Cold-rolled precipitation hardening steel and manufacturing method therefor

Abstract

The present invention provides a method for manufacturing cold-rolled precipitation hardened steel, comprising: a hot-rolling step of hot-rolling a slab to manufacture a hot-rolled sheet; a cold-rolling step of rolling the hot-rolled sheet to manufacture a cold-rolled sheet; and a cold-rolling annealing step of annealing the cold-rolled sheet to manufacture a cold-rolled annealed sheet, wherein the volume fraction of ferrite included in the cold-rolled annealed sheet and having an average crystal size of 2.7-4.2 μm is 98 % or more.

Description

Cold-rolled precipitation-hardening steel and its manufacturing method

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

[0002] Cold-rolled precipitation hardening steel uses a ferrite structure of low weight carbon and low weight manganese as the base structure, and includes trace amounts of precipitated elements such as titanium (Ti), niobium (Nb), vanadium (V), and molybdenum (Mo), thereby improving strength by utilizing a mechanism that increases yield strength through grain refinement and precipitation hardening effects.

[0003] However, if we look at the current specifications for cold-rolled precipitation hardening steel, the maximum strength range is 460 to 560 MPa in the VDA (verband der Automobilindustrie; German Automobile Association) specifications, and the maximum strength range is 550 to 650 MPa in some automobile specifications.

[0004] When higher strength is required, hot-rolled precipitation hardening steel or cold-rolled transformed structure steel is used.

[0005] Korea places particular importance on the formability, elongation, and tensile strength of steel plates, and has been using a lot of transformed phase steel, especially dual phase steel (DP steel), which has an excellent balance of elongation and tensile strength, for use in the production of automobile parts, etc.

[0006] Europe and the Americas have been using cold-rolled precipitation-hardening steel separately where bending, welding, and high yield properties are required and where high tensile strength and formability are required. Where bending, welding, and high yield properties are required, cold-rolled precipitation-hardening steel is used, and where high tensile strength and formability are required, abnormal structure steel is used separately.

[0007] Meanwhile, cold-rolled precipitation hardening steel has the advantage of being able to be produced at a lower cost than dual phase steel (DP steel).

[0008] However, when cold-rolled precipitation hardening steel was produced in the past through a continuous annealing process, the precipitates became coarse and it was difficult to obtain fine precipitates, so it was often produced through a box annealing process.

[0009] However, since the box annealing process has lower productivity than the continuous annealing process, it is desirable to produce steel plates in a continuous annealing process for more economical production.

[0010] Therefore, cold-rolled precipitation hardening steel with high yield strength and production capability through a continuous annealing process is required for use in automobile parts with low production costs and high safety.

[0011] The present invention is intended to solve various problems including the above-described problems, and according to one embodiment of the present invention, a cold-rolled precipitation hardening steel having a high yield strength and capable of being produced in a continuous annealing process and a method for producing the same are provided.

[0012] However, these tasks are exemplary and the scope of the present invention is not limited thereby.

[0013] According to one aspect of the present invention, a method for manufacturing a cold-rolled precipitation hardening steel is provided, comprising: carbon (C) 0.04 wt% or more and 0.07 wt% or less, silicon (Si) 0.1 wt% or more and 0.3 wt% or less, manganese (Mn) 1.1 wt% or more and 1.5 wt% or less, phosphorus (P) 0 to 0.02 wt% or less, sulfur (S) 0 to 0.02 wt% or less, nitrogen (N) 0 to 0.005 wt% or less, aluminum (Al) 0.015 wt% or more and 0.060 wt% or less, niobium (Nb) 0.02 wt% or more and 0.04 wt% or less, titanium (Ti) 0.17 wt% or more and 0.30 wt% or less, chromium (Cr) 0 to 0.1 wt% or less, boron (B) 0 to 0.001 wt% or less, molybdenum (Mo) A method for manufacturing a cold-rolled precipitation hardened steel is provided, comprising: a hot rolling step of manufacturing a hot-rolled sheet by hot-rolling a slab containing 0 or more and 0.06 wt% or less of iron (Fe) and other unavoidable impurities; a cold rolling step of manufacturing a cold-rolled sheet by rolling the hot-rolled sheet; and a cold-rolling annealing step of manufacturing a cold-rolled annealed sheet by annealing the cold-rolled sheet; wherein the cold-rolled annealed sheet contains ferrite having an average crystal size of 2.7 ㎛ or more and 4.2 ㎛ and has a volume fraction of 98% or more.

