Ultrahigh-strength cold-rolled steel sheet and method for producing same

Abstract

The ultra-high strength cold-rolled steel sheet of the present invention comprises, in wt%: 0.23 to 0.40 wt% of carbon (C); from 0.05 wt% to 1.0 wt% of silicon (Si); 0.5 wt% to 3.0 wt% of manganese (Mn); 0.01% to 0.12% by weight of vanadium (V); 0.01 wt% to 0.3 wt% of aluminum (Al); 0 wt% (exclusive) to 0.5 wt% (inclusive) chromium (Cr); 0 wt% (exclusive) to 0.1 wt% (inclusive) of titanium (Ti); 0 wt% (exclusive) to 0.02 wt% (inclusive) phosphorus (P); 0 wt% (exclusive) to 0.01 wt% (inclusive) of sulfur (S); from 0.001% by weight to 0.005% by weight of boron (B); the remainder being iron (Fe) and other unavoidable impurities, wherein the final microstructure of the steel sheet includes tempered martensite at a volume fraction of 90% or more; the average pitch between precipitates in the tempered martensite is 300 nm or more, the average size of the precipitates is 200 nm or less, and the number of precipitates having an average size of 40 nm or less is 25 or more based on the area of 20 [mu] m2 in the final microstructure.

Description

Technical Field

[0001] The technical concept of the present invention relates to a cold-rolled steel sheet, and more specifically, to an ultra-high strength cold-rolled steel sheet having excellent bendability and hydrogen embrittlement resistance and a method for manufacturing the same. Background Art

[0002] Recently, various environmental and energy use regulations have led to a demand for high-strength steel sheets to improve fuel efficiency and durability. In particular, high yield strength and tensile strength are important for manufacturing parts such as front bumpers and side beams with automotive structural steel. Here, the microstructure used to manufacture high-strength steel sheets should be composed of low-temperature structures such as martensite and bainite. This structure is prone to embrittlement fracture due to hydrogen remaining inside the steel or hydrogen flowing in from the outside. This is called hydrogen embrittlement. Hydrogen embrittlement is characterized in that the strength at which fracture occurs is lower than the strength at which fracture generally occurs, and becomes more sensitive as the strength increases. In addition, since the material can be broken by hydrogen embrittlement even under very small stress, measures to address this problem are being sought.

[0003] As a related document, there is Korean Patent Application No. 10-2018-0047388. Summary of the invention

[0004] Technical issues

[0005] Therefore, the present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide an ultra-high strength cold-rolled steel sheet having excellent bendability and hydrogen embrittlement resistance and a method for manufacturing the same.

[0006] It should be understood that the technical problems are only provided as examples, and the technical ideas of the present invention are not limited thereto.

[0007] Technical Solution

[0008] According to one aspect of the present invention, the above and other objects can be achieved by providing an ultra-high strength cold-rolled steel sheet having excellent bendability and hydrogen embrittlement resistance and a method for manufacturing the same.

[0009] According to another aspect of the present invention, there is provided an ultra-high strength cold-rolled steel sheet, which comprises, by weight %, carbon (C): 0.23 weight % to 0.40 weight %; silicon (Si): 0.05 weight % to 1.0 weight %; manganese (Mn): 0.5 weight % to 3.0 weight %; vanadium (V): 0.01 weight % to 0.12 weight %; aluminum (Al): 0.01 weight % to 0.3 weight %; chromium (Cr): greater than 0 weight % and less than or equal to 0.5 weight %; titanium (Ti): greater than 0 weight % and less than or equal to 0 .1 wt%; phosphorus (P): greater than 0 wt% and less than or equal to 0.02 wt%; sulfur (S): greater than 0 wt% and less than or equal to 0.01 wt%; and boron (B): 0.001 wt% to 0.005 wt%; the remainder is Fe and other inevitable impurities, wherein the final microstructure includes tempered martensite with a volume fraction of 90% or more, wherein the average spacing between precipitates in the tempered martensite is 300 nm or more, the average size of the precipitates is 200 nm, and the final microstructure is 20 μm. 2 Based on the area of ​​, the number of precipitates having an average size of 40 nm was 25 or more.

[0010] According to an embodiment of the present invention, the surface layer of the cold rolled steel sheet may include a soft region having a hardness less than 85% of an average hardness of a base material, wherein a ratio of a thickness of the soft region to a thickness of the base material is 0.03 to 0.10.

[0011] According to an embodiment of the present invention, the cold rolled steel sheet may have a prior austenite grain size (PAGS) of 12 μm.

[0012] According to an embodiment of the present invention, the cold rolled steel sheet may further include, in terms of weight %, molybdenum (Mo): 0.01 weight % to 0.3 weight % or niobium (Nb): 0.01 weight % to 0.1 weight %.

[0013] According to an embodiment of the present invention, the value of [C]+[V]+[Cr]+[Mo]+[Nb] of the cold rolled steel sheet (wherein [C], [V], [Cr], [Mo] and [Nb] are the weight % values ​​of carbon, vanadium, chromium, molybdenum and niobium) can be less than 0.63.

[0014] According to an embodiment of the present invention, the ultra-high strength cold-rolled steel sheet may have a yield strength (YP) of 1200 MPa or more, a tensile strength (TS) of 1500 MPa or more, an elongation (El) of 7.0% or more, a yield ratio of 7.0% or more, a bendability (R / t) of 2.5 or less, and a hydrogen embrittlement elongation reduction rate of 35% or less.

