High-strength steel plate and manufacturing method thereof

Abstract

The present invention relates to a steel sheet containing C, Si, Mn, Cr, Al, Nb, Ti, B, P, S, N, the balance being Fe and other unavoidable impurities, and the contents of C, Si and Al satisfy the following mathematical formula (1), and the microstructure contains, in area fraction, more than 1% and not more than 4% retained austenite, more than 10% and not more than 20% fresh martensite, 5% or less (excluding 0%) ferrite, more than 50% and not more than 70% tempered martensite, and the balance being bainite, and the number density of the retained austenite is 0.25 particles / μm 2 The present invention provides a high-strength steel sheet having an effective diameter smaller than the average effective diameter of 0.2 to 0.4 μm, and a method for manufacturing the same, in which the proportion of the retained austenite having an effective diameter smaller than the average effective diameter is more than 60%. [Formula (1)] [C] + ([Si] + [Al]) / 5≦0.35wt.% (Here, [C], [Si], and [Al] represent the weight percentages of C, Si, and Al, respectively.)

Description

[Technical Field]

[0001] The present invention relates to a high-strength steel plate with high hole expandability and a manufacturing method thereof. [Background technology]

[0002] Recently, efforts to secure high-strength steel sheet manufacturing technologies have been promoted to reduce the weight of automobiles. Among these, steel sheets that combine high strength and formability can increase productivity, are economically advantageous, and are also advantageous in terms of the safety of final parts. In particular, steel sheets with high tensile strength (TS) have a high load capacity before fracture, so there is a growing demand for steel materials with a tensile strength of 1180 MPa or higher. While many attempts have been made to improve the strength of existing steel materials, it has been discovered that simply increasing strength results in a decrease in ductility and hole expansion ratio (HER).

[0003] On the other hand, one prior art that overcomes the above drawbacks is TRIP (Transformation Induced Plasticity) steel sheet, which contains large amounts of Si and Al. However, while TRIP steel sheet can achieve an elongation of 14% or more at a TS of 1180 MPa, the large amounts of Si and Al added result in poor LME (Liquid Metal Embrittlement) resistance and poor weldability, limiting its practical use as an automotive structural material.

[0004] Furthermore, while various yield ratios are being pursued for the same tensile strength grade depending on the application and purpose, it is not easy to create steel sheets with high hole expandability when the yield ratio is low. This is because, while it is usually necessary to introduce martensite or ferrite phases as second phases to lower the yield ratio, these structural characteristics are factors that impair hole expandability.

[0005] Patent Document 1 discloses a high-strength cold-rolled steel sheet that combines yield ratio, strength, hole expandability, and delayed fracture resistance, and has a high elongation of 17.5% or more. However, Patent Document 1 has the drawback that the addition of a high amount of Si causes LME, resulting in poor weldability. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Korean Patent Publication No. 2017-7015003 Summary of the Invention [Problem to be solved by the invention]

[0007] The present invention is intended to solve the limitations of the conventional art described above, and has an object to provide a high-strength steel plate having high strength and a low yield ratio while also having an elongation suitable for processing, high hole expandability, and good weldability.

[0008] The object of the present invention is not limited to the above-mentioned content, and a person having ordinary skill in the art to which the present invention pertains will have no difficulty in understanding further object of the present invention from the overall matters of the specification of the present invention. [Means for solving the problem]

[0009] One aspect of the present invention is a steel sheet comprising, by weight, C: 0.12% or more and less than 0.17%, Si: 0.3 to 0.8%, Mn: 2.5 to 3.0%, Cr: 0.4 to 1.1%, Al: 0.01 to 0.3%, Nb: 0.01 to 0.03%, Ti: 0.01 to 0.03%, B: 0.001 to 0.003%, P: 0.04% or less, S: 0.01% or less, N: 0.01% or less, the balance being Fe and other impurities. The steel contains unavoidable impurities, and the contents of C, Si, and Al satisfy the following mathematical formula (1). The microstructure has, in area fraction, retained austenite of more than 1% and not more than 4%, fresh martensite of more than 10% and not more than 20%, ferrite of 5% or less (excluding 0%), tempered martensite of more than 50% and not more than 70%, and the balance is bainite. The number density of the retained austenite is 0.25 particles / μm 2 The high-strength steel plate has an average effective diameter of the retained austenite of 0.2 to 0.4 μm, and the proportion of the retained austenite having an effective diameter smaller than the average effective diameter exceeds 60%.

[0010] [Formula (1)] [C] + ([Si] + [Al]) / 5≦0.35wt.% (Here, [C], [Si], and [Al] represent the weight percentages of C, Si, and Al, respectively.)

[0011] A cementite phase can be precipitated as a second phase between the bainite laths or at the lath or grain boundaries of the tempered martensite phase and distributed at an area fraction of 1% to 3%.