[0014] According to one aspect of the present invention, a cold-rolled precipitation hardening steel is provided, comprising: carbon (C) 0.04 wt% or more and 0.07 wt% or less, silicon (Si) 0.1 wt% or more and 0.3 wt% or less, manganese (Mn) 1.1 wt% or more and 1.5 wt% or less, phosphorus (P) 0 to 0.02 wt% or less, sulfur (S) 0 to 0.02 wt% or less, nitrogen (N) 0 to 0.005 wt% or less, aluminum (Al) 0.015 wt% or more and 0.060 wt% or less, niobium (Nb) 0.02 wt% or more and 0.04 wt% or less, titanium (Ti) 0.17 wt% or more and 0.30 wt% or less, chromium (Cr) 0 to 0.1 wt% or less, boron (B) 0 to 0.001 wt% or less, molybdenum (Mo) 0 to 0. A cold-rolled precipitation hardening steel is provided, which contains 0.06 wt% or less of iron (Fe) and other unavoidable impurities, and in which the volume fraction of ferrite, which has an average crystal size of 2.7 ㎛ or more and 4.2 ㎛, is 98% or more.

[0015] According to one embodiment of the present invention, which is achieved as described above, a cold-rolled precipitation-hardened steel having high yield strength and capable of being produced through a continuous annealing process, and a method for manufacturing the same, can be provided. Of course, the scope of the present invention is not limited by these effects.

[0016] Figure 1 is a flowchart showing a method for manufacturing cold-rolled precipitation hardening steel according to one embodiment of the present invention.

[0017] Figure 2 is a graph showing the results of potential density analysis according to one embodiment of the present invention.

[0018] Figure 3 is a graph showing the results of potential density analysis according to one embodiment of the present invention.

[0019] Figure 4 is a graph showing the results of potential density analysis according to one embodiment of the present invention.

[0020] Figure 5 is a graph showing the results of precipitate distribution analysis according to one embodiment of the present invention.

[0021] Figure 6 is a graph showing the results of precipitate distribution analysis according to one embodiment of the present invention.

[0022] Figure 7 is a graph showing the results of precipitate distribution analysis according to one embodiment of the present invention.

[0023] Figure 8 is a graph showing the annealing temperature and annealing time conditions of an embodiment of the present invention.

[0024] In one embodiment, the cold rolling annealing step may anneal the cold rolled sheet at a temperature of 740°C or higher and 800°C or lower.

[0025] In one embodiment, the cold rolling annealing step may anneal the cold rolled sheet for a time of 30 seconds or more and 300 seconds or less.

[0026] In one embodiment, the cold rolling annealing step may have an annealing temperature and annealing time that satisfy the following equation 1.

[0027] <Formula 1>

[0028] ((898.03)*(x-5) (-0.0338) )-5 ≤ y ≤ ((898.03)*(x+5) (-0.0338) )+5

[0029] (At this time, 30 ≤ x ≤ 300, 740 ≤ y ≤ 800)

[0030] (Here, x represents the cold rolling annealing time (s), and y represents the cold rolling annealing temperature (℃))

[0031] In one embodiment, the reduction ratio in the cold rolling step may be 45% or more and 60% or less.

[0032] In one embodiment, the hot rolling step may include: a step of reheating the slab; a step of finish-rolling the reheated slab; and a step of coiling the finish-rolled slab at a temperature of 500°C or more and 550°C or less.

[0033] In one embodiment, the grain size of the cold-rolled annealed sheet may be 2.7 ㎛ or more and 4.2 ㎛ or less.

[0034] In one embodiment, the recrystallization fraction of the cold-rolled annealed sheet may be 15% or more and 22% or less.

[0035] In one embodiment, the dislocation density of the cold-rolled annealed sheet is 6.5*10 14 / m 2 Ideal 7.5*10 14 / m 2 It could be as follows:

[0036] In one embodiment, the average size of the precipitates included in the cold-rolled annealed sheet may be 6 nm or more and 10 nm or less.

[0037] In one embodiment, the yield strength of the cold-rolled precipitation hardening steel may be 800 MPa or more, the yield ratio may be 95% or more, and the elongation may be 8% or more.

