, V1 represents the average cooling rate during primary cooling, V2 represents the average cooling rate during secondary cooling, and t represents the thickness of the cold-rolled steel sheet.)

[0090] According to an embodiment of the present invention, after the reheating step, the reheated steel sheet may be further subjected to hot-dip galvanizing, galvannealing, and skin pass rolling, if necessary. Specifically, the method may further include a step of galvanizing the reheated steel sheet in a galvanizing bath at 450 to 470°C.

[0091] Furthermore, according to one embodiment of the present invention, the method may further include, if necessary, subjecting the plated steel sheet to an alloying heat treatment at a temperature in the range of 470 to 550°C. The alloying heat treatment is performed to obtain an appropriate alloying level, and the temperature is determined depending on the surface condition of the steel sheet. However, by controlling the surface condition of the steel material, the alloying heat treatment temperature must not exceed 550°C in order to prevent softening of the steel sheet and loss of retained austenite due to excessive tempering. Meanwhile, in order to rapidly proceed with alloying, the alloying heat treatment temperature is preferably higher than the hot-dip galvanizing temperature, and therefore the lower limit is controlled to 470°C. Furthermore, after the alloying heat treatment, the method may further include a step of cooling the alloying heat-treated steel sheet to room temperature and then temper rolling it at a reduction of less than 1% in order to correct the shape of the steel sheet and adjust its yield strength. [Example]

[0092] The present invention will be described in more detail below through examples. However, it should be noted that the following examples are intended to illustrate and embody the present invention, and are not intended to limit the scope of the present invention. The scope of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred therefrom.

[0093] (Example) Slabs having the alloy compositions shown in Table 1 below were prepared, then reheated at 1180 to 1220°C, and subjected to hot rolling, coiling, annealing, primary cooling, secondary cooling, reheating, and hot-dip galvanizing (GI) processes under the conditions shown in Table 2 below to produce cold-rolled steel sheets. Some of the steel sheets were also subjected to alloying heat treatment under the alloying heat treatment temperature (GA) conditions shown in Table 2 below. In this case, the cooling rate after finish hot rolling was 30 to 50°C / s, the cold reduction was 33 to 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.

[0094] The tensile properties, hole expandability, and LME evaluation results of the spot welds of the steel sheets manufactured in this way are shown in Table 3 below. The tensile strength (TS), yield strength (YS), and elongation (EL) were measured through tensile tests in the direction perpendicular to the rolling direction, and the test specimen specifications were 50 mm gauge length and 25 mm width. The hole expandability was measured according to the ISO 16330 standard, and the hole was sheared using a 10 mm diameter punch with a 12% clearance.

[0095] AE values ​​were measured by spot welding plated steel sheets, and the results are shown in Table 3 below. The AE value is the minimum current value at which LME occurs minus the minimum current value at which expulsion occurs. The spot welding test was performed by increasing the current in 0.5 kA increments, starting from a low current value, and allowing a cooling period between each current value to prevent excessive heat input to the material. After increasing the current value in this manner, the minimum current value at which expulsion of the weld nugget occurred was measured. At the same time, the minimum current value at which LME occurred was measured by observing the surface and cross section of the weld. The results are shown in Table 3 below. When observing the surface of the weld at 10x magnification and the cross section at 100x magnification, the weld was deemed to have passed if no cracks due to LME were visually observed.

[0096] Furthermore, the results of measuring the microstructure of the cold-rolled steel sheets produced as above and the calculation results of the relational expressions 1 to 3 used in the present invention are shown in Table 4.

[0097] The microstructure was measured by the point counting method from scanning electron microscope (SEM) photographs, and the fraction of retained austenite was measured by XRD.

[0098] [Table 1]

[0099] [Table 2]

[0100] [Table 3]

[0101] [Table 4]

[0102] As can be seen from Tables 1 to 4 above, Invention Examples 1 to 4, which were manufactured using Steel Types A to D, satisfied the alloy compositions, Relational Formula 1, and manufacturing conditions proposed by the present invention, thereby ensuring the microstructure that the present invention aims to achieve, and ensuring the targeted tensile strength of 980 to 1150 MPa, yield strength of 740 to 950 MPa, hole expandability (HER) of 45% or more, and elongation of 12% or more, and were also confirmed to have excellent LME properties.

[0103] In particular, in order to observe the martensite islands (MA) present inside the bainite in the cold-rolled steel sheet obtained in Example 1 of the present application, a photograph of a cross section in the thickness direction was taken with a scanning electron microscope (SEM) at a magnification of 5,000 times, and is shown in Figure 1. In Figure 1, the martensite islands (MA) present inside the bainite are indicated by arrows.

[0104] On the other hand, in the case of Comparative Examples 1 and 2, the ferrite phase fraction exceeds 10%, and it is found that the tensile strength, hole expandability, and hardness deviation are poor.

[0105] In the case of Comparative Examples 3 and 6, the fraction of the ferrite or martensite phase was outside the range limited by the present invention, and the required quality could not be obtained.

[0106] Furthermore, in the case of Comparative Examples 4 and 5, the required material was not obtained because the process was outside the range limited by the present invention.

[0107] Furthermore, Comparative Examples 7 and 8 contained a large amount of Si, which was outside the range of elements restricted by the present invention, and Comparative Example 9 contained an amount of C and other elements outside the range. In the LME evaluation, the minimum current at which LME occurred was lower than the minimum current at which expulsion occurred, indicating that spot weldability LME was weak.