[0012] The steel plate may further contain, by weight percent, one or more of Cu: 0.1% or less, Ni: 0.1% or less, Mo: 0.3% or less, and V: 0.03% or less.

[0013] The steel sheet can have a tensile strength of 1180 MPa or more, a yield strength of 740 MPa to 980 MPa, a yield ratio of 0.65 to 0.85, a hole expandability (HER) of 25% or more, and an elongation of 7 to 14%.

[0014] The steel sheet may be a cold-rolled steel sheet. A hot-dip galvanized layer may be formed on at least one surface of the steel sheet. The steel sheet may have a galvannealed layer formed on at least one surface thereof.

[0015] Another aspect of the present invention is a steel sheet comprising, by weight, C: 0.12% or more and less than 0.17%, Si: 0.3 to 0.8%, Mn: 2.5 to 3.0%, Cr: 0.4 to 1.1%, Al: 0.01 to 0.3%, Nb: 0.01 to 0.03%, Ti: 0.01 to 0.03%, B: 0.001 to 0.003%, P: 0.04% or less, S: 0.01% or less, N: 0.01% or less, and the balance Fe and other inevitable impurities, and preparing a slab in which the C, Si, and Al contents satisfy the following mathematical formula (1); reheating the slab to a temperature range of 1150 to 1250°C; finish hot rolling the reheated slab at a finish rolling temperature (FDT) range of 900 to 980°C; and cooling the slab at an average cooling rate of 10 to 100°C / sec after the finish hot rolling. a coiling step in a temperature range of 500 to 700°C; a cold rolling step of 30 to 60% cold reduction; a continuous annealing step of the cold-rolled steel sheet in a temperature range of (Ac3+20°C to Ac3+50°C) while controlling the furnace atmosphere by filling a gas containing 95% or more nitrogen and the remainder hydrogen; a primary cooling step of the continuously annealed steel sheet at an average cooling rate of 10°C / s or less to a primary cooling end temperature of 560 to 700°C, and a secondary cooling step of 10°C / s or more by cooling using a high-hydrogen gas with a maximum fraction of 65% to a secondary cooling end temperature of 280 to 350°C; and a reheating step of the cooled steel sheet to a temperature range of 380 to 480°C at a heating rate of 5°C / s or less.

[0016] [Formula (1)] [C] + ([Si] + [Al]) / 5≦0.35wt.% (Here, [C], [Si], and [Al] represent the weight percentages of C, Si, and Al, respectively.)

[0017] The slab may further contain, by weight percent, one or more of Cu: 0.1% or less, Ni: 0.1% or less, Mo: 0.3% or less, and V: 0.03% or less. After the reheating step, the method may further include a step of hot dip galvanizing at a temperature range of 480 to 540°C. After the hot dip galvanizing step, an alloying heat treatment may be performed, followed by cooling to room temperature. After cooling to room temperature, temper rolling of less than 1% can be carried out. [Effects of the Invention]

[0018] According to the present invention, it is possible to provide a high-strength steel sheet that has a high tensile strength of 1180 MPa or more, a yield strength of 740 MPa to 980 MPa, and a low yield ratio of 0.65 to 0.85, while also exhibiting a high hole expandability of 25% or more and an elongation of 7% to 14%.

[0019] Furthermore, a galvanized steel sheet manufactured using the high-strength steel sheet of the present invention has the effect of exhibiting excellent LME (Liquid Metal Embrittlement) resistance after galvanization and excellent weldability.

[0020] The various beneficial advantages and effects of the present invention are not limited to the above, but can be more easily understood in the course of describing specific embodiments of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0021] The terminology used herein is for the purpose of referring to particular embodiments only and is not intended to limit the invention. As used herein, the singular forms "a," "an," and "the" include the plural forms unless the term clearly dictates otherwise. The meaning of "comprising" as used in this specification embodies certain properties, regions, integers, steps, operations, elements and / or components, and does not exclude the presence or addition of other certain properties, regions, integers, steps, operations, elements, components and / or groups.

[0022] Unless otherwise defined, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by a person of ordinary skill in the art to which this invention pertains. Terms defined in commonly used dictionaries are additionally interpreted as having a meaning consistent with the relevant technical literature and the presently disclosed content, and are not interpreted as having an ideal or very formal meaning unless defined.

[0023] Hereinafter, a high-strength steel sheet according to one aspect of the present invention will be described in detail. It should be noted that in the present invention, the content of each element is expressed in terms of weight percent unless otherwise specified. The proportion of crystals or structures is based on area unless otherwise specified.

[0024] First, the chemical composition of the high strength steel sheet according to one aspect of the present invention will be described in detail. A high-strength steel plate according to one aspect of the present invention contains, by weight%, C: 0.12% or more and less than 0.17%, Si: 0.3 to 0.8%, Mn: 2.5 to 3.0%, Cr: 0.4 to 1.1%, Al: 0.01 to 0.3%, Nb: 0.01 to 0.03%, Ti: 0.01 to 0.03%, B: 0.001 to 0.003%, P: 0.04% or less, S: 0.01% or less, N: 0.01% or less, the balance being Fe and other unavoidable impurities, and the contents of C, Si, and Al can satisfy the following mathematical formula (1).