[0015] According to another aspect of the present invention, there is provided a method for manufacturing an ultra-high strength cold-rolled steel sheet, the method comprising: hot rolling steel to provide a hot-rolled steel sheet, wherein the steel comprises, in terms of weight percent, carbon (C): 0.23 weight percent to 0.40 weight percent; silicon (Si): 0.05 weight percent to 1.0 weight percent; manganese (Mn): 0.5 weight percent to 3.0 weight percent; vanadium (V): 0.01 weight percent to 0.12 weight percent; aluminum (Al): 0.01 weight percent to 0.3 weight percent; chromium (Cr): greater than 0 weight percent and less than or equal to 0.5 weight percent; titanium (Ti): greater than 0 weight percent and less than or equal to 0.1 weight percent; phosphorus (P): greater than 0 weight percent and less than or equal to 0.5 weight percent; 0 wt % and less than or equal to 0.02 wt %; sulfur (S): greater than 0 wt % and less than or equal to 0.01 wt %; boron (B): 0.001 wt % to 0.005 wt %; the rest is Fe and other inevitable impurities; cold rolling the hot rolled steel sheet to provide a cold rolled steel sheet; heating the cold rolled steel sheet; annealing the heated cold rolled steel sheet; cooling the annealed cold rolled steel sheet; and reheating and tempering the cooled cold rolled steel sheet, wherein, when the cold rolled steel sheet is annealed, the annealing temperature is gradually increased within a temperature range of 750° C. to 950° C., without a section for maintaining a constant temperature until the cold rolled steel sheet is cooled.

[0016] According to an embodiment of the present invention, in the annealing of the cold-rolled steel sheet, the annealing temperature (T) may satisfy the relationship between the following formula 1 and the time (t):

[0017] [Formula 1]

[0018] Annealing temperature (T) = A × t 2 +B×t+C (where A, B and C constants satisfy -0.007 <A<-0.005,2<B<3,500<C<700)。

[0019] According to an embodiment of the present invention, in the heating of the cold rolled steel sheet, the heating rate may be 3° C. / sec or more, and in the annealing of the cold rolled steel sheet, the heating rate may be less than 3° C. / sec.

[0020] According to an embodiment of the present invention, hot rolling may be performed under the conditions of a reheating temperature (SRT) of 1180 to 1300°C, a finishing outlet temperature (FDT) of 800 to 950°C, and a coiling temperature (CT) of 500 to 700°C.

[0021] According to an embodiment of the present invention, cooling the annealed cold-rolled steel sheet may include: slowly cooling the annealed cold-rolled steel sheet to 700 to 800°C at a cooling rate of 3°C / second to 15°C / second; performing primary rapid cooling of the slowly cooled cold-rolled steel sheet to 300 to 350°C at a cooling rate of 80°C / second to 150°C / second; and performing secondary rapid cooling of the cold-rolled steel sheet after the primary rapid cooling to room temperature to 300°C at a cooling rate of 30°C / second to 90°C / second.

[0022] According to an embodiment of the present invention, the reheating and tempering of the cooled cold-rolled steel sheet may include reheating the cold-rolled steel sheet subjected to secondary rapid cooling and tempering it at 150° C. to 350° C. for 30 seconds to 300 seconds.

[0023] Beneficial Effects

[0024] According to the technical idea of ​​the present invention, an ultra-high strength cold-rolled steel sheet with excellent bendability and hydrogen embrittlement resistance and a method for manufacturing the same can be realized. Specifically, by replacing the annealing holding section with a temperature rise according to a quadratic function equation, an appropriate soft (decarburized) area can be obtained, thereby obtaining an ultra-high strength steel sheet that meets the target bendability (R / t) of 2.5 or less, and the steel sheet meets the elongation reduction rate of 35% or less after hydrogenation due to the suppression of the coarsening of the original austenite grain size (PAGS). In addition, by the primary and secondary rapid cooling after slow cooling, tempering during the cooling process can be suppressed, and uniform tempered martensite can be achieved by subsequent tempering. In addition, an ultra-high strength steel sheet with a yield strength of 1200MPa or more, a tensile strength of 1500MPa or more, and a high yield ratio (70% or more) can be realized, and replacing the holding section in the annealing section with a partial temperature rise can shorten the process time by about 10% or more, thereby improving productivity.

[0025] The effects of the present invention are described only as examples, and the scope of the present invention is not limited to these effects. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 is a process flow chart schematically showing a method for manufacturing an ultra-high strength cold-rolled steel sheet according to an embodiment of the present invention.

[0027] Figure 2 is a time-temperature graph showing annealing, cooling, and tempering heat treatment processes after cold rolling in a method of manufacturing an ultra-high strength cold-rolled steel sheet according to each comparative example and an example of the present invention.

[0028] Figure 3 A photograph showing a representative final microstructure of an ultra-high strength cold-rolled steel sheet according to an embodiment of the present invention.

[0029] Figure 4 A photograph showing precipitates observed in the microstructure of an ultra-high strength cold-rolled steel sheet according to an embodiment of the present invention is shown. DETAILED DESCRIPTION

[0030] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. The embodiments of the present invention are provided to explain the technical ideas of the present invention more comprehensively to those skilled in the art. The following embodiments may be changed in many different forms, and the scope of the technical spirit of the present invention is not limited to the following embodiments. On the contrary, embodiments are provided to make the present invention thorough and complete, and to fully convey the technical ideas of the present invention to those skilled in the art. The same reference numerals in the specification represent the same elements. In addition, the various elements and regions in the accompanying drawings are schematically drawn. Therefore, the technical ideas of the present invention are not limited by the relative sizes or spacings drawn in the accompanying drawings.

