Cold rolled steel sheet and method of manufacturing same

Abstract

The purpose of one aspect of the present invention is to provide a cold rolled steel sheet and a method of manufacturing same. The purpose of a preferred aspect of the present invention is to provide a cold rolled steel sheet having excellent low-temperature tempering hardness and room-temperature aging resistance, and a method of manufacturing same.

Description

Cold rolled steel sheet and its manufacturing method

[0001] The present invention relates to a cold rolled steel sheet and a method for manufacturing the same.

[0002] Recently, there has been a continuous demand for increased strength and reduced thickness of steel sheets to achieve weight reduction and improve automotive fuel efficiency. Bake-hardening steels are known to be the most suitable steel for exterior plating applications. Bake-hardening refers to the phenomenon in which dislocations created during pressing are fixed to activated carbon and nitrogen during paint firing, resulting in an increase in yield strength. Steels with excellent bake-hardening properties facilitate forming before paint firing and offer enhanced dent resistance in the final product, ensuring both superior formability and strength.

[0003] However, since there is a risk that aging deterioration, such as yield point elongation, may occur when the steel is kept at room temperature for a long period of time due to the dissolved elements in the steel, it is required to have room temperature aging resistance to ensure aging for a certain period of time or longer.

[0004] In general, cold-rolled steel sheets with bake hardening properties are known to have a bake hardening amount of approximately 40 to 50 MPa by applying low-temperature coiling at 400 to 500°C and subsequent annealing to low-carbon P-added Al-killed steel. This is because it is easier to achieve both formability and bake hardening properties through subsequent annealing. In the case of P-added Al-killed steel produced by continuous annealing, bake hardening properties are easily secured because a relatively fast cooling rate is used. However, there is a problem that formability deteriorates due to rapid heating and short-time annealing, and thus its use is limited to automobile exterior panels that do not require high workability.

[0005] Recent rapid advances in steelmaking technology have enabled the control of optimal dissolved element content in steel. Furthermore, the availability of Al-killed steel sheets containing strong carbonitride-forming elements such as titanium and niobium has made it possible to produce bake-hardenable cold-rolled steel sheets with superior formability. The use of these cold-rolled steel sheets for automotive exterior panels requiring dent resistance continues to grow.

[0006] Generally, automotive sub-treatment is performed at 170℃, and steel sheets that can secure a sub-temperature hardening value (BH value) suitable for this temperature have been manufactured so far.

[0007] However, some automakers are recently considering lowering the baking temperature after painting as a way to reduce costs and CO2 emissions. In particular, there is a growing trend of automakers using non-ferrous lightweight materials such as aluminum or plastic (or CFRP) instead of steel for exterior panels to reduce vehicle weight. Until now, steel and non-ferrous materials have been separately press-processed, painted and baked, and then parts made of the two materials are assembled in the final stage. However, there is a growing trend of press-processing steel and non-ferrous materials, assembling them, and baking them at the same temperature for the purposes of improving production process efficiency, reducing energy costs, and protecting the environment. This low-temperature baking causes a sharp drop in the baking hardening value (BH value) obtained at the existing 170℃, which raises the problem that the proper dent resistance, which is the purpose of using bake-hardened steel, cannot be secured.

[0008]

[0009] Typically, lowering the bake hardening temperature of steel sheets results in a decrease in the amount of carbon and nitrogen incorporated into the steel sheet and a delay in the time required for fixation, which in turn reduces the bake hardening property. For bake hardening, a pre-strain of several percent is typically applied followed by a heat treatment at 170°C for 20 minutes after painting, and the required hardening amount is at least 30 MPa. Therefore, in order to secure an appropriate level of bake hardening while lowering the bake hardening temperature, bake hardening must be maximized at high temperatures. However, if the bake hardening property of the steel sheet increases beyond a certain level, the aging resistance of the steel sheet deteriorates, which increases the possibility of surface defects during processing of parts. Therefore, the most desirable approach is to minimize the difference between the amount of bake hardening obtained in the conventional bake treatment at 170°C and the BH value obtained in the low-temperature, i.e., 140°C, bake heat treatment. However, considering that the dissolved elements in the steel are exponentially dependent on temperature, it is very difficult to reduce the difference in BH values ​​at low temperatures of 140°C and high temperatures of 170°C.

[0010] In other words, in order to secure appropriate curability at low temperatures, a manufacturing technology is required that can simultaneously secure superior curability and corresponding aging resistance under normal conditions of 170°C.

[0011] [Prior Art Literature]

[0012] (Patent Document 1) Japanese Patent Laid-Open Publication No. 7-75803

[0013] (Patent Document 2) Japanese Patent Publication No. 2001-140038

[0014] One aspect of the present invention is to provide a cold rolled steel sheet and a method for manufacturing the same.

[0015] A preferred aspect of the present invention is to provide a cold-rolled steel sheet having excellent low-temperature hardening properties and room-temperature aging resistance, and a method for manufacturing the same.