[0025] [Formula (1)] [C] + ([Si] + [Al]) / 5≦0.35wt.% (Here, [C], [Si], and [Al] represent the weight percentages of C, Si, and Al, respectively.)

[0026] Carbon (C): 0.12% or more and less than 0.17% Carbon (C) is a fundamental element that supports the strength of steel through solid solution strengthening and precipitation strengthening. If the C content is less than 0.12%, it is difficult to ensure a tempered martensite fraction of 50% or more, making it difficult to obtain a tensile strength (TS) equivalent to 1180 MPa. On the other hand, if the C content is 0.17% or more, it is difficult to obtain high LME resistance, and under severe spot weldability conditions, cracks may occur due to penetration of molten Zn during the welding process. Furthermore, a high carbon content deteriorates arc weldability and laser weldability, increasing the risk of weld cracking due to low-temperature embrittlement, and making it difficult to achieve the target hole expandability. Therefore, in the present invention, the C content is preferably limited to 0.12% or more and less than 0.17%. A preferred lower limit of the C content may be 0.122%, and a more preferred lower limit of the C content may be 0.125%. A preferred upper limit of the C content may be 0.168%, and a more preferred upper limit of the C content may be 0.165%.

[0027] Silicon (Si): 0.3-0.8% Silicon (Si) is a key element in TRIP (Transformation Induced Plasticity) steel, which inhibits cementite precipitation in the bainite region, thereby increasing the retained austenite fraction and elongation. If the Si content is less than 0.3%, almost no retained austenite remains, resulting in excessively low elongation. On the other hand, if the Si content exceeds 0.8%, 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, in the present invention, the Si content is preferably limited to 0.3 to 0.8%. The lower limit of the Si content may be preferably 0.35%, and more preferably 0.4%. The upper limit of the Si content may be preferably 0.78%, and more preferably 0.75%.

[0028] Manganese (Mn): 2.5-3.0% In the present invention, the manganese (Mn) content may be 2.5 to 3.0%. If the Mn content is less than 2.5%, it becomes difficult to ensure strength. On the other hand, if the Mn content exceeds 3.0%, the bainite transformation rate slows, and excessive fresh martensite is formed, making it difficult to obtain high hole expandability. Furthermore, if the Mn content is high, the martensite formation start temperature decreases, and the cooling end temperature required to obtain the initial martensite phase in the annealing water cooling stage becomes excessively low. Therefore, in the present invention, the Mn content is preferably limited to 2.5 to 3.0%. The lower limit of the Mn content may be preferably 2.55%, and more preferably 2.6%. The upper limit of the Mn content may be preferably 2.95%, and more preferably 2.9%.

[0029] Chromium (Cr): 0.4 to 1.1% In the present invention, the chromium (Cr) content may be 0.4 to 1.1%. If the Cr content is less than 0.4%, it becomes difficult to obtain the target tensile strength, and if the Cr content exceeds the upper limit of 1.1%, the bainite transformation rate becomes slow, making it difficult to obtain high hole expandability. Therefore, in the present invention, the Cr content is preferably limited to 0.4 to 1.1%. A preferred lower limit of the Cr content may be 0.5%, and a more preferred lower limit of the Cr content may be 0.6%. A preferred upper limit of the Cr content may be 1.05%, and a more preferred upper limit of the Cr content may be 1.0%.

[0030] Aluminum (Al): 0.01 to 0.3% In the present invention, the aluminum (Al) content may be 0.01 to 0.3%. If the Al content is less than 0.01%, the deoxidation of the steel material is insufficient, resulting in a loss of cleanliness. On the other hand, if Al is added in excess of 0.3%, the castability of the steel material is impaired. Therefore, in the present invention, the Al content is preferably limited to 0.01 to 0.3%. A preferred lower limit of the Al content may be 0.015%, and a more preferred lower limit of the Al content may be 0.02%. A preferred upper limit of the Al content may be 0.28%, and a more preferred upper limit of the Al content may be 0.25%.

[0031] Niobium (Nb): 0.01-0.03% In the present invention, 0.01 to 0.03% niobium (Nb) can be added to improve the strength and hole expandability of steel through grain refinement and precipitate formation. If the Nb content is less than 0.01%, the microstructure refinement effect disappears and the amount of precipitation strengthening becomes insufficient. On the other hand, if the Nb content exceeds 0.03%, the castability of the steel deteriorates. Therefore, in the present invention, the Nb content is preferably limited to 0.01 to 0.03%. The lower limit of the Nb content may be preferably 0.012%, and more preferably 0.014%. The upper limit of the Nb content may be preferably 0.025%, and more preferably 0.023%.