[0038] In one embodiment, the recrystallization fraction of the cold-rolled precipitation hardening steel may be 15% or more and 22% or less.

[0039] In one embodiment, the dislocation density of the cold rolled precipitation hardening steel is 6.5*10 14 / m 2 Ideal 7.5*10 14 / m 2 It could be as follows:

[0040] In one embodiment, the average size of the precipitate included in the cold-rolled precipitation hardening steel may be 6 nm or more and 10 nm or less.

[0041] In one embodiment, the yield strength of the cold-rolled precipitation hardening steel may be 800 MPa or more, and the tensile strength may be 800 MPa or more.

[0042] In one embodiment, the yield ratio of the cold rolled precipitation hardening steel may be 95% or greater.

[0043] In one embodiment, the elongation of the cold-rolled precipitation hardening steel may be 8% or more.

[0044] Hereinafter, the present invention will be described in detail. When describing the present invention, if it is determined that a detailed description of a related known technology or configuration may unnecessarily obscure the gist of the present invention, such detailed description will be omitted.

[0045] In the examples below, the terms first, second, etc. are not used in a limiting sense, but are used for the purpose of distinguishing one component from another.

[0046] In the examples below, singular expressions include plural expressions unless the context clearly indicates otherwise.

[0047] In the following examples, when various components such as layers, films, regions, and plates are said to be “on” other components, this includes not only cases where they are “directly on” other components, but also cases where other components are interposed between them.

[0048] For convenience of explanation, the sizes of components in the drawings may be exaggerated or reduced. For example, the sizes and thicknesses of each component shown in the drawings are arbitrarily shown for convenience of explanation, and the present invention is not necessarily limited to what is shown.

[0049] In the following examples, terms such as “include” or “have” mean that a feature or component described in the specification is present, and do not preclude the possibility that one or more other features or components may be added.

[0050] In this specification, “A and / or B” refers to the case where it is A, or B, or both A and B. In addition, in this specification, “at least one of A and B” refers to the case where it is A, or B, or both A and B.

[0051] Although not defined otherwise, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by those of ordinary skill in the technical field to which the present invention pertains. Terms defined in commonly used dictionaries are further interpreted to have meanings consistent with the relevant technical literature and the present disclosure, and are not to be construed as ideal or overly formal unless otherwise defined.

[0052] Hereinafter, embodiments of the present invention will be described in detail so that those skilled in the art can easily implement them. However, the present invention may be implemented in various different forms and is not limited to the embodiments described herein.

[0053] Figure 1 is a flowchart showing a method for manufacturing cold-rolled precipitation hardening steel according to one embodiment of the present invention.

[0054] Referring to FIG. 1, a method for manufacturing cold-rolled precipitation hardening steel according to one embodiment of the present invention may include a hot rolling step (S100), a cold rolling step (S200), and a cold rolling annealing step (S300).

[0055] In a method for manufacturing cold-rolled precipitation hardening steel according to one embodiment of the present invention, a semi-finished product to be hot-rolled may be a slab.

[0056] The slab contains carbon (C) 0.04 wt% or more and 0.07 wt% or less, silicon (Si) 0.1 wt% or more and 0.3 wt% or less, manganese (Mn) 1.1 wt% or more and 1.5 wt% or less, phosphorus (P) 0 to 0.02 wt% or less, sulfur (S) 0 to 0.02 wt% or less, nitrogen (N) 0 to 0.005 wt% or less, aluminum (Al) 0.015 wt% or more and 0.060 wt% or less, niobium (Nb) 0.02 wt% or more and 0.04 wt% or less, titanium (Ti) 0.17 wt% or more and 0.30 wt% or less, chromium (Cr) 0 to 0.1 wt% or less, boron (B) 0 to 0.001 wt% or less, molybdenum (Mo) 0 to 0.06 wt% or less, and the remainder May contain iron (Fe) and other unavoidable impurities.

[0057] Below, the reasons why the numerical range of components included in cold-rolled precipitation hardening steel is limited are explained.

[0058] carbon (C)

[0059] Carbon is an essential element in steelmaking and must be included to ensure strength and control the microstructure. Carbon can form carbides such as TiC and NbC.

[0060] In cold-rolled precipitation hardening steel, if the carbon content in the slab is less than 0.04 wt%, the precipitation effect is minimal, making it difficult to secure the target yield strength, and the carbides may become coarse, reducing the grain refinement effect.