[0016] One embodiment of the present invention provides a cold-rolled steel sheet comprising, in wt%, C: 0.010 to 0.020%, Si: 0.0050 to 0.10%, Mn: 0.50 to 1.450%, Cr: 0.30 to 1.50%, P: 0.030% or less (excluding 0%), S: 0.010% or less (excluding 0%), N: 0.0020 to 0.010%, Al: 0.010 to 0.040%, the remainder being Fe and other inevitable impurities, the microstructure including columnar ferrite and the remainder being a transformation structure, the transformation structure including at least one or more of martensite, bainite, and ferritic bainite, the transformation structure being 1.0 area% or less (excluding 0%), and having an average grain size of 1000 nm or less.

[0017] The above cold rolled steel sheet may have a tensile strength of 350 MPa or more, a yield strength of 200 to 300 MPa, an elongation of 30% or more, a baking hardening amount after heat treatment at 140°C for 20 minutes of 30 MPa or more, a difference between the baking hardening amount after heat treatment at 140°C for 20 minutes and the baking hardening amount after heat treatment at 170°C for 20 minutes of 20 MPa or less, and a yield point elongation (AI) after heat treatment at 100°C for 1 hour of 0.2% or less.

[0018] The above cold-rolled steel sheet may have a hot-dip zinc-based plating layer or an alloyed hot-dip zinc-based plating layer formed on at least one surface.

[0019] Another embodiment of the present invention comprises the steps of: reheating a slab comprising, in wt%, C: 0.010 to 0.020%, Si: 0.0050 to 0.10%, Mn: 0.50 to 1.450%, Cr: 0.30 to 1.50%, P: 0.030% or less (excluding 0%), S: 0.010% or less (excluding 0%), N: 0.0020 to 0.010%, Al: 0.010 to 0.040%, with the remainder being Fe and other unavoidable impurities; finishing hot-rolling the reheated slab to obtain a hot-rolled steel sheet; cooling the hot-rolled steel sheet to 500 to 700°C and then coiling it; pickling the coiled hot-rolled steel sheet and then cold-rolling it to obtain a cold-rolled steel sheet; A method for manufacturing a cold-rolled steel sheet is provided, comprising: a step of first heating the cold-rolled steel sheet at 760 to 830°C; a step of first cooling the first-heated cold-rolled steel sheet to a cooling stop temperature of 200 to 400°C at an average cooling rate of 20°C / s or more; and a step of second heating the first-cooled cold-rolled steel sheet to 400 to 600°C at a heating rate of 5°C / s or less.

[0020] The above slab reheating can be performed at 1100 to 1250°C.

[0021] The above finishing hot rolling can be performed at 880°C or higher.

[0022] The above cold rolling can be performed at a reduction ratio of 60 to 90%.

[0023] The above primary heating can be performed for 30 seconds or more.

[0024] The method may further include a step of subjecting the second-heated cold-rolled steel sheet to temper rolling at a reduction ratio of 0.5 to 2% using a skin pass roll having a surface roughness (Ra) of 1.0 to 2.2 ㎛.

[0025] After the second heating, a step of immersing the second-heated cold-rolled steel sheet in a zinc plating bath at 440 to 500°C may be additionally included.

[0026] After immersion in the zinc plating bath, a step of temper rolling the cold rolled steel sheet at a reduction ratio of 0.5 to 2% using a skin pass roll having a roughness (Ra) of 1.0 to 2.2 ㎛ may be additionally included.

[0027] Afterwards, a step of alloying the cold rolled steel sheet at 450 to 540°C may be additionally included.

[0028] According to one aspect of the present invention, a cold rolled steel sheet and a method for manufacturing the same can be provided.

[0029] According to a preferred aspect of the present invention, a cold-rolled steel sheet having excellent low-temperature hardening properties and room-temperature aging resistance and a method for manufacturing the same can be provided.

[0030] Hereinafter, a cold-rolled steel sheet according to one embodiment of the present invention will be described. First, the alloy composition will be described. Unless otherwise specified, the contents of the alloy composition described below are in weight percent.

[0031] C: 0.010~0.020%

[0032] Carbon (C) effectively contributes to securing the strength of steel as an interstitial solid solution element. In addition, since it is an important element for securing the fraction of martensite by increasing the hardenability of steel, a certain level of C or more must be added in order to secure a certain amount of transformation structure aimed at by the present invention. If the C content is less than 0.010%, it is difficult to sufficiently obtain the above-mentioned effect. If the C content exceeds 0.020%, the transformation structure is excessively formed, resulting in an increase in strength and a decrease in elongation, which increases the possibility of occurrence of bending defects on the product surface during part processing by the customer. Therefore, the C content is preferably in the range of 0.010 to 0.020%. The lower limit of the C content is more advantageously 0.0110%, more advantageously 0.0120%, and most advantageously 0.0130%. The upper limit of the above C content is more advantageous when it is 0.0190%, more advantageous when it is 0.0180%, and most advantageous when it is 0.0170%.