[0032] Titanium (Ti): 0.01-0.03%, Boron (B): 0.001-0.003% In the present invention, 0.01 to 0.03% titanium (Ti) and 0.001 to 0.003% boron (B) can be added to enhance the hardenability of steel. If the Ti content is less than 0.01%, B will bond with N, eliminating the hardenability-enhancing effect of B. If the Ti content exceeds 0.03%, the castability of the steel will deteriorate. On the other hand, if the B content is less than 0.001%, no effective hardenability-enhancing effect will be obtained, and if the B content exceeds 0.003%, boron carbides may be formed, which may actually impair the hardenability. Therefore, in the present invention, it is preferable to limit the Ti content to 0.01 to 0.03% and the B content to 0.001 to 0.003%.

[0033] Phosphorus (P): 0.04% or less Phosphorus (P) exists in steel as an impurity, and while it is advantageous to control its content as low as possible, it is also added intentionally to increase the strength of steel. However, if excessive P is added, the toughness of the steel deteriorates, so in the present invention, to prevent this, it is preferable to limit the upper limit to 0.04%.

[0034] Sulfur (S): 0.01% or less Sulfur (S), like P, exists in steel as an impurity, and it is advantageous to control its content as low as possible. Furthermore, because S deteriorates the ductility and impact properties of steel, it is preferable to limit its upper limit to 0.01%.

[0035] Nitrogen (N): 0.01% or less In the present invention, nitrogen (N) is added to the steel material as an impurity, and the upper limit is limited to 0.01% or less. In addition to the above-mentioned contents of C, Si, and Al, C, Si, and Al can satisfy the following mathematical formula (1).

[0036] [Formula (1)] [C] + ([Si] + [Al]) / 5≦0.35wt.% (Here, [C], [Si], and [Al] represent the weight percentages of C, Si, and Al, respectively.)

[0037] Liquid metal embrittlement (LME) in coated steel sheets occurs when the zinc applied during spot welding becomes liquid, creating tensile stress at the austenite grain boundaries of the steel sheet, causing the liquid zinc to penetrate the austenite grain boundaries. This LME phenomenon is particularly pronounced in steel sheets containing added Si and Al. Therefore, in the present invention, the amounts of Si and Al added are controlled using the above formula (1). Furthermore, a high C content lowers the A3 temperature of the steel, expanding the austenite region vulnerable to LME and weakening the toughness of the material. Therefore, the amount of C added is limited using the above formula (1).

[0038] If the value of the above formula (1) exceeds 0.35%, as mentioned above, LME resistance during spot welding is poor, and LME cracks are present after spot welding, impairing fatigue properties and structural safety. On the other hand, the smaller the value of the above formula (1), the better the spot weldability and LME resistance, so there is no need to separately set a lower limit. However, if the value is less than 0.20, although spot weldability and LME resistance are improved, it becomes difficult to obtain high tensile strength of 1180 MPa class together with excellent hole expandability. Therefore, in some cases, the lower limit can be limited to 0.20%.

[0039] In addition to the above-mentioned alloying elements, the high-strength steel sheet according to one aspect of the present invention may further include one or more of Cu: 0.1 wt % or less, Ni: 0.1 wt % or less, Mo: 0.3 wt % or less, and V: 0.03 wt % or less.

[0040] Copper (Cu): 0.1% or less, Nickel (Ni): 0.1% or less, Molybdenum (Mo): 0.3% or less Copper (Cu), nickel (Ni), and molybdenum (Mo) are elements that increase the strength of steel. In the present invention, they are included as optional elements, with the upper limits of their addition limited to 0.1%, 0.1%, and 0.3%, respectively. While these elements increase the strength and hardenability of steel, adding excessive amounts of these elements may exceed the target strength grade. Because these elements are expensive, it is preferable to limit their addition to 0.1% or 0.3% from an economical perspective. However, because Cu, Ni, and Mo act as solid-solution strengthening elements, adding less than 0.03% may result in minimal solid-solution strengthening. Therefore, if added, the lower limit can be limited to 0.03% or more.

[0041] Vanadium (V): 0.03% or less Vanadium (V) is an element that increases the yield strength of steel through precipitation hardening, and in the present invention, it can be selectively added to increase the yield strength. However, excessive V content can excessively reduce the elongation and induce brittleness in the steel, so in the present invention, the upper limit of V is limited to 0.03% or less. On the other hand, V induces precipitation hardening, so even a small amount is effective, but if added at less than 0.005%, the effect may be minimal. Therefore, if added, the lower limit can be limited to 0.005% or more.

[0042] In addition to the above-described steel composition, the present invention may contain the remainder Fe and unavoidable impurities. Since unavoidable impurities may be unintentionally mixed in during a normal steel manufacturing process, they cannot be completely eliminated, and this meaning is easily understood by engineers in the field of normal steel manufacturing. However, the present invention does not completely exclude the addition of other components than the above-described steel composition.