[0061] When the carbon content in the slab exceeds 0.07 wt%, carbon acts as an austenite stabilizing element, which increases the pearlite fraction during recrystallization annealing after hot rolling and cold rolling, which may cause a decrease in yield ratio and bending properties, and when annealing at high temperatures after cold rolling, austenite transformation may occur in some pearlite structures, which may create a two-phase structure.

[0062] Silicon (Si)

[0063] When silicon is added, the effect of strengthening the solid solution during steel production is excellent, and since silicon is a ferrite stabilizing element, it is advantageous for grain refinement during ferrite transformation and can suppress the formation of pearlite, thereby playing a role in increasing the reactivity with solid solution carbon and precipitated elements.

[0064] If the silicon content in the slab is less than 0.1 wt%, the effect of reinforcing the solution is minimal, and if the silicon content in the slab exceeds 0.3 wt%, the surface properties of the steel plate may deteriorate.

[0065] manganese (Mn)

[0066] Manganese has a solid-solution strengthening effect in steelmaking, making it an essential element for improving strength and enhancing toughness. However, excessive addition can cause center segregation, reducing the toughness of the steel sheet.

[0067] If the manganese content in the slab is less than 1.1 wt%, it is difficult to secure high tensile strength due to solid solution strengthening, making it difficult to obtain cold-rolled precipitation hardening steel with a yield strength of 800 MPa or higher.

[0068] While manganese content exceeding 1.5 wt% in the slab is beneficial for securing strength, it can also lead to the formation of inclusions or segregation, which reduces workability, reduces delayed fracture resistance, and increases carbon equivalent, thereby reducing weldability, a key advantage of precipitation-hardening steels. Furthermore, it can induce transformation structures during high-temperature annealing and cooling after cold rolling.

[0069] Person (P)

[0070] Phosphorus is an element that increases strength while minimizing loss of elongation. However, if the phosphorus content in a slab exceeds 0.02 wt%, grain boundary segregation can increase brittleness after hot rolling and reduce workability even after cold annealing.

[0071] Yellow (S)

[0072] Sulfur is a representative impurity, and if the sulfur content in the slab exceeds 0.005 wt%, non-metallic inclusions may be formed, which may reduce toughness and weldability.

[0073] Nitrogen (N)

[0074] Nitrogen can form nitrides such as TiN and NbN. In cold-rolled precipitation-hardening steel, if the nitrogen content in the slab exceeds 0.005 wt%, large TiN precipitates are formed, which adversely affects workability and makes it difficult to secure the target elongation. In particular, TiN may not melt even when the slab is reheated to 1250°C because it is thermodynamically stable.

[0075] Aluminum (Al)

[0076] Aluminum can be used as a deoxidizer to purify ferrite.

[0077] Therefore, if the aluminum content in the slab is less than 0.015 wt%, the deoxidation effect is minimal, and if the aluminum content in the slab exceeds 0.060 wt%, nitrides may be formed during slab manufacturing, which may cause cracks during casting or hot rolling.

[0078] niobium (Nb)

[0079] Niobium is a powerful carbonitride-forming element. It combines with carbon and nitrogen in steel during hot rolling and annealing to form carbides and nitrides. These niobium-based carbonitrides suppress recrystallization and grain growth during annealing after cold rolling, thereby refining grains and improving the strength and toughness of steel.

[0080] If the niobium content in the slab is less than 0.02 wt%, the precipitation strengthening effect is minimal, and if the niobium content in the slab exceeds 0.04 wt%, a problem may arise in which the annealing time or temperature must be increased to secure the elongation of the steel due to the recrystallization delay effect of niobium.

[0081] titanium (Ti)

[0082] Titanium is a strong carbonitride-forming element. It combines with carbon and nitrogen in steel during hot rolling and annealing to form carbides and nitrides. These titanium-based carbonitrides suppress recrystallization and grain growth during annealing after cold rolling, thereby refining grains and improving the strength and toughness of steel.

[0083] If the titanium content in the slab is less than 0.17 wt%, the precipitation strengthening effect is minimal, and if the titanium content in the slab exceeds 0.30 wt%, a problem may arise in which the annealing time or temperature must be increased to secure the elongation of the steel due to the recrystallization delay effect of titanium.