[0033] Si: 0.0050~0.10%

[0034] Silicon (Si) is an element that generally contributes to increasing the strength of steel through solid solution strengthening. When the Si content is less than 0.0050%, it is difficult to sufficiently obtain the above-described effect. However, in the present invention, the lower limit of the Si content can be satisfied even if Si is not intentionally added. When the Si content exceeds 0.10%, there is a problem of deteriorating the plating surface properties. Therefore, the Si content is preferably in the range of 0.0050 to 0.10%. The lower limit of the Si content is more advantageously 0.00550%, more advantageously 0.0070%, and most advantageously 0.010%. The upper limit of the Si content is more advantageously 0.090%, more advantageously 0.070%, and most advantageously 0.050%.

[0035] Mn: 0.50~1.450%

[0036] Manganese (Mn) not only contributes to increasing the strength of steel as a solid solution strengthening element, but also can play a role in precipitating S in the steel as MnS. In the present invention, the Mn, together with C and Cr, can increase the hardenability of the steel, thereby contributing to securing the fraction of the transformation structure desired in the steel of the present invention. When the content of the Mn is less than 0.50%, it may be difficult to secure an appropriate level of the transformation structure fraction and thereby secure low-temperature bake hardenability and room-temperature aging resistance. When the content of the Mn exceeds 1.450%, the transformation structure may be excessively formed, making it difficult to secure low-temperature bake hardenability. In addition, excessive addition of manganese (Mn) may cause the formation of annealing oxides, which may cause problems on the surface of the plated product, and may cause problems such as a decrease in elongation, which may result in poor workability. Therefore, the content of the Mn is preferably in the range of 0.50 to 1.450%. The lower limit of the above Mn content is more advantageously 0.60%, more advantageously 0.70%, and most advantageously 0.80%. The upper limit of the above Mn content is more advantageously 1.40%, more advantageously 1.350%, and most advantageously 1.30%.

[0037] Cr: 0.30~1.50%

[0038] Chromium (Cr) is a solid solution strengthening element and is one of the very important elements in the present invention along with the previously mentioned C, Mn, and N. The Cr increases the hardenability of steel and effectively contributes to the formation of martensite. In addition, the Cr is added during hot rolling. 23By forming coarse Cr-based carbides such as C6, the amount of dissolved C in the steel is controlled below an appropriate level, thereby suppressing the occurrence of yield point elongation (YPel), thereby enabling the production of composite phase steel with a low yield ratio. In addition, Cr is also an element that effectively contributes to securing the elongation of composite phase steel by minimizing the decrease in elongation compared to the increase in strength. If the content of Cr is less than 0.30%, the hardenability of the steel is lowered, making it difficult to secure the fraction of the transformation structure targeted in the present invention. This results in a decrease in the BH140 value, and also makes it impossible to satisfy the condition that the BH170-BH140 value (the difference between the amount of bake hardening after heat treatment at 170°C for 20 minutes and the amount of bake hardening after heat treatment at 140°C for 20 minutes) is 20 MPa or less. When the content of Cr exceeds 1.50%, martensite may be excessively formed, thereby exceeding the transformation structure fraction suggested in the present invention, which may lower the BH140 value and lower the elongation. Therefore, the content of Cr is preferably in the range of 0.30 to 1.50%. The lower limit of the Cr content is more advantageously 0.40%, more advantageously 0.50%, and most advantageously 0.60%. The upper limit of the Cr content is more advantageously 1.40%, more advantageously 1.30%, and most advantageously 1.20%.

[0039] P: 0.030% or less (excluding 0%)

[0040] Phosphorus (P) is an element that is inevitably included as an impurity in steel. However, it is also an effective element for securing the strength of steel through solid solution strengthening without significantly impairing the drawability even in very small amounts. On the other hand, if the P content exceeds 0.030%, the possibility of brittle fracture increases, which may not only cause slab breakage during hot rolling, but also significantly deteriorate the surface properties of the coated steel sheet. Therefore, the P content is preferably in the range of 0.030% or less (excluding 0%). The P content is more advantageously 0.020% or less, even more advantageously 0.010% or less, and most advantageously 0.0050% or less. Meanwhile, the present invention does not specifically limit the lower limit of the P content, but as an example, it may be 0.0010%.

[0041] S: 0.010% or less (excluding 0%)

[0042] Sulfur (S) is an impurity that is inevitably included in steel, and in order to secure excellent welding properties, it is desirable to manage its content as low as possible. In particular, since the S can cause red-hot embrittlement, it is desirable that the S content be in the range of 0.010% or less (excluding 0%). The S content is more advantageously 0.0080% or less, even more advantageously 0.0060% or less, and most advantageously 0.0050% or less. Meanwhile, the present invention does not specifically limit the lower limit of the S content, but as an example, it may be 0.0010%.