[0043] Meanwhile, a high-strength steel sheet according to one aspect of the present invention that satisfies the above-mentioned steel composition has a microstructure that includes, in area fractions, more than 1% and not more than 4% retained austenite, more than 10% and not more than 20% fresh martensite, not more than 5% (excluding 0%) ferrite, more than 50% and not more than 70% tempered martensite, and the remainder being bainite.

[0044] Furthermore, a cementite phase can be precipitated and distributed as a second phase between the bainite laths or at the lath or grain boundaries of the tempered martensite phase at an area fraction of 1% to 3%.

[0045] In a high-strength steel sheet according to one aspect of the present invention, the contents of Si and Al, which inhibit cementite growth and stabilize austenite, are limited to the condition of Equation 1, so that some cementite precipitates and grows within the microstructure. This cementite precipitates at martensite laths or grain boundaries when martensite formed during secondary cooling is reheated, or is formed in carbon-enriched areas between bainitic ferrite laths when bainite transformation occurs during reheating after secondary cooling.

[0046] In the high-strength steel sheet according to the present invention, by limiting the upper limits of Si and Al according to Equation (1), cementite precipitates at an area fraction of 1% or more. However, despite this, the presence of some Si and Al causes austenite to remain, and carbon is distributed within the retained austenite, resulting in a cementite precipitation amount of less than 3 area%. Furthermore, because some Si and Al are added, retained austenite exists at a level of more than 1 area% and less than 4 area%, but the high fraction of retained austenite distributed is not observed, as in typical TRIP steels with very high Si and Al contents.

[0047] In the present invention, fresh martensite is introduced at a level of more than 10% by area and not more than 20% by area to obtain a low yield ratio. If the austenite phase fraction is high after secondary cooling and reheating, the carbon content in the austenite is low, resulting in insufficient stability, and some of it will transform into fresh martensite during the subsequent cooling process, resulting in a low yield ratio.

[0048] In the present invention, the ferrite structure is poor in hole expandability, but can be present at a level of more than 0% and not more than 5% by area during the manufacturing process. Alternatively, the microstructure of the present invention may be composed of bainite.

[0049] The tempered martensite phase has a fine internal structure, making it an advantageous steel structure for ensuring the hole expandability of steel. If the tempered martensite fraction is less than 50 area %, it is difficult to achieve the target hole expandability. If the amount of tempered martensite is insufficient, the amount of phase transformation before the final cooling stage is insufficient, and ultimately, excessive fresh martensite is formed, impairing both the elongation and hole expandability of the steel. On the other hand, if the tempered martensite fraction exceeds 70 area %, the yield ratio and yield strength of the steel exceed the upper limit of the present invention, making it difficult to form the steel and potentially causing problems such as springback after forming.

[0050] The number density of the retained austenite in the microstructure is 0.25 particles / μm 2 The average effective diameter of the retained austenite may be 0.2 to 0.4 μm, and the proportion of the retained austenite having an effective diameter smaller than the average effective diameter may be more than 60%. 。

[0051] If the number and size distribution of retained austenite grains per unit area do not meet the above conditions, Zn penetration through austenite grain boundaries during welding is promoted, making LME cracking more likely to occur. The greater the number of retained austenite grains and the larger the size of each individual retained austenite grain, the worse the LME resistance. Here, number density can be defined as the number of individually separated retained austenite grains present within a unit area, and effective diameter can be defined as the diameter of a circle with the same area as the cross-sectional area of ​​a retained austenite grain. The average in the average effective diameter refers to the usual arithmetic mean. Furthermore, if the size and fraction of retained austenite are large in steel with the same carbon content, the stability of the retained austenite decreases and it easily transforms into martensite even under small stress, resulting in a low HER value and poor stretch flangeability.

[0052] By having the above-described chemical composition and microstructure, the high-strength steel sheet of the present invention can exhibit a tensile strength of 1180 MPa or more, a yield strength of 740 MPa to 980 MPa, and a high hole expandability of 25% or more even with a low yield ratio of 0.65 to 0.85.

[0053] As described above, the reason why the yield ratio of the high-strength steel sheet according to the present invention is low is due to the introduction of fresh martensite. However, the present inventors have confirmed that, under the alloy composition and structural control conditions according to the present invention, a hole expandability of 25% or more can be obtained even in the presence of fresh martensite.

[0054] Furthermore, the high strength steel sheet according to the present invention has a weak TRIP effect due to the limited Si and Al contents, and exhibits an elongation of 7% or more and 14% or less.

[0055] The high strength steel sheet according to the present invention may be a cold rolled steel sheet. The high-strength steel sheet according to the present invention may have a hot-dip galvanized layer formed on at least one surface thereof by a hot-dip galvanizing method. The present invention does not particularly limit the configuration of the hot-dip galvanized layer, and any hot-dip galvanized layer that is commonly used in the technical field can be preferably applied to the present invention.