[0031] Hereinafter, a method for manufacturing an ultra-high strength cold-rolled steel sheet according to the technical concept of the present invention will be described in detail.

[0032] Figure 1 is a process flow chart schematically showing a method for manufacturing an ultra-high strength cold-rolled steel sheet according to an embodiment of the present invention, Figure 2 1 is a time-temperature graph showing annealing, cooling and tempering heat treatment processes after cold rolling using a method for manufacturing an ultra-high strength cold-rolled steel sheet according to each comparative example and an example of the present invention.

[0033] See also Figure 1 , the method for manufacturing an ultra-high strength cold-rolled steel sheet according to an embodiment of the present invention includes: a step of hot rolling steel to provide a hot-rolled steel sheet (S10); a step of cold rolling the hot-rolled steel sheet to provide a cold-rolled steel sheet (S20); a step of heating the cold-rolled steel sheet (S31); a step of annealing the heated cold-rolled steel sheet (S32); a step of cooling the annealed cold-rolled steel sheet (S40, S50, S60); and a step of reheating and tempering the cooled cold-rolled steel sheet (S70).

[0034] Hot rolling step (S10)

[0035] The ultra-high strength cold-rolled steel sheet according to the embodiment of the present invention comprises, by weight%, carbon (C): 0.23 weight % to 0.40 weight %; silicon (Si): 0.05 weight % to 1.0 weight %; manganese (Mn): 0.5 weight % to 3.0 weight %; vanadium (V): 0.01 weight % to 0.12 weight %; aluminum (Al): 0.01 weight % to 0.3 weight %; chromium (Cr): greater than 0 weight % and less than or equal to 0.5 weight %; titanium (Ti): greater than 0 weight % and less than or equal to 0.1 weight %; phosphorus (P): greater than 0 weight % and less than or equal to 0.02 weight %; sulfur (S): greater than 0 weight % and less than or equal to 0.01 weight %; and boron (B): 0.001 weight % to 0.005 weight %; the remainder is Fe and other inevitable impurities.

[0036] Hereinafter, the functions and contents of each component contained in the ultra-high strength cold-rolled steel sheet according to the present invention will be described. Here, the content of each component element refers to weight % based on the total weight of the steel sheet.

[0037] Carbon (C): 0.23% to 0.40%

[0038] Carbon is added to ensure the strength of the steel, and as the carbon content in the martensitic structure increases, the strength also increases. The carbon content is preferably 0.23% to 0.40%. When the carbon content is low, i.e., below 0.23%, it is difficult to obtain the target strength. When it is greater than 0.40%, the carbon equivalent (Ceq) increases, which may be detrimental to weldability, bendability, and hydrogen embrittlement resistance.

[0039] Silicon (Si): 0.05% to 1.0%

[0040] Silicon is a ferrite stabilizing element that delays the formation of carbides in ferrite and has a solid solution strengthening effect. The amount of silicon added is preferably 0.05% to 1.0%. If it is less than 0.05%, the effect is very small. If it exceeds 1.0%, oxides such as Mn2SiO4 may be formed during the manufacturing process, and the carbon equivalent will increase, which will reduce weldability. In addition, carbide precipitation will be excessively suppressed, which leads to deviation from the effect of the present invention.

[0041] Manganese (Mn): 0.5% to 3.0%

[0042] Manganese has a solid solution strengthening effect, and contributes to improving strength by improving hardenability. The amount of manganese added is preferably 0.5% to 3.0%. When the content is less than 0.5%, hardenability is insufficient, making it difficult to ensure strength. When the content is more than 3.0%, the formation or segregation of inclusions such as MnS may cause deterioration of workability and deterioration of hydrogen embrittlement resistance, and reduce weldability by increasing carbon equivalent.

[0043] Vanadium (V): 0.01% to 0.12%

[0044] Vanadium is the main element precipitated in the form of carbides (VC) in steel, has the effect of refining the original austenite grain size (PAGS) and helps to improve the yield strength. Based on the total weight of the ultra-high strength cold-rolled steel sheet according to the embodiment of the present invention, vanadium is preferably added in a content ratio of 0.01% to 0.12%. When the content of vanadium is less than 0.01%, it is difficult to expect the precipitate effect. When the content of vanadium is greater than 0.12%, problems such as material degradation, increased manufacturing costs, grain coarsening due to the formation of coarse carbonates, and uneven structure due to excessively high recrystallization temperatures will occur.

[0045] Aluminum (Al): 0.01% to 0.3%

[0046] Aluminum is used as a deoxidizer and can help purify ferrite. When the content of aluminum is less than 0.01%, the deoxidation effect may be insufficient. When the content of aluminum is more than 0.3%, AlN may be formed during slab manufacturing, which may cause cracks during casting or hot rolling. Therefore, the preferred addition content of aluminum is 0.01% to 0.3% based on the total weight of the steel plate.

[0047] Chromium (Cr): greater than 0% and less than or equal to 0.5%

[0048] Chromium is a ferrite stabilizing element, improves the solid solution strengthening and hardenability of steel, and helps improve strength by refining carbides. When the chromium content is greater than 0.5%, weldability is impeded, thereby increasing the manufacturing cost of steel. Therefore, the preferred addition content of chromium is greater than 0% and less than or equal to 0.5% based on the total weight of the steel plate.

[0049] Titanium (Ti): greater than 0% and less than or equal to 0.1%

[0050] Titanium is a precipitate-forming element and has the effect of precipitating TiN and TiC and refining grains. In particular, the nitrogen content in steel can be reduced by the precipitation of TiN, and when added together with boron, the precipitation of BN can be prevented. The preferred addition content of titanium is greater than 0% and less than or equal to 0.1%. When the content of titanium is greater than 0.1%, it increases the manufacturing cost of steel.