[0043] N: 0.0020~0.010%

[0044] Nitrogen (N) is an element that is inevitably included as an impurity in steel. Therefore, it is generally desirable to control its content as low as possible. However, in the present invention, it is a very important element that is effective in securing low-temperature bake hardenability and room-temperature aging resistance even in very small amounts. However, N has a very fast diffusion rate and can cause bake hardenability and aging degradation at the same time. Therefore, in order to suppress aging degradation due to N, a certain amount of transformed structure fraction must be secured by an appropriate combination of C, Mn, and Cr, and in order to obtain low-temperature bake hardenability at 100°C in steel containing such transformed structure, a certain amount of N or more is required. If the N content is less than 0.0020%, it may be difficult to sufficiently obtain the above-mentioned effect. If the N content exceeds 0.010%, it becomes difficult to simultaneously secure low-temperature bake hardenability and room-temperature aging resistance. Therefore, the N content is preferably in the range of 0.0020 to 0.010%. The lower limit of the above N content is more advantageously 0.0030%, more advantageously 0.0040%, and most advantageously 0.0050%. The upper limit of the above N content is more advantageously 0.0090%, more advantageously 0.0080%, and most advantageously 0.0070%.

[0045] Al: 0.010~0.040%

[0046] Aluminum (Al) is a component added to refine grain size and deoxidize steel. In the present invention, in order to manufacture Al-killed steel in a stable state, the lower limit of the Al content may be limited to 0.010%. When the Al content exceeds 0.040%, while the strength increases due to grain refinement, excessive inclusions may be formed during steelmaking / continuous casting operations, which may not only deteriorate the surface quality of the steel sheet but also lead to an increase in manufacturing costs. Therefore, the Al content is preferably in the range of 0.010 to 0.040%. The lower limit of the Al content is more advantageously 0.0150%, and 0.020% is even more advantageous. The upper limit of the Al content is more advantageously 0.0350%, and 0.030% is even more advantageous.

[0047] In addition to the composition described above, the cold-rolled steel sheet of the present invention may contain remaining iron (Fe) and unavoidable impurities. Unavoidable impurities can be unintentionally introduced during the typical manufacturing process, and thus cannot be excluded. Since these impurities are readily apparent to anyone skilled in the art of steel manufacturing, their full content is not specifically discussed in this specification.

[0048] The microstructure of the cold-rolled steel sheet of the present invention preferably includes columnar ferrite and the remainder transformation structure. At this time, the transformation structure may include at least one of martensite, bainite, and ferritic bainite, and the transformation structure is preferably 1.0 area% or less (excluding 0%) and has an average grain size of 1000 nm or less. The present invention is characterized in that the transformation structure among the microstructure is utilized to secure an appropriate level of low-temperature bake hardenability. The aforementioned C, Mn, and Cr are advantageous in forming the transformation structure, and since a large number of mobile dislocations exist around the transformation structure, the movement of interstitial elements such as C and N can be hindered. That is, by appropriately utilizing N and C and the transformation structure, aging does not occur at room temperature, and bake hardenability (BH property) can be obtained even at a low bake temperature. If the transformation structure does not exist, sufficient mobile dislocations cannot be obtained, and the BH140 value cannot satisfy the level proposed by the present invention. In addition, the added C may affect not only the bake hardening (BH) property but also the aging property, thereby increasing the BH170 value, which may cause not only an increase in the BH170-BH140 value but also a deterioration in the room temperature aging property. When the fraction of the above-mentioned transformed structure exceeds 1.0 area%, the aging property becomes excellent due to the excessive transformed structure, but a problem of a decrease in the BH140 value may occur. Therefore, the fraction of the above-mentioned transformed structure is preferably in the range of 1.0 area% or less (excluding 0%). The lower limit of the above-mentioned transformed structure fraction is more advantageously 0.010%, more advantageously 0.0150%, and most advantageously 0.020%. The upper limit of the above-mentioned transformed structure fraction is more advantageously 0.090%, more advantageously 0.080%, and most advantageously 0.070%. Meanwhile, if the average crystal grain size of the above-mentioned transformation structure exceeds 1000 nm, there may be a disadvantage in that the strength increases but the formability decreases.Meanwhile, in the present invention, there is no particular limitation on the lower limit of the average crystal grain size of the above-mentioned transformation structure, but as an example, it may be 100 nm.

[0049] As described above, the cold-rolled steel sheet of the present invention can secure excellent low-temperature bake hardening properties and room-temperature aging resistance, such as a tensile strength of 350 MPa or more, a yield strength of 200 to 300 MPa, an elongation of 30% or more, a bake hardening amount (BH140) after heat treatment at 140°C for 20 minutes of 30 MPa or more, a difference between the bake hardening amount (BH170) after heat treatment at 170°C for 20 minutes and the bake hardening amount (BH140) after heat treatment at 140°C for 20 minutes of 20 MPa or less, and a yield point elongation (AI) after heat treatment at 100°C for 1 hour of 0.2% or less. In the present invention, the tensile strength, elongation, and BH140 are advantageous as their values ​​are higher, so their upper limits are not particularly limited. However, as an example, it is difficult for the tensile strength, elongation, and BH140 values ​​to exceed 400 MPa, 40%, and 50 MPa, respectively. In addition, in the present invention, the lower limits of the BH170-BH140 values ​​and the yield point elongation after heat treatment at 100°C for 1 hour are advantageous as they are lower, so their lower limits are not particularly limited. As an example, the lower limits of the BH170-BH140 values ​​and the yield point elongation after heat treatment at 100°C for 1 hour may be 0 MPa and 0%, respectively. Meanwhile, the BH value refers to the amount of baking hardening at a specific temperature, and refers to a value obtained by measuring the increase in the lower yield strength compared to room temperature after baking at a specific temperature for a certain period of time based on the Flow Stress after pre-strain of 2%.