[0084] molybdenum (Mo)

[0085] Molybdenum acts when niobium and titanium combine with carbon to form precipitates, slowing the formation and growth of fine precipitates and thereby enhancing the yield strength of precipitation-hardening steels. However, molybdenum is expensive, and if the molybdenum content exceeds 0.06 wt% in a slab, its price competitiveness may decline.

[0086] Hot rolling stage (S100)

[0087] In the hot rolling step (S100), the above slab can be hot rolled to manufacture a hot-rolled plate.

[0088] The hot rolling step (S100) may include a step of reheating the slab, a step of rolling the reheated slab, and a step of coiling the rolled slab.

[0089] First, the slab can be reheated in the hot rolling stage (S100).

[0090] In one embodiment, the slab reheating temperature (SRT) in the hot rolling step (S100) may be 1,200°C or more and 1,250°C or less.

[0091] If the slab reheating temperature exceeds 1,250 ℃, austenite grain growth may occur, which may deteriorate the properties of the final cold-rolled steel sheet.

[0092] If the slab reheating temperature is below 1,200°C, precipitates such as NbC and TiC will not be dissolved, and the precipitates may become coarser during subsequent processing. Consequently, the desired strength enhancement effect may not be achieved.

[0093] At this time, the reheating time can be between 40 minutes and 2 hours. If the reheating time is less than 40 minutes, precipitates such as NbC and TiC will not be dissolved, and the precipitates may become coarser during subsequent processing. Therefore, the desired strength enhancement effect may not be achieved. If the reheating time exceeds 2 hours, the austenite grain size may increase, reducing strength.

[0094] After reheating the slab, the heated slab can be rolled at a predetermined finishing delivery temperature (FDT). The finishing delivery temperature of the hot rolling step (S100) can be performed at a temperature above the Ar3 point.

[0095] After rolling the slab at a predetermined finishing rolling temperature, it can be cooled to a predetermined coiling temperature (CT) and then coiled.

[0096] At this time, the cooling rate in the coiling step may be 10 ℃ / s or more and 100 ℃ / s or less.

[0097] If the cooling rate is less than 10 ℃ / s, the crystal grain size of ferrite increases during phase transformation, which may deteriorate the material of the final cold-rolled steel sheet.

[0098] If the cooling rate exceeds 100 ℃ / s, the operation of the cooling device equipment may not be easy.

[0099] In the hot rolling stage (S100), the coiling temperature may be 500°C or higher and 550°C or lower. If the coiling temperature is lower than 500°C, the strength may increase after hot rolling, which may cause plate breakage or cracks during cold rolling.

[0100] If the coiling temperature exceeds 550℃, the hot-rolled precipitation fraction increases, which may cause the precipitates to coarsen during cold-rolled annealing, making it difficult to obtain the target strength.

[0101] Cold rolling stage (S200)

[0102] In the cold rolling step (S200), a hot-rolled sheet that has gone through the hot rolling step (S100) can be rolled at a cold rolling reduction ratio of 45% or more and 60% or less to manufacture a cold-rolled sheet.

[0103] Cold rolling reduction is used to roll hot-rolled steel sheets to a target thickness, and making the surface of the steel sheet more attractive can have a significant impact on the recrystallization behavior after cold rolling recrystallization.

[0104] If the cold reduction ratio is less than 45%, there is a problem that the accumulated energy for recrystallization is low, so the recrystallization temperature increases significantly.

[0105] When the cold reduction ratio exceeds 60%, dislocations increase, leading to an increase in recrystallization nucleation sites and precipitation sites. Consequently, the recrystallization rate increases during the subsequent annealing process, which may actually decrease the dislocation density and make it difficult to obtain the desired microstructure.

[0106] Therefore, when the process is carried out at the above cold reduction ratio, recrystallization can be delayed, so that the desired dislocation density can be obtained, and thus a cold-rolled precipitation hardening steel with high yield strength can be manufactured.

[0107] Cold rolling annealing stage (S300)

[0108] In the cold rolling annealing step (S300), the cold rolled sheet that has gone through the cold rolling step (S200) is annealed at a predetermined temperature to produce a cold rolled annealed sheet.

[0109] In one embodiment, the cold rolled sheet may be annealed for a time of 30 seconds or more and 300 seconds or less.