[0051] Phosphorus (P): greater than 0% and less than or equal to 0.02%

[0052] Phosphorus is an impurity contained in the steel manufacturing process. Although it can improve strength through solid solution strengthening, it will cause low-temperature embrittlement when contained in large amounts. Therefore, it is preferred to limit the phosphorus content to more than 0% and less than or equal to 0.02% based on the total weight of the steel plate.

[0053] Sulfur (S): greater than 0% and less than or equal to 0.01%

[0054] Sulfur is an impurity contained in the steel manufacturing process, and forms non-metallic inclusions such as FeS and MnS, thereby reducing bendability, toughness and weldability. Therefore, it is preferred to limit the sulfur content to more than 0% and less than or equal to 0.01% based on the total weight of the steel plate.

[0055] Boron (B): 0.001% to 0.005%

[0056] Boron is a hardening element that greatly contributes to the formation of martensite during the cooling process after annealing. The preferred addition content of boron is 0.001% to 0.005%. When the content is less than 0.001%, the effect is insufficient, so it is difficult to ensure martensite. When the content is more than 0.005%, the toughness of the steel may decrease.

[0057] The remaining component of the ultra-high strength cold rolled steel sheet is iron (Fe). However, since unexpected impurities from raw materials or the surrounding environment are inevitably mixed in the normal steelmaking process, impurities cannot be excluded. Since these impurities are known to any technician in the field of ordinary manufacturing processes, they are not specifically mentioned in this specification.

[0058] Meanwhile, the ultra high strength cold rolled steel sheet according to the modified embodiment of the present invention may further include at least one of elements having the following composition ranges in addition to the above-mentioned alloy elements.

[0059] Molybdenum (Mo): 0.01% to 0.3%

[0060] Molybdenum has the effect of improving hydrogen embrittlement resistance, has a solid solution strengthening effect and improves hardenability, thereby helping to improve strength. When the content of molybdenum is greater than 0.3%, the manufacturing cost of steel will increase. Therefore, the preferred addition content of molybdenum is 0.01% to 0.3% based on the total weight of the steel plate.

[0061] Niobium (Nb): 0.01% to 0.1%

[0062] Niobium has the effect of refining the prior austenite grain size (PAGS) and contributes to improving the yield strength by forming precipitates in the form of NbC. That is, niobium is a precipitate-forming element that improves the toughness and strength of steel by precipitation and grain refinement. The preferred addition content of niobium is 0.01% to 0.1%. When the addition content is greater than 0.1%, a low-temperature transformation structure may occur due to the delayed ferrite transformation effect during hot rolling, which may adversely affect the impact properties and increase the manufacturing cost of the steel.

[0063] The ultra-high strength cold-rolled steel sheet having the above composition range according to the present invention can satisfy the following relational expression 1:

[0064] Relationship 1: [C]+[V]+[Cr]+[Mo]+[Nb]<0.63

[0065] Wherein [C], [V], [Cr], [Mo] and [Nb] are weight % values ​​of carbon, vanadium, chromium, molybdenum and niobium. When relational expression 1 is not satisfied, precipitates may coarsen, thereby increasing hydrogen embrittlement.

[0066] Meanwhile, the steel having the above composition is hot rolled to provide a hot rolled steel sheet. The hot rolling may be performed under the following conditions: reheating temperature (SRT): 1180°C to 1300°C, finishing outlet temperature (FDT): 800°C to 950°C, coiling temperature (CT): 500°C to 700°C.

[0067] The slab containing the above alloy components and the rest being iron and inevitable impurities is reheated to 1180°C to 1300°C. The slab is manufactured in the form of a semi-finished product by continuously casting the molten steel obtained through the steelmaking process. By reheating the slab, the component segregation occurring during the casting process is homogenized to prepare for the hot rolling of the slab. When the slab reheating temperature (SRT) is lower than 1180°C, there is a problem that the slab segregation is not fully reused. When the SRT is higher than 1300°C, the austenite grain size increases, and the process cost may increase due to the increase in temperature. The slab can be reheated for 1 to 4 hours. When the reheating time is less than 1 hour, the homogenization of the segregation is insufficient. When the reheating time is longer than 4 hours, the austenite grain size increases and the process cost may increase.

[0068] The reheated slab is hot rolled. Hot rolling is performed at a finishing exit temperature (FDT) of 800°C to 900°C. When the FDT is lower than 800°C, the rolling load increases rapidly, thereby reducing productivity. When the FDT is higher than 900°C, the grain size increases, which reduces strength. After hot rolling, it is cooled to 500°C to 700°C and then coiled. When the coiling temperature is lower than 500°C, the strength increases and the rolling load during cold rolling increases. When the coiling temperature is higher than 700°C, defects may occur due to surface oxidation in subsequent processes.

[0069] Cold rolling step (S20)

[0070] The hot rolled steel sheet is pickled to remove the surface oxide scale, and cold rolled. Milling is performed to remove the surface oxide scale remaining after pickling. In order to achieve a uniform surface oxide scale or completely remove the surface oxide scale, milling is performed from 0.3 to 1 mm from the initial thickness. If the milling thickness is less than 0.3 mm, the surface oxide scale generated during the hot rolling process may remain, and it may be difficult to control a uniform decarburized layer during the subsequent heat treatment. If milling is performed over 1 mm, the scrap portion increases, which may reduce the recovery rate and increase the process cost. The thickness reduction rate during the cold rolling process is about 40% to 70%.