[0050] In addition, the cold-rolled steel sheet of the present invention may have a hot-dip zinc-based plating layer or an alloyed hot-dip zinc-based plating layer formed on at least one surface.

[0051] Hereinafter, a method for manufacturing a cold-rolled steel sheet according to one embodiment of the present invention will be described.

[0052] First, the slab satisfying the aforementioned alloy composition is reheated. The reheating step may be performed to smoothly perform the subsequent hot rolling step and sufficiently obtain the target physical properties of the steel plate. The slab reheating may be performed at 1100 to 1250°C. If the slab reheating temperature is lower than 1100°C, slab inclusions, etc. may not be sufficiently re-dissolved, which may cause material deviations or surface defects after hot rolling. If the slab reheating temperature exceeds 1250°C, the strength may be reduced due to excessive growth of austenite grains, and excessive scale may be formed, which may deteriorate the surface quality of the steel plate. Therefore, the slab reheating temperature may be 1100 to 1250°C. The lower limit of the reheating temperature of the above slab is more advantageously 1110°C, more advantageously 1120°C, and most advantageously 1130°C. The upper limit of the reheating temperature of the above slab is more advantageously 1240°C, more advantageously 1230°C, and most advantageously 1220°C.

[0053] Thereafter, the reheated slab is subjected to a finish hot rolling to obtain a hot-rolled steel sheet. When the finish hot rolling is performed in the austenite single-phase region, pancake-shaped austenite grains and a deformation zone are formed, which may be advantageous in terms of refining the final structure. The finish hot rolling may be performed at 880°C or higher. When the finish hot rolling temperature is lower than 880°C, two-phase rolling of austenite and ferrite may occur, causing material non-uniformity and resulting in excessive rolling load. Therefore, the finish hot rolling temperature may be controlled to 880°C or higher so that hot rolling is completed in the austenite single-phase region. The finish hot rolling temperature is more advantageously 890°C or higher, more advantageously 900°C or higher, and most advantageously 910°C or higher. Meanwhile, the present invention does not specifically limit the upper limit of the finish hot rolling temperature. However, as an example, the finishing hot rolling temperature may be 950°C or lower.

[0054] Thereafter, the hot-rolled steel sheet is cooled to 500 to 700°C and then coiled. If the coiling temperature is lower than 500°C, the shape of the steel sheet may be poor, and a large amount of low-temperature transformation phases such as martensite or bainite may be formed, which may result in excessive strength increase of the steel sheet. If the coiling temperature exceeds 700°C, coarse ferrite grains may be formed, and coarse carbides and nitrides may easily be formed, which may deteriorate the quality of the steel. In addition, due to the high coiling temperature, oxides of the hot-rolled steel sheet such as manganese and silicon may increase, and even if some of the oxides remain during the pickling process or are completely removed, concentrated compounds may be formed on the surface of the steel sheet, which may cause surface defects during plating. Therefore, the coiling temperature is preferably in the range of 500 to 700°C. The lower limit of the above coiling temperature is more advantageously 510°C, more advantageously 520°C, and most advantageously 550°C. The upper limit of the above coiling temperature is more advantageously 690°C, more advantageously 680°C, and most advantageously 670°C.

[0055] Afterwards, the coiled hot-rolled steel sheet is pickled and then cold-rolled to obtain a cold-rolled steel sheet. After the coiling, a pickling process may be additionally performed before performing the cold rolling process, which is a subsequent process, to remove surface scale. In the present invention, the conditions for the pickling process are not particularly limited, and all conditions commonly used in the relevant technical field may be applied. The cold rolling may be performed at a reduction ratio of 60 to 90%. If the reduction ratio during the cold rolling is less than 60%, the recrystallization driving force by the cold rolling is not sufficient, so that the recrystallization of ferrite is not completed, and a problem may occur in which unrecrystallized ferrite structure remains. If the reduction ratio during the cold rolling exceeds 90%, the load on the rolling roll may be very severe, which may cause a problem in that the shape of the steel sheet deteriorates. In particular, cracks may occur at the edge of the steel sheet, which may cause a load of the cold rolling. Therefore, the reduction ratio during the cold rolling can be 60 to 90%. The lower limit of the reduction ratio during the cold rolling is more advantageously 65%, and 70% is even more advantageous. The upper limit of the reduction ratio during the cold rolling is more advantageously 85%, and 80% is even more advantageous.