[0110] If the cold rolling annealing time is less than 30 seconds, it may be difficult to secure elongation in the final product. If the cold rolling annealing time exceeds 300 seconds, it may be difficult to secure yield strength and yield ratio.

[0111] In one embodiment, the cold rolled sheet can be continuously annealed at a cold rolling annealing temperature of 740°C or higher and 800°C or lower.

[0112] If the cold rolling annealing temperature is below 740°C, it may be difficult to secure elongation in the final product. If the cold rolling annealing temperature exceeds 800°C, it may be difficult to secure yield strength and yield ratio.

[0113] In the case of continuous annealing, the nature of the process typically prevents the establishment of separate time standards. In this case, temperature should be a variable. Control should be implemented so that when the continuous annealing line's production speed increases (annealing time decreases), the annealing temperature increases, and when the continuous annealing line's production speed decreases (annealing time increases), the annealing temperature decreases.

[0114] In this case, the temperature and time of the annealing plate must be adjusted so that they can be applied within the temperature-time working condition range presented in Equation 1 below so that the recrystallization fraction can be adjusted.

[0115] Therefore, the cold rolling annealing time and cold rolling annealing temperature can satisfy the relationship shown in Equation 1 below.

[0116] <Formula 1>

[0117] ((898.03)*(x-5) (-0.0338) )-5 ≤ y ≤ ((898.03)*(x+5) (-0.0338) )+5

[0118] (At this time, 30 ≤ x ≤ 300, 740 ≤ y ≤ 800)

[0119] (Here, x represents the cold rolling annealing time (s), and y represents the cold rolling annealing temperature (℃))

[0120] In one embodiment, when the cold rolling annealing temperature is 785°C or higher and 789°C or lower, the cold rolling annealing time may be performed for 50 seconds.

[0121] In one embodiment, when the cold rolling annealing temperature is 765°C or higher and 772°C or lower, the cold rolling annealing time may be performed for 100 seconds.

[0122] In one embodiment, when the cold rolling annealing temperature is 736°C or higher and 745°C or lower, the cold rolling annealing time may be performed for 300 seconds.

[0123] In other words, when the annealing time is 50 seconds, the cold rolling annealing temperature can be set to 785 ℃ or more and 789 ℃ or less, when the annealing time is 100 seconds, the cold rolling annealing temperature can be set to 765 ℃ or more and 772 ℃ or less, and when the annealing time is 300 seconds, the cold rolling annealing temperature can be set to 736 ℃ or more and 745 ℃ or less.

[0124] That is, as the annealing time increases, the minimum and maximum values ​​of the cold rolling annealing temperature that can be set may decrease.

[0125] However, even in this case, as described above, the time for annealing the cold-rolled sheet must be 30 seconds or more and 300 seconds or less, and the temperature for annealing the cold-rolled sheet must be 740 ℃ or more and 800 ℃ or less.

[0126] In the cold-rolled annealing step (S300), the cold-rolled annealed sheet can be cooled at a cooling rate of 5°C / s or more and 100°C / s or less. Cooling under the above conditions allows for easy securing of the grain size in the final product.

[0127] The dominant crystal structure in the cold-rolled annealed sheet that has gone through the cold-rolled annealing step (S300) may be ferrite.

[0128] The volume fraction of ferrite having an average crystal size of 2.7 ㎛ or more and 4.2 ㎛ or more contained in the cold-rolled annealed plate described above may be 98% or more, and the volume fraction of pearlite may be 2% or less.

[0129] Additionally, the average size of the precipitates included in the cold-rolled annealed sheet may be 6 nm or more and 10 nm or less.

[0130] In the present invention, the average crystal size is a measure indicating the average size of crystal grains, and may refer to the crystal size measured based on a misorientation angle of 15 degrees or more for each crystal grain using electron backscatter diffraction (EBSD) according to ASTM standards.

[0131] Recrystallization of the rolled structure occurs in the cold-rolled annealing step (S300), and in one embodiment, the recrystallization fraction of the cold-rolled annealed sheet may be 15% or more and 22% or less.

[0132] A method for manufacturing cold-rolled precipitation hardening steel according to one embodiment of the present invention can control the dislocation density of the steel sheet by delaying recrystallization to obtain the desired yield strength and yield ratio. In one embodiment, the dislocation density of the cold-rolled annealed sheet is 6.5*10 14 / m 2 Ideal 7.5*10 14 / m 2 It could be as follows:

[0133] In the present invention, the recrystallization fraction and dislocation density were measured through electron backscatter diffraction (EBSD) analysis.