[0056] The hot-dip galvanized layer may be an alloyed hot-dip galvanized layer alloyed with a portion of the alloying components of the steel sheet.

[0057] Next, a method for producing a high strength steel plate according to another aspect of the present invention will be described in detail. A high-strength steel sheet according to one aspect of the present invention can be manufactured by undergoing the steps of preparing a steel slab that satisfies the above-described steel composition and formula (1), reheating the slab, hot rolling, coiling, cold rolling, continuous annealing, primary and secondary cooling, and reheating, as detailed below.

[0058] First, a slab having the above-described alloy composition and satisfying formula (1) is prepared, and the slab is reheated to a temperature of 1150°C to 1250°C. If the slab temperature is below 1150°C, it may be impossible to perform the next step, hot rolling. On the other hand, if the slab temperature exceeds 1250°C, a lot of energy is unnecessarily consumed to increase the slab temperature. Therefore, it is preferable to limit the heating temperature to a temperature of 1150°C to 1250°C.

[0059] The reheated slab is hot-rolled to a thickness suitable for the intended purpose at a finish rolling temperature (FDT) of 900 to 980°C. If the finish rolling temperature (FDT) is less than 900°C, the rolling load is large, resulting in an increase in shape defects and reduced productivity. On the other hand, if the finish rolling temperature exceeds 980°C, the surface quality deteriorates due to an increase in oxides caused by excessively high-temperature operation. Therefore, it is preferable to perform hot rolling at a finish rolling temperature of 900 to 980°C.

[0060] After hot rolling, the steel sheet is cooled to the coiling temperature at an average cooling rate of 10 to 100° C. / sec, and coiled in a temperature range of 500 to 700° C. After coiling, the steel sheet is cold rolled at a cold reduction rate of 30 to 60% to obtain a cold-rolled steel sheet.

[0061] If the cold rolling reduction is less than 30%, it is difficult to ensure the target thickness accuracy and also difficult to correct the shape of the steel sheet. On the other hand, if the cold rolling reduction exceeds 60%, there is a high possibility that cracks will occur at the edge of the steel sheet, and the cold rolling load will become excessively large. Therefore, it is preferable to limit the cold rolling reduction to 30 to 60%.

[0062] Cold-rolled steel sheet is subjected to continuous annealing in a temperature range of (Ac3 + 20°C to Ac3 + 50°C) (hereinafter referred to as "SS" or "continuous annealing temperature"), filled with a gas containing 95% or more nitrogen and the remainder hydrogen, while controlling the furnace atmosphere. The continuous annealing stage heats the steel to the austenite single-phase region to form nearly 100% austenite, and then the steel is used for phase transformation. If the continuous annealing temperature is less than Ac3 + 20°C, sufficient austenite transformation does not occur, and the desired martensite and bainite fractions cannot be achieved after annealing. On the other hand, if the continuous annealing temperature exceeds Ac3 + 50°C, productivity decreases, coarse austenite is formed, which can deteriorate the material quality, and the size of retained austenite in the final structure increases.

[0063] In cases where it is difficult to determine the Ac3 temperature of a steel sheet during actual production, continuous annealing can be carried out in a temperature range of 810 to 850° C. Furthermore, the continuous annealing can be carried out in a continuous alloying hot-dip galvanizing continuous furnace.

[0064] The continuously annealed steel sheet is first cooled to a first cooling finish temperature (SCS) of 560-700°C at an average cooling rate of 10°C / s or less, and then secondarily cooled to a second cooling finish temperature (RCS) of 280-350°C at an average cooling rate of 10°C / s or more to introduce martensite into the steel sheet microstructure. The first cooling finish temperature can be defined as the point at which quenching equipment not used in the first cooling is added and quenching begins. Dividing the cooling process into primary and secondary cooling stages allows for uniform temperature distribution in the steel sheet during the slow cooling stage, reducing the final temperature and material deviations and also advantageously achieving the desired phase composition.

[0065] The primary cooling is performed slowly at an average cooling rate of 10°C / s or less, and the cooling end temperature may be in the range of 560 to 700°C. If the primary cooling end temperature is lower than 560°C, excessive ferrite phase precipitates, resulting in poor final hole expandability. On the other hand, if the temperature exceeds 700°C, excessive load is placed on the secondary cooling, and the sheet threading speed on the continuous annealing line must be slowed, which may reduce productivity.

[0066] The secondary cooling can be performed using a quenching facility that was not used in the primary cooling. As a preferred example, a hydrogen quenching facility using H gas can be used. More specifically, cooling can be performed using a high hydrogen gas with a maximum fraction of 65%, but is not limited thereto.