[0056] Thereafter, the cold-rolled steel sheet is first heated at 760 to 830°C. If the first heating temperature is lower than 760°C, recrystallization may not be sufficiently completed, which may result in the generation of an unrecrystallized structure. In addition, since it corresponds to a ferrite single-phase annealing, there is a problem that the microstructure desired in the present invention cannot be secured. If the first heating temperature exceeds 830°C, the austenite fraction becomes excessively high, which reduces the stability of the austenite in the ideal region, resulting in reverse transformation into ferrite during cooling, making it difficult to secure the target martensite fraction in the final structure, making it difficult to expect sufficient bake hardenability and room temperature aging resistance. Therefore, the first heating temperature is preferably in the range of 760 to 830°C. The lower limit of the first heating temperature is more advantageously 770°C, and even more advantageously 780°C. The upper limit of the first heating temperature is preferably 820°C, and 810°C is even more advantageous. The first heating may be performed for 30 seconds or longer. If the first heating time is less than 30 seconds, recrystallization and growth may not sufficiently occur, which may cause a problem in that the structure becomes uneven. In the present invention, the upper limit of the first heating time is not particularly limited, but as an example, the upper limit may be 50 seconds.

[0057] Thereafter, the first heated cold rolled steel sheet is first cooled to a cooling stop temperature of 200 to 400°C at an average cooling rate of 20°C / s or more. The first cooling is to form martensite. If the first cooling stop temperature is less than 200°C, a lot of energy is required to raise the temperature during subsequent reheating, which may result in a disadvantage of reduced productivity. If the first cooling stop temperature exceeds 400°C, the Ms temperature may not be reached, and thus the martensite required for the present invention may not be formed. Therefore, the first cooling stop temperature is preferably in the range of 200 to 400°C. The lower limit of the first cooling stop temperature is more advantageously 210°C, more advantageously 220°C, and most advantageously 230°C. The upper limit of the above first cooling stop temperature is preferably 390°C, more preferably 380°C, and most preferably 370°C. If the above first cooling rate is less than 20°C / s, martensite may not be formed. The present invention does not specifically limit the upper limit of the above first cooling rate, but as an example, the upper limit may be 100°C / s.

[0058] Thereafter, the first-cooled cold-rolled steel sheet is secondarily heated to 400 to 600°C at a heating rate of 5°C / s or less. If the second heating temperature is less than 400°C, there may be a disadvantage in that the plating bath inlet temperature cannot be secured. If the second heating temperature exceeds 600°C, there may be a disadvantage in that the secured martensite phase is tempered during the heat treatment process, so that the desired physical properties cannot be secured. Therefore, the second heating temperature is preferably in the range of 400 to 600°C. The lower limit of the second heating temperature is more advantageously 410°C, more advantageously 420°C, and most advantageously 430°C. The upper limit of the second heating temperature is more advantageously 590°C, more advantageously 580°C, and most advantageously 570°C. If the secondary heating rate exceeds 5°C / s, there may be a disadvantage in that sufficient maturation is not achieved. In the present invention, there is no particular limitation on the lower limit of the secondary heating rate, but as an example, the lower limit may be 3°C / s.

[0059] After the secondary heating process, a step of subjecting the secondary heated cold rolled steel sheet to temper rolling at a reduction ratio of 0.5 to 2% using a skin pass roll having a surface roughness (Ra) of 1.0 to 2.2 μm may be additionally included. If the surface roughness (Ra) of the skin pass roll is less than 1.0 μm, the surface roughness (Ra) of the final product may be lowered, which may result in a disadvantage of reduced press formability. If the surface roughness (Ra) of the skin pass roll exceeds 2.2 μm, the sharpness of the product surface may be lowered. Therefore, the surface roughness (Ra) of the skin pass roll may be 1.0 to 2.2 μm. The lower limit of the surface roughness (Ra) of the skin pass roll is more advantageously 1.1 μm, more advantageously 1.2 μm, and most advantageously 1.3 μm. The upper limit of the surface roughness (Ra) of the above skin pass roll is more advantageously 2.1 μm, more advantageously 2.0 μm, and most advantageously 1.9 μm. If the reduction ratio during the temper rolling is less than 0.5%, sufficient dislocations may not be formed, which is disadvantageous in terms of plate shape, and there is a risk of surface defects occurring during plating. In addition, it may be disadvantageous in terms of aging resistance. If the reduction ratio during the temper rolling exceeds 2%, not only may material deterioration occur due to excessive increase in dislocation density in the surface layer, but also side effects such as plate breakage may occur due to limitations in equipment capacity. Therefore, the reduction ratio during the temper rolling may be 0.5 to 2%. The lower limit of the reduction ratio during the temper rolling is more advantageously 0.6%, more advantageously 0.7%, and most advantageously 0.8%. In the above temper rolling, the upper limit of the reduction ratio is more advantageous when it is 1.9%, more advantageous when it is 1.8%, and most advantageous when it is 1.7%.

[0060] Meanwhile, after the secondary heating process or temper rolling process, a plating process can be performed according to the required purpose.