[0071] Cold rolling heat treatment (S31 to S70)

[0072] Refer to Figure 1 and Figure 2 successively perform the steps of heating the cold-rolled steel sheet (S31); annealing the heated cold-rolled steel sheet (S32); cooling the annealed cold-rolled steel sheet (S40, S50, S60); and reheating and tempering the cooled cold-rolled steel sheet (S70).

[0073] In the step of heating the cold-rolled steel sheet (S31), the heating rate can be 3 °C / second or more. In the heating step (S31), the cold-rolled steel sheet can reach a temperature of A3 to A3 + 30 °C. When the heating rate in the heating step (S31) is less than 3 °C / second, it takes a long time to reach the target annealing temperature, so the production efficiency may decrease and the grain size may increase.

[0074] In the step of annealing the heated cold-rolled steel sheet (S32), the annealing temperature is characterized in that it gradually increases within the temperature range (ΔT) of 750 °C to 950 °C and there is no section of maintaining a constant temperature before performing the step of cooling the cold-rolled steel sheet (S40).

[0075] The step of annealing the cold-rolled steel sheet (S32) is characterized in that the annealing temperature (T) satisfies the relationship between the following formula 1 and time (t):

[0076] <Formula 1>

[0077] Annealing temperature (T) = A × t 2 + B × t + C (where A, B, and C are constants satisfying -0.007 < A < -0.005, 2 < B < 3, and 500 < C < 700)

[0078] The annealing temperature should be increased to A3 or higher to produce a single austenite phase, thereby producing the final structure, that is, tempered martensite. In the present invention, the annealing temperature varies depending on the type of steel, and the annealing temperature of most steels satisfies 750 °C to 950 °C. In the present invention, by eliminating the holding section in the ordinary annealing section and replacing it with a temperature increase according to Formula 1, the following advantages can be obtained.

[0079] It is possible to eliminate the disadvantage that an appropriate degree of softening (decarburization) cannot be obtained when maintaining a low temperature and the adverse effect of hydrogen embrittlement caused by the coarsening of the prior austenite grain size (PAGS) that may occur when maintaining a high temperature, reduce the heat treatment time of the entire annealing section by about 10%, and thus improve the productivity.

[0080] Next, the annealed cold-rolled steel sheet is subjected to a step of slow cooling (S40). Step (S40) is a section in which cooling is performed at a cooling rate of 3°C / sec to 15°C / sec after annealing. For example, the annealed cold-rolled steel sheet is slowly cooled to 700°C to 800°C at a cooling rate of 3°C / sec to 15°C / sec. Here, when the slow cooling termination temperature (i.e., the rapid cooling start temperature) drops below 700°C or lower, ferrite transformation occurs, resulting in a decrease in strength, thereby failing to achieve the target strength.

[0081] The cold-rolled steel sheet after slow cooling is subjected to a primary rapid cooling process (S50). For example, the cold-rolled steel sheet after slow cooling can be subjected to a primary rapid cooling process until a temperature of 300°C to 350°C at a cooling rate of 80°C / sec to 150°C / sec. The primary rapid cooling section is a temperature section from the slow cooling termination temperature to the Ms transformation point or lower. In this section, it is important to suppress the transformation of ferrite and bainite by rapid cooling. In the alloy system of the present embodiment, the cooling rate for suppressing the transformation is about 80°C / sec or greater. When the cooling rate is lower than 80°C / sec, the strength may be reduced due to the transformation of ferrite and bainite.

[0082] Next, the cold-rolled steel sheet that has undergone the primary rapid cooling process is subjected to a secondary rapid cooling treatment step (S60). For example, the cold-rolled steel sheet that has undergone the primary rapid cooling process can be subjected to a secondary cooling treatment at a cooling rate of 30°C / second to 90°C / second until the temperature reaches 300°C from room temperature. The secondary rapid cooling section is a temperature section from the Ms transformation point to room temperature. In this section, martensitic transformation occurs. In the alloy system of the present embodiment, the Ms temperature is around 350°C, and the martensitic transformation begins at 300°C to 400°C. If this temperature range is maintained for a long time, tempering may occur, resulting in the formation of a tempered martensitic structure with larger carbides, and a decrease in strength due to the softening of the tempered martensite. In the present embodiment, in order to prevent this tempering, a cooling rate of 30°C / second or more is required in the secondary cooling section. However, when the average cooling rate of the primary rapid cooling section and the secondary rapid cooling section is 70°C / second or more, cooling can be performed without grading the rapid cooling section.

[0083] The step of reheating and tempering the cooled cold-rolled steel sheet (S70). For example, the tempering step is to reheat the cold-rolled steel sheet that has undergone secondary cooling treatment and hold it at 150°C to 350°C for 30 seconds to 300 seconds. The tempering step is a section where the generated martensite is transformed into tempered martensite. When tempering is performed at a temperature below 150°C, the temperature is too low to see the tempering effect. When tempering is performed at a temperature above 300°C, the size of the carbide may become coarse, which may lead to a decrease in strength. The tempering holding time has little effect compared to the tempering temperature. However, if it is less than 30 seconds, it is difficult to obtain a stable tempering effect. If it exceeds 5 minutes, the heat treatment efficiency may decrease, the carbide size may increase, and the strength may decrease, so it is limited to 5 minutes or less.