[0067] At this time, it is important to control the cooling end temperature of the secondary cooling to 280 to 350°C, which allows an appropriate initial martensite fraction to be obtained. However, if the cooling end temperature is lower than 280°C, the initial martensite fraction transformed during the secondary cooling becomes excessively high, leaving no room for the various phase transformations required in subsequent processes, resulting in poor shape and workability of the steel sheet. On the other hand, if the secondary cooling end temperature exceeds 350°C, the initial martensite fraction will be low, making it difficult to obtain high hole expandability, and the average size of the retained austenite will also increase.

[0068] The cooled steel sheet is reheated again to a temperature range of 380 to 480°C (hereinafter also referred to as the "annealing reheating temperature" or "RHS") at a heating rate of 5°C / s or less to temper the martensite obtained in the previous step, induce bainite transformation, and enrich carbon in the untransformed austenite adjacent to the bainite.

[0069] At this time, it is important to control the reheating temperature to 380 to 480°C. If the reheating temperature is lower than 380°C or higher than 480°C, the amount of bainite phase transformation is small, and excessively large amounts of fresh martensite are formed during the final cooling process, resulting in significant impairment of elongation and hole expandability.

[0070] If necessary, the reheated steel sheet can be subjected to a hot-dip galvanizing treatment in a temperature range of 480 to 540°C to form a hot-dip galvanized layer on at least one surface of the steel sheet. Furthermore, if necessary, after the hot dip galvanizing treatment, in order to obtain an alloyed hot dip galvanized layer, an alloying heat treatment can be carried out and then cooled to room temperature.

[0071] Furthermore, the method may further include a step of subsequently correcting the shape of the steel sheet, cooling it to room temperature, and then performing temper rolling of less than 1% in order to adjust the yield strength. [Example]

[0072] The present invention will be described in more detail below through examples. However, it should be noted that the following examples are only intended to illustrate and embody the present invention and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred therefrom.

[0073] (Example) First, five types of steel sheets, A to E, satisfying the chemical composition shown in Table 1 below were prepared. Furthermore, for each example, the steel sheet thickness, FDT (finish rolling temperature), CT (hot rolling coiling temperature) process conditions, and continuous alloying hot dip annealing conditions SS (continuous annealing temperature), SCS (first cooling end temperature), RCS (second cooling end temperature), and RHS (annealing reheating temperature) were used to measure the material properties and phase fractions. The cooling rate, cold reduction, and heating rate after finish rolling, which are not separately shown in Table 2 below, were all controlled within ranges satisfying the conditions of the present invention. The Ac3 temperature for each example was calculated using Termocalc, a commonly used thermodynamics software.

[0074] The materials and phase fraction measuring methods used in this example are as follows. In this example, the tensile strength (TS), yield strength (YS), and elongation (EL) were measured by a tensile test in the direction perpendicular to the rolling direction, and the gauge length was 50 mm and the width of the tensile test specimen was 25 mm.

[0075] The hole expandability was measured according to the ISO 16330 standard, and the holes were sheared using a 10 mm diameter punch with a 12% clearance.

[0076] The phase fractions of each example were measured using the point counting method from scanning electron microscope (SEM) photographs, and the fraction of retained austenite was measured using XRD. The number density and effective diameter of retained austenite were also obtained using EBSD analysis with the scanning electron microscope. The remainder of the phases other than those listed in Table 3 below was bainite.

[0077] [Table 1]

[0078] [Table 2]

[0079] [Table 3]

[0080] First, Comparative Examples 1 and 2 are cases where steel types A and B were used, respectively. Steel types A and B had carbon (C) or manganese (Mn) contents lower than the range of the present invention, and did not achieve a tensile strength (TS) of 1180 MPa.

[0081] In Comparative Examples 3 and 4, the tempered martensite fraction did not exceed 50 area % and the fresh martensite fraction exceeded 20 area %, but the hole expandability (HER) value was low and the yield ratio was also less than 0.65. In Comparative Examples 3 and 4, the continuous annealing temperature and RCS temperature were high, the average size of the retained austenite was large, and the number was even larger, so the proportion of the effective grain size finer than the average size did not reach 60%.

[0082] In the case of Comparative Example 5, although the carbon (C) content of steel type E exceeded the range of the components of the present invention and other conditions were satisfied, the carbon (C) and silicon (Si) contents were high, and both the number density and size of the retained austenite were high, resulting in a low hole expandability (HER) value of less than 25%, and the LME resistance was also low.

[0083] In contrast to the above comparative examples, inventive examples 1 to 3, steel types C and D satisfying the alloy composition of the present invention were applied, and when all process conditions were met, hole expandability of 25% or more and elongation of 7% to 14%, appropriate for processing, were obtained with a low yield ratio of 0.65 to 0.85.

[0084] Although the present invention has been described with reference to the preferred embodiments, it should be understood that those skilled in the art can make various modifications and changes to the present invention without departing from the spirit and scope of the present invention as set forth in the claims below.