[0061] That is, the secondarily heated cold-rolled steel sheet can be immersed in a zinc plating bath. The present invention does not specifically limit the temperature of the zinc plating bath, and a temperature range commonly used in the relevant technical field can be applied. The temperature of the zinc plating bath can range from 440 to 500°C, as an example.

[0062] In addition, after the zinc plating bath immersion, a step of temper rolling the cold rolled steel sheet at a reduction ratio of 0.5 to 2% using a skin pass roll having a surface roughness (Ra) of 1.0 to 2.2 ㎛ may be additionally included. Meanwhile, as an example, the temper rolling process may be performed when temper rolling is not performed after the secondary heating process.

[0063] Thereafter, an additional step of alloying the cold-rolled steel sheet may be included. The present invention does not specifically limit the alloying temperature, and any temperature range commonly used in the relevant technical field may be applied. For example, the alloying temperature may range from 450 to 540°C.

[0064] Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are intended only to illustrate and explain the present invention in more detail and are not intended to limit the scope of the present invention.

[0065] (Example)

[0066] Slabs having the alloy compositions described in Table 1 below were reheated, finish hot-rolled, coiled, and cold-rolled under the conditions described in Table 2 below. Thereafter, primary heating, primary cooling, and secondary heating were performed under the conditions described in Table 2 below. Thereafter, after furnace cooling, the slabs were immersed in a molten zinc plating bath at 460°C to perform hot-dip galvanizing, and then temper rolling was performed at a reduction ratio of 1.2% using a skin pass roll having a surface roughness (Ra) of 2.0 μm. Meanwhile, Comparative Examples 1 and 2 were furnace cooled after the primary heating.

[0067] The microstructure and mechanical properties of the steel plate manufactured in this manner were measured, and the results are shown in Table 3 below.

[0068] The microstructure was measured using an optical microscope at 1 / 4 of the plate thickness after Le Pera etching the steel plate.

[0069] Among the mechanical properties, tensile strength, yield strength, and elongation were measured by conducting a tensile test using the ASTM-L standard.

[0070] Among the mechanical properties, the amount of hardening was measured by heat-treating the same standard specimens used in the above tensile test at 140°C and 170°C for 20 minutes after 2% pre-strain, and then measuring the increase in the lower yield strength compared to room temperature.

[0071] Among mechanical properties, the yield point elongation (AI), which is representative of room temperature aging resistance, was measured by performing a tensile test on the specimen after heat treatment at 100°C for 1 hour.

[0072] Steel grade No. Alloy composition (weight %) CSiMnPSAlCrN10.0190.0181.00.0130.0020.0300.490.00220.0150.0111.20.0120.0020.0220.490.00330.0200.0070.80.0160.0030.0270.50.00540.0320.0171.00.0150.0020.0290.40.00350.0060.0122.20.0120.0020.0251.20.00560.0160.0110.80.0130.0030.0211.70.007

[0073] Classification Steel grade No. Reheating temperature (℃) Finishing hot rolling temperature (℃) Coiling temperature (℃) Cold reduction ratio (%) 1st heating temperature (℃) 1st heating time (s) 1st cooling speed (℃ / s) 1st cooling stop temperature (℃) 2nd heating speed (℃ / s) 2nd heating temperature (℃) Invention example 1 1 1 2 0 0 9 1 0 6 5 0 7 0 8 1 0 3 5 2 2 3 0 0 4.0 460 Invention example 2 1 1 2 0 0 9 1 0 6 5 0 7 0 8 1 0 3 5 2 2 3 0 0 4.0 450 Invention example 3 2 1 2 0 0 9 0 6 5 0 7 0 8 1 0 3 4 2 5 3 1 5 4.0 455 Comparative example 1 2 1 2 0 0 900650708703325---Invention Example 4311809106507080040223054.1466Invention Example 5311909105507079035263054.0461Comparative Example 231180920550708103124---Comparative Example 3411809205507081035253154.2466Comparative Example 4511809205507081039243023.8455Comparative Example 5611809205507081037283104.0465

[0074] ClassificationMicrostructureYield strength(MPa)Tensile strength(MPa)Elongation(%)BH140(MPa)BH170-BH140(MPa)AI(%)Transformed structureFraction(area%)Transformed structureAverage grain size(nm)FerriteInvention example10.23300Remainder262386353140Invention example20.12500Remainder239391343910Invention example30.28250Remainder2573853838140.1Comparative example100Remainder2793983312260Invention example40.60550Remainder2203903 83490Invention Example 50.53350Residue 247388343560Comparative Example 200Residue 213379352280Comparative Example 31.0450Residue 3124152732160Comparative Example 40.051100Residue 2683903321240.3Comparative Example 50.68800Residue 2984201535300Transformation Structure: At least one of martensite, bainite, and ferritic bainite BH140: Bake hardening amount after heat treatment at 140℃ for 20 minutes BH170: Bake hardening amount after heat treatment at 170℃ for 20 minutes

[0075] As can be seen from Tables 1 to 3 above, in the case of Invention Examples 1 to 5 that satisfy the alloy composition and manufacturing conditions of the present invention, it can be seen that excellent mechanical properties are secured by securing the microstructure targeted by the present invention.