[0084] The ultra-high strength cold-rolled steel sheet according to the embodiment of the present invention obtained by performing the above steps contains, in weight%, carbon (C): 0.23 weight % to 0.40 weight %; silicon (Si): 0.05 weight % to 1.0 weight %; manganese (Mn): 0.5 weight % to 3.0 weight %; vanadium (V): 0.01 weight % to 0.12 weight %; aluminum (Al): 0.01 weight % to 0.3 weight %; chromium (Cr): greater than 0 weight % and less than or equal to 0.5 weight %; titanium (Ti): greater than 0 weight % and less than or equal to 0.1 weight %; phosphorus (P): greater than 0 weight % and less than or equal to 0.02 weight %; sulfur (S): greater than 0 weight % and less than or equal to 0.01 weight %; boron (B): 0.001 weight % to 0.005 weight %, and the rest is Fe and other inevitable impurities.

[0085] The final microstructure contains tempered martensite with a volume fraction of 90% or more, and the remainder may be martensite and bainite. In the final microstructure of the embodiment, the volume fraction of tempered martensite may be 100%. In the final microstructure of another embodiment, the volume fraction of tempered martensite may be 90%, and the volume fraction of martensite and bainite may be 10%.

[0086] The invention is characterized in that the average spacing between precipitates in the tempered martensite is 300 nm or more, the average size of the precipitates is 200 nm or less, and the average size of the precipitates is 20 μm or less in the final microstructure. 2 The number of precipitates with an average size of 40 nm or less is 25 or more. When the number of tempered martensite in the ultra-high strength cold-rolled steel sheet base material of this embodiment is A and the number of precipitates is B, the relationship of B / A<0.01 is satisfied.

[0087] The prior austenite grain size (PAGS) of the ultra-high strength cold-rolled steel sheet according to the embodiment of the present invention achieved by performing the above steps may be 12 μm or less. If it is coarser than this, it is difficult to confirm the effect of improving hydrogen embrittlement characteristics by grain refinement.

[0088] In order to satisfy the target tensile properties and hydrogen embrittlement characteristics of the ultra-high strength cold-rolled steel sheet according to an embodiment of the present invention, PAGS is composed of fine tempered martensite (90% or more) and martensite and bainite (remainder) structures.

[0089] The ultra-high strength cold-rolled steel sheet according to the embodiment of the present invention achieved by performing the above steps has a soft area in the surface layer, the hardness of the soft area is less than 85% of the average hardness of the base material of the cold-rolled steel sheet, and the ratio of the thickness of the soft area to the thickness of the base material can be 0.03 to 0.10. The hardness of the steel sheet is measured at intervals of 30 μm from the surface to the center of the cross section of the plate thickness using a Vickers tester with a load of 50 g. When the hardness of the base material is Hv M The hardness of the soft part (decarburized part) is Hv S When the soft area of ​​the steel plate satisfies the relation Hv M / Hv S <0.85.

[0090] That is, the soft area of ​​the steel plate surface layer has a hardness less than 0.85×Hv M In the area, due to the M / Hv S When the hardness difference between the surface layer and the base material is greater than 0.85, it is difficult to obtain the improvement effect of the bending properties, so it is set to less than 0.85. The hardness of the steel plate base material is measured using the average value of 5 points in the 1 / 4 plate thickness area. In the present invention, when the thickness of the base material is tm and the thickness of the soft (decarburized) area is ts, the relationship 0.03 is satisfied. <t s / t m <0.1. When the thickness of the soft region is too thin (0.03 or less), it is difficult to obtain the effect of improving the bendability, and when the thickness is greater than 0.1, it is difficult to ensure the target characteristics.

[0091] It is confirmed that the ultra-high strength cold-rolled steel sheet according to the embodiment of the present invention achieved by performing the above steps has the following characteristics: yield strength (YP): 1200 MPa or more, tensile strength (TS): 1500 MPa or more, elongation (El): 7.0% or more, yield ratio: 70% or more, bendability (R / t): 2.5 or less, hydrogen embrittlement elongation reduction rate: 35% or less.

[0092] Experimental Examples

[0093] Hereinafter, preferred experimental examples are given to help understand the present invention. However, the following experimental examples are only used to help understand the present invention, and the present invention is not limited to the following experimental examples.

[0094] Table 1 shows the composition of the steel according to the experimental example of the present invention (unit: weight %), and the rest is iron.

[0095] Referring to Table 1, the steel according to Example 1 satisfies the following composition ranges in terms of weight %: carbon (C): 0.23 weight % to 0.40 weight %; silicon (Si): 0.05 weight % to 1.0 weight %; manganese (Mn): 0.5 weight % to 3.0 weight %; vanadium (V): 0.01 weight % to 0.12 weight %; aluminum (Al): 0.01 weight % to 0.3 weight %; chromium (Cr): greater than 0 weight % and less than or equal to 0.5 weight %; titanium (Ti): greater than 0 weight % and less than or equal to 0.1 weight %; phosphorus (P): greater than 0 weight % and less than or equal to 0.02 weight %; sulfur (S): greater than 0 weight % and less than or equal to 0.01 weight %; boron (B): 0.001 weight % to 0.005 weight %, and the remainder is iron (Fe). However, compared with Example 1 and Comparative Examples 3 and 4, the carbon content in Comparative Examples 1 and 2 is relatively low. Meanwhile, unlike Example 1, Comparative Examples 1 to 4 do not include vanadium.

[0096] At the same time, it is confirmed that the value of [C]+[V]+[Cr]+[Mo]+[Nb] (wherein [C], [V], [Cr], [Mo] and [Nb] in Example 1 are the weight % values ​​of carbon, vanadium, chromium, molybdenum and niobium) is less than 0.63, but the value of [C]+[V]+[Cr]+[Mo]+[Nb] of Comparative Examples 3 and 4 is greater than 0.63 (Comparative Example 3: 0.686, Comparative Example 4: 0.631).