Claims

1. The alloy contains, by weight, C: 0.10 to 0.16%, Si: 0.3 to 0.8%, Al: 0.01 to 0.5%, Mn: 2.0 to 3.0%, Cr: 0.001 to 0.5%, Mo: 0.001 to 0.5%, B: 0.0001 to 0.001%, Nb: 0.001 to 0.05%, Ti: 0.001 to 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 balance being Fe and other unavoidable impurities; The microstructure contains, by area%, ferrite: 10% or less (excluding 0%), retained austenite: more than 1% and 5% or less, martensite: 25% or more and less than 50%, and bainite: 35% or more and less than 70%; The cold-rolled steel sheet has an average size of martensite islands (MA) present inside the bainite of 0.35 to 0.55 μm.

2. The cold-rolled steel sheet according to claim 1, wherein the value defined by the following relational expression 1 satisfies 70 or more. [Relationship 1] 234×[C]-29×[Si]-128×[Al]+29×[Mn]+10×[Cr]-17×[Mo]-37×[Nb]-49×[Ti]+100×[B] (In the above Relational Formula 1, the [C], [Si], [Al], [Mn], [Cr], [Mo], [Nb], [Ti], and [B] represent the weight percent content of each element in parentheses.)

3. The cold-rolled steel sheet according to claim 1, wherein a value defined by the following relational expression 2 satisfies 270 or more and 330 or less. [Relationship 2] 270×[C]+90×[Mn]+70×[Cr]+80×[Mo] (In the above Relational Formula 2, the [C], [Mn], [Cr], and [Mo] represent the weight percent content of each element in parentheses.)

4. The cold-rolled steel sheet according to claim 1, wherein the value defined by the following relational expression 3 satisfies 1.8 or less. [Relationship 3] 5×[C]+[Si]+0.5×[Al] (In the above Relational Formula 3, [C], [Si], and [Al] represent the weight percent content of each element in parentheses.)

5. The cold-rolled steel sheet according to claim 1, wherein the microstructure contains 2 to 7% by area of ​​ferrite.

6. The cold-rolled steel sheet according to claim 1, wherein the microstructure contains, in area percentage, 2 to 4% retained austenite.

7. The cold-rolled steel sheet according to claim 1, wherein the microstructure contains 45 to 63% by area of ​​bainite.

8. The cold-rolled steel sheet according to claim 1, wherein the microstructure contains 29 to 49% by area of ​​martensite.

9. The cold rolled steel sheet according to claim 1, wherein the tensile strength is 980 to 1150 MPa and the yield strength is 740 to 950 MPa.

10. The cold rolled steel sheet according to claim 1, wherein the hole expandability (HER) is 45% or more.

11. heating a steel slab containing, by weight, C: 0.10 to 0.16%, Si: 0.3 to 0.8%, Al: 0.01 to 0.5%, Mn: 2.0 to 3.0%, Cr: 0.001 to 0.5%, Mo: 0.001 to 0.5%, B: 0.0001 to 0.001%, Nb: 0.001 to 0.05%, Ti: 0.001 to 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 balance being Fe and other unavoidable impurities; Finish hot rolling the heated slab at 830 to 980°C to obtain a hot-rolled steel sheet; coiling the hot-rolled steel sheet at 450 to 700°C; cold rolling the coiled hot-rolled steel sheet; continuous annealing the cold-rolled steel sheet at a temperature of 790 to 830°C; subjecting the continuously annealed steel sheet to a primary cooling end temperature of 450 to 600°C at an average cooling rate of less than 10°C / s; Secondarily cooling the primarily cooled steel sheet to a secondary cooling end temperature of 250 to 350°C at an average cooling rate of 10°C / s or more; reheating the second-cooled steel sheet to a temperature in the range of 350 to 480°C; Including, The following relational expression 4 is satisfied: The steel sheet obtained by the above steps has a microstructure containing, in area percentages, ferrite: 10% or less (excluding 0%), retained austenite: more than 1% and 5% or less, martensite: 25% or more and less than 50%, and bainite: 35% or more and less than 70%, and the average size of martensite islands (MA) present in the bainite is 0.35 to 0.55 μm. [Relationship 4] V1 / V2×t>0.5 (In the above-mentioned relational expression 4, V1 represents the average cooling rate during primary cooling, V2 represents the average cooling rate during secondary cooling, and t represents the thickness of the cold-rolled steel sheet.)

12. The method for producing a cold-rolled steel sheet according to claim 11, wherein the cold rolling reduction rate is in the range of 30 to 60% during the cold rolling.

13. The method for producing a cold-rolled steel sheet according to claim 11, further comprising the step of plating the reheated steel sheet in a galvanizing bath at 450 to 470°C.

14. The method for manufacturing a cold rolled steel sheet according to claim 13, further comprising the step of subjecting the plated steel sheet to an alloying heat treatment at a temperature in the range of 470 to 550°C.

15. The method for manufacturing a cold-rolled steel sheet according to claim 14, further comprising the step of cooling the alloyed heat-treated steel sheet to room temperature and then temper-rolling the steel sheet at a reduction of less than 1%.

16. The method for producing a cold-rolled steel sheet according to claim 11, wherein an average temperature rising rate during the reheating is 0.5 to 2.5 ° C. / s.

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