Claims

1. The steel sheet contains, by weight, C: 0.12% or more and less than 0.17%, Si: 0.3 to 0.8%, Mn: 2.5 to 3.0%, Cr: 0.4 to 1.1%, Al: 0.01 to 0.3%, Nb: 0.01 to 0.03%, Ti: 0.01 to 0.03%, B: 0.001 to 0.003%, P: 0.04% or less, S: 0.01% or less, N: 0.01% or less, the balance being Fe and other unavoidable impurities; The contents of C, Si, and Al satisfy the following formula (1): The microstructure is composed of retained austenite in an area fraction of 1% to 4% and fresh martensite in an area fraction of 1% to 4%. Site 10% to 20% or less, Ferrite 5% or less (excluding 0%), Tempered Martensitic more than 50% and not more than 70% ferrite, the remainder including bainite, The number density of the retained austenite is 0.25 particles / μm ２ is as follows: The high-strength steel plate has an average effective diameter of the retained austenite of 0.2 to 0.4 μm, and the proportion of the retained austenite having an effective diameter smaller than the average effective diameter exceeds 60%. [Formula (1)] [C]+([Si]+[Al]) / 5≦0.35wt. % (Here, [C], [Si], and [Al] represent the weight percentages of C, Si, and Al, respectively. ）

2. Between bainite laths or tempered martensite laths or Cementite phase is precipitated as a second phase at the grain boundaries with an area fraction of 1% to 3%. The high strength steel plate according to claim 1 .

3. In weight percent, Cu: 0.1% or less, Ni: 0.1% or less, Mo: 0.3% or less, and V: The high strength steel plate according to claim 1, further comprising one or more of the following: 0.03% or less.

4. Tensile strength of 1180 MPa or more, yield strength of 740 to 980 MPa, 0.65 to 0.8 5, a hole expandability (HER) of 25% or more, and an elongation of 7 to 14%. The high-strength steel plate according to claim 1.

5. The high strength steel sheet according to claim 1 , wherein the steel sheet is a cold rolled steel sheet.

6. 2. The steel sheet according to claim 1, wherein a hot-dip galvanized layer is formed on at least one surface of the steel sheet. High strength steel plate.

7. 2. The steel sheet according to claim 1, wherein a galvannealed layer is formed on at least one surface of the steel sheet. High strength steel plate as described.

8. A method for producing the high strength steel plate of claim 1, preparing a slab containing, by weight %, C: 0.12% or more and less than 0.17%, Si: 0.3 to 0.8%, Mn: 2.5 to 3.0%, Cr: 0.4 to 1.1%, Al: 0.01 to 0.3%, Nb: 0.01 to 0.03%, Ti: 0.01 to 0.03%, B: 0.001 to 0.003%, P: 0.04% or less, S: 0.01% or less, N: 0.01% or less, the balance being Fe and other inevitable impurities, wherein the contents of C, Si, and Al satisfy the following mathematical formula (1): reheating the slab to a temperature range of 1150 to 1250°C; The reheated slab is finished at a finish rolling temperature (FDT) range of 900 to 980°C. hot rolling; After the finish hot rolling, cooling at an average cooling rate of 10 to 100°C / sec; a coiling step at a temperature in the range of 500 to 700°C; cold rolling at a cold reduction of 30 to 60%; The cold-rolled steel sheet was heated to a temperature range of (Ac3+20°C to Ac3+50°C) to reduce the nitrogen concentration to 9%. The furnace is filled with a gas containing 5% or more of carbon and the remainder hydrogen, and continuous annealing is performed while controlling the atmosphere inside the furnace. the step of: The continuously annealed steel sheet is cooled to a primary cooling end temperature of 560 to 700°C at a rate of 10°C / s or less. The primary cooling is performed at an average cooling rate, and the maximum fraction is 65% up to the secondary cooling end temperature of 280 to 350°C. By cooling using high hydrogen gas up to 10000kJ / s, the average cooling rate is 10°C / s or more. Cooling; and The cooled steel plate is reheated to a temperature range of 380 to 480°C at a heating rate of 5°C / s or less. a heating step; A manufacturing method for high-strength steel plates. [Formula (1)] [C]+([Si]+[Al]) / 5≦0.35wt. % (Here, [C], [Si], and [Al] represent the weight percentages of C, Si, and Al, respectively. ）

9. The slab contains, by weight, Cu: 0.1% or less, Ni: 0.1% or less, Mo: 0.3% The high strength steel according to claim 8, further comprising one or more of: S: 0.03% or less; and V: 0.03% or less. How the board is manufactured.

10. After the reheating step, hot dip galvanizing is performed at a temperature range of 480 to 540°C. The method for producing a high strength steel plate according to claim 8, further comprising:

11. After the hot dip galvanizing step, alloying heat treatment is performed and then cooling to room temperature is performed. The method for producing a high strength steel plate according to claim 10, further comprising the steps of:

12. The high strength steel sheet according to claim 10, wherein the steel sheet is subjected to temper rolling of less than 1% after being cooled to room temperature. Manufacturing method.

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