[0097] In the experimental examples of the present invention, Comparative Examples 1 to 4 and Example 1 were subjected to the same conditions within the above-mentioned ranges of hot rolling (S10), cold rolling (S20) and heat treatment after cold rolling (S40, S50, S60). However, the annealing process (S32) of Comparative Examples 1 and 2 corresponds to Figure 2 The process ① shown in the figure, the annealing process (S32) of Comparative Examples 3 and 4 and Example 1 corresponds to Figure 2 The process shown in ②.

[0098] [Table 1]

[0099]

[0100] Tables 2 and 3 show the results of property evaluation of the steel according to the experimental examples of the present invention. In Table 3, the hydrogen embrittlement elongation reduction ratio (%) was measured under the conditions of 5 mA and 1 hour of hydrogen injection.

[0101] [Table 2]

[0102] YS(MPa) TS(MPa) T.EL(%) YR(%) R / t Comparative Example 1 1290 1571 6.1 82.1 0.7 Comparative Example 2 1285 1543 5.8 83.3 0.5 Comparative Example 3 1324 1749 7.2 75.7 2.2 Comparative Example 4 1339 1745 7.8 76.7 2.0 Example 1 1290 1726 8.3 74.8 2.0

[0103] [Table 3]

[0104]

[0105]

[0106] Referring to Table 2 and Table 3, compared with the alloy content of Example 1, the carbon (C) content of Comparative Examples 1 and 2 is low, and thus has low physical properties. In addition, the main feature is that no heat treatment is used during the annealing section. Figure 2 The process shown in ② is different from Figure 2 The general heat treatment is performed as in the process ① shown in . As a result, the bendability is good because the hardness difference of the soft part is large and the thickness occupied by the soft part is large, but by performing the general maintenance heat treatment, the process time is increased, and the original austenite grain size (PAGS) increases due to being kept at a relatively high temperature, and the hydrogen embrittlement elongation reduction rate is large. Comparative Examples 3 and Comparative Examples 4 show smaller original austenite grain sizes (PAGS) than Comparative Examples 1 and 2 due to the increase in the content of Nb and Mo. The reason why hydrogen embrittlement is not improved even if the original austenite grain size (PAGS) is reduced is as follows. Precipitates are generally precipitated by Mo, Nb, V, Ti, etc., but as the content of the corresponding elements increases, the precipitates do not dissolve. It is confirmed that the values ​​of [C] + [V] + [Cr] + [Mo] + [Nb] of Comparative Examples 3 and 4 exceed 0.63 (Comparative Example 3: 0.686, Comparative Example 4: 0.631). That is, as the carbides insoluble in water increase during the reheating process, coarse precipitates are precipitated. As the precipitates become coarser, the average distance becomes shorter and the number of precipitates decreases, which does not significantly contribute to improving hydrogen embrittlement.

[0107] On the other hand, it was confirmed that in Example 1, fine precipitates of relatively uniform size were uniformly distributed, satisfying the conditions proposed in the present invention (yield strength (YP): 1200 MPa or more, tensile strength (TS): 1500 MPa or more, elongation (El): 7.0% or more, yield ratio: 70% or more, bendability (R / t): 2.5 or less), and hydrogen embrittlement was significantly improved (hydrogen embrittlement elongation reduction rate: 35% or less).

[0108] Figure 3is a photograph of a representative final microstructure of an ultra-high strength cold-rolled steel sheet according to Example 1 of the present invention, Figure 4 This is a photograph of precipitates observed in the microstructure of the ultra-high strength cold-rolled steel sheet according to Example 1 of the present invention.

[0109] Reference Figure 3 and Figure 4 It can be confirmed that the final microstructure of the ultra-high strength cold-rolled steel sheet according to Example 1 of the present invention includes tempered martensite with a volume fraction of 90% or more, the average spacing between precipitates inside the tempered martensite is 300nm or more, the average size of the precipitates is 200nm or less, and the final microstructure is 20μm. 2 Based on the area of ​​, the number of precipitates having an average size of 40 nm or less is 25 or more.

[0110] The technical idea of ​​the ultra-high strength cold rolled steel sheet and the method for manufacturing the same according to the present invention is described above. The embodiment of the present invention relates to a method for manufacturing ultra-high strength steel using martensite similar to the comparative embodiment, but has the following differences and advantages.

[0111] By replacing the annealing holding section with a temperature rise according to a quadratic function equation, an appropriate soft (decarburized) region can be obtained, thereby obtaining an ultra-high strength steel plate that meets the target bendability (R / t) of 2.5 or less, and hydrogenates the steel plate to meet an elongation reduction rate of 35% or less due to the suppression of coarsening of the original austenite grain size (PAGS). In addition, tempering during cooling can be suppressed by primary and secondary rapid cooling after slow cooling, and uniform tempered martensite can be achieved by subsequent tempering. In addition, an ultra-high strength steel plate with a yield strength of 1200 MPa or more, a tensile strength of 1500 MPa or more, and a high yield ratio (70% or more) can be achieved, and replacing the holding section in the annealing section with a partial temperature rise can shorten the process time by about 10% or more, thereby improving productivity.

[0112] It is obvious to those skilled in the art that the present invention described above is not limited to the above embodiments and drawings, and various substitutions, modifications and changes may be made without departing from the technical concept of the present invention.