[0076] In the case of Comparative Example 1, where the first heating temperature was not satisfied and the first cooling and second heating were not performed, it can be seen that the microstructure targeted by the present invention was not secured, so BH140 was low and BH170-BH140 was large.

[0077] In the case of Comparative Example 2, where primary cooling and secondary heating were not performed, it can be seen that the BH140 is at a low level because the microstructure targeted by the present invention was not secured.

[0078] In the case of comparative example 3, which does not satisfy the C content, it can be seen that the yield strength is high and the elongation is low.

[0079] In the case of Comparative Example 4, which does not satisfy the C content and Mn content, it can be seen that the microstructure targeted by the present invention is not secured, so BH140 is low and BH170-BH140 and AI are large.

[0080] In the case of comparative example 5, which does not satisfy the Cr content, it can be seen that the elongation is low and BH170-BH140 is at a large level.

Claims

1. Contains, in wt%, C: 0.010 to 0.020%, Si: 0.0050 to 0.10%, Mn: 0.50 to 1.450%, Cr: 0.30 to 1.50%, P: 0.030% or less (excluding 0%), S: 0.010% or less (excluding 0%), N: 0.0020 to 0.010%, Al: 0.010 to 0.040%, and the remainder is composed of Fe and other unavoidable impurities. The microstructure includes columnar ferrite and residual transformation structure. The above transformation structure includes at least one of martensite, bainite and ferritic bainite, A cold rolled steel sheet having the above-mentioned metamorphic structure of 1.0 area% or less (excluding 0%) and an average crystal grain size of 1000 nm or less.

2. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet having a tensile strength of 350 MPa or more, a yield strength of 200 to 300 MPa, an elongation of 30% or more, a baking hardening amount of 30 MPa or more after heat treatment at 140°C for 20 minutes, a difference between the baking hardening amount after heat treatment at 140°C for 20 minutes and the baking hardening amount after heat treatment at 170°C for 20 minutes of 20 MPa or less, and a yield point elongation (AI) of 0.2% or less after heat treatment at 100°C for 1 hour.

3. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet having a hot-dip zinc-based plating layer or an alloyed hot-dip zinc-based plating layer formed on at least one surface.

4. A step of reheating a slab comprising, by weight%, C: 0.010 to 0.020%, Si: 0.0050 to 0.10%, Mn: 0.50 to 1.450%, Cr: 0.30 to 1.50%, P: 0.030% or less (excluding 0%), S: 0.010% or less (excluding 0%), N: 0.0020 to 0.010%, Al: 0.010 to 0.040%, with the remainder being Fe and other unavoidable impurities; A step of final hot rolling the above reheated slab to obtain a hot rolled steel sheet; A step of cooling the above hot-rolled steel plate to 500 to 700°C and then coiling it; A step of obtaining a cold rolled steel sheet by pickling the above-mentioned hot rolled steel sheet and then cold rolling it; A step of first heating the above cold rolled steel plate at 760 to 830°C; A step of first cooling the first heated cold rolled steel sheet to a cooling stop temperature of 200 to 400°C at an average cooling rate of 20°C / s or more; and A method for manufacturing a cold rolled steel sheet, comprising the step of secondarily heating the first-cooled cold rolled steel sheet to 400 to 600°C at a heating rate of 5°C / s or less.

5. In claim 4, The above slab reheating is a method for manufacturing cold rolled steel sheets, which is performed at 1100 to 1250°C.

6. In claim 4, The above finishing hot rolling is a method for manufacturing cold rolled steel sheets, performed at 880℃ or higher.

7. In claim 4, The above cold rolling is a method for manufacturing cold rolled steel sheets, which is performed at a reduction ratio of 60 to 90%.

8. In claim 4, A method for manufacturing a cold rolled steel sheet, wherein the above primary heating is performed for 30 seconds or longer.

9. In claim 4, A method for manufacturing a cold rolled steel sheet, further comprising the step of subjecting the secondarily heated cold rolled steel sheet to temper rolling at a reduction ratio of 0.5 to 2% using a skin pass roll having a surface roughness (Ra) of 1.0 to 2.2 ㎛.

10. In claim 4, A method for manufacturing a cold rolled steel sheet, further comprising, after the second heating, a step of immersing the second heated cold rolled steel sheet in a zinc plating bath at 440 to 500°C.

11. In claim 9, A method for manufacturing a cold rolled steel sheet, further comprising the step of subjecting the cold rolled steel sheet to temper rolling at a reduction ratio of 0.5 to 2% using a skin pass roll having a roughness (Ra) of 1.0 to 2.2 ㎛ after immersion in the zinc plating bath.

12. In claim 10 or 11, A method for manufacturing a cold rolled steel sheet, further comprising the step of alloying the cold rolled steel sheet at 450 to 540°C.