Claims

1. An ultra-high strength cold-rolled steel sheet, comprising, by weight %, carbon (C): 0.23 wt % to 0.40 wt %; silicon (Si): 0.05 wt % to 1.0 wt %; manganese (Mn): 0.5 wt % to 3.0 wt %; vanadium (V): 0.01 wt % to 0.12 wt %; Aluminum (Al): 0.01 wt% to 0.3 wt%; Chromium (Cr): greater than 0 wt% and less than or equal to 0.5 wt%; Titanium (Ti): greater than 0 wt% and less than or equal to 0.1 wt%; Phosphorus (P): greater than 0 wt% and less than or equal to 0.02 wt%; Sulfur (S): greater than 0 wt% and less than or equal to 0.01 wt%; and Boron (B): 0.001 wt% to 0.005 wt%; The rest is Fe and other inevitable impurities. wherein the final microstructure comprises 90% by volume or more of tempered martensite, wherein the average spacing between precipitates in the tempered martensite is 300 nm or more, the average size of the precipitates is 200 nm or less, and the average size of the precipitates is 200 nm or less in the final microstructure. 2 Based on the area of ​​, the number of precipitates having an average size of 40 nm or less is 25 or more.

2. The ultra-high strength cold-rolled steel sheet according to claim 1, wherein: The surface layer of the cold-rolled steel sheet comprises a soft region, the hardness of the soft region is less than 85% of the average hardness of the base material, wherein the ratio of the thickness of the soft region to the thickness of the base material is 0.03 to 0.

10.

3. The ultra-high strength cold-rolled steel sheet according to claim 1, wherein: The prior austenite grain size (PAGS) of the cold rolled steel sheet is 12 μm or less. 4 . The ultra-high strength cold-rolled steel sheet according to claim 1 , further comprising, in terms of weight %, molybdenum (Mo): 0.01 weight % to 0.3 weight % or niobium (Nb): 0.01 weight % to 0.1 weight %.

5. The ultra-high strength cold-rolled steel sheet according to claim 1 or 4, wherein: The value of [C]+[V]+[Cr]+[Mo]+[Nb] of the cold rolled steel sheet (wherein [C], [V], [Cr], [Mo] and [Nb] are the weight % values ​​of carbon, vanadium, chromium, molybdenum and niobium) is less than 0.

63.

6. The ultra-high strength cold-rolled steel sheet according to claim 1, wherein: The ultra-high strength cold-rolled steel sheet has a yield strength (YP) of 1200 MPa or more, a tensile strength (TS) of 1500 MPa or more, an elongation (El) of 7.0% or more, a yield ratio of 7.0% or more, a bendability (R / t) of 2.5 or less, and a hydrogen embrittlement elongation reduction rate of 35% or less.

7. A method for manufacturing an ultra-high strength cold-rolled steel sheet, the method comprising: Hot rolling the steel to provide a hot rolled steel sheet, the steel comprising, in wt %, carbon (C): 0.23 wt % to 0.40 wt %; Silicon (Si): 0.05 wt% to 1.0 wt%; Manganese (Mn): 0.5 wt% to 3.0 wt%; Vanadium (V): 0.01 wt% to 0.12 wt%; Aluminum (Al): 0.01 wt% to 0.3 wt%; Chromium (Cr): greater than 0 wt% and less than or equal to 0.5 wt%; Titanium (Ti): greater than 0 wt% and less than or equal to 0.1 wt%; Phosphorus (P): greater than 0 wt% and less than or equal to 0.02 wt%; Sulfur (S): greater than 0 wt% and less than or equal to 0.01 wt%; and Boron (B): 0.001 wt% to 0.005 wt%; The rest is Fe and other unavoidable impurities; cold rolling the hot rolled steel sheet to provide a cold rolled steel sheet; Heating the cold rolled steel sheet; Annealing the heated cold-rolled steel sheet; Cooling the cold-rolled steel sheet after annealing; and The cooled cold rolled steel sheet is reheated and tempered. When the cold-rolled steel sheet is annealed, the annealing temperature is gradually increased within a temperature range of 750° C. to 950° C., and there is no section for maintaining a constant temperature until the cold-rolled steel sheet is cooled.

8. The method according to claim 7, wherein: In the annealing of the cold-rolled steel sheet, the annealing temperature (T) satisfies the relationship between the following formula 1 and the time (t): [Formula 1] Annealing temperature (T) = A × t 2 +B×t+C(where, A, B and C constants meet -0.007 <A<-0.005,2<B<3,500<C<700)。 9. The method according to claim 7, wherein: In the heating of the cold-rolled steel sheet, the heating rate is 3° C. / second or more, and When annealing the cold rolled steel sheet, the heating rate is less than 3°C / second.

10. The method according to claim 7, wherein: The hot rolling is performed under the following conditions: a reheating temperature (SRT) of 1180°C to 1300°C, a finishing outlet temperature (FDT) of 800°C to 950°C, and a coiling temperature (CT) of 500°C to 700°C.

11. The method according to claim 7, wherein: Cooling the cold-rolled steel sheet after annealing includes: The annealed cold-rolled steel sheet is slowly cooled at a cooling rate of 3°C / second to 15°C / second until the temperature reaches 700°C to 800°C; The slowly cooled cold-rolled steel sheet is first rapidly cooled at a cooling rate of 80°C / sec to 150°C / sec to 300°C to 350°C; and The cold-rolled steel sheet that has undergone primary rapid cooling is subjected to secondary rapid cooling at a cooling rate of 30° C. / sec to 90° C. / sec until the temperature reaches 300° C. via room temperature.

12. The method according to claim 7, wherein: Reheating and tempering the cooled cold-rolled steel sheet includes: reheating the cold-rolled steel sheet after secondary rapid cooling, and tempering it at 150° C. to 350° C. for 30 seconds to 300 seconds.

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