[0134] Cold rolled precipitation hardening steel

[0135] A cold-rolled precipitation hardening steel according to one embodiment of the present invention can be manufactured by the cold-rolled precipitation hardening steel manufacturing method described above.

[0136] The dominant crystal structure of the cold-rolled precipitation hardening steel manufactured by the method for manufacturing cold-rolled precipitation hardening steel according to one embodiment of the present invention may be ferrite.

[0137] According to one embodiment of the present invention, the volume fraction of ferrite having an average crystal size of 2.7 ㎛ or more and 4.2 ㎛ included in the cold-rolled precipitation hardening steel may be 98% or more, and the volume fraction of pearlite may be 2% or less.

[0138] When manufacturing cold-rolled precipitation hardening steel by the above-described method for manufacturing cold-rolled precipitation hardening steel, recrystallization of the rolled structure occurs, and in one embodiment, the recrystallization fraction of the cold-rolled precipitation hardening steel may be 15% or more and 22% or less.

[0139] When manufacturing cold-rolled precipitation hardening steel by the above-described cold-rolled precipitation hardening steel manufacturing method, recrystallization is delayed, and the dislocation density can be controlled to obtain the desired yield strength and yield ratio. In one embodiment, the dislocation density of the cold-rolled precipitation hardening steel is 6.5*10 14 / m 2 Ideal 7.5*10 14 / m 2 It could be as follows:

[0140] Since the cold-rolled precipitation hardening steel has the microstructure, recrystallization fraction, dislocation density, etc. as described above, the cold-rolled precipitation hardening steel can exhibit properties such as a yield strength of 800 MPa or more, a tensile strength of 800 MPa or more, a yield ratio of 95% or more, and an elongation of 8% or more.

[0141] Hereinafter, the structure and operation of the present invention will be described in more detail through preferred embodiments of the present invention. However, these are presented as preferred examples of the present invention and should not be construed as limiting the present invention in any way.

[0142] Experimental example

[0143] Below, the present invention will be described in more detail through experimental examples. However, the following experimental examples are intended to further illustrate the present invention, and the scope of the present invention is not limited by these examples. Those skilled in the art may appropriately modify or alter the following experimental examples within the scope of the present invention.

[0144]

[0145]

[0146]

[0147]

[0148] Example 1

[0149] (1) A slab consisting of component A shown in Table 1 above, the remainder iron (Fe), and unavoidable impurities was prepared.

[0150] (2) The above slab was reheated at a temperature of 1,210°C for 60 minutes, finished rolled at a temperature of Ar3 +30°C, cooled at a cooling rate of 10°C / s, and then coiled at 500°C to produce a hot-rolled plate.

[0151] (3) The hot-rolled plate was rolled at a cold rolling reduction rate of 50% to produce a cold-rolled plate.

[0152] (4) The cold-rolled sheet was cold-rolled and annealed under the conditions of 800°C and 30 seconds, which are the cold-rolled annealing temperature and cold-rolled annealing time according to Table 2, and the cold-rolled annealed sheet was cooled at a cooling rate of 5°C / s to produce cold-rolled precipitation hardened steel.

[0153] Example 2

[0154] It was manufactured in the same manner as Example 1, except that it was annealed at a cold rolling annealing temperature of 770°C according to Table 2 above for a cold rolling annealing time of 100 seconds.

[0155] Example 3

[0156] It was manufactured in the same manner as Example 1, except that it was annealed at a cold rolling annealing temperature of 740°C according to Table 2 above for a cold rolling annealing time of 300 seconds.

[0157] Comparative Example 1

[0158] It was manufactured in the same manner as Example 1, except that it was annealed at a cold rolling annealing temperature of 740°C according to Table 2 above for a cold rolling annealing time of 100 seconds.

[0159] Comparative Example 2

[0160] It was manufactured in the same manner as Example 1, except that it was annealed at a cold rolling annealing temperature of 740°C according to Table 2 above for a cold rolling annealing time of 500 seconds.

[0161] Comparative Example 3

[0162] It was manufactured in the same manner as Example 1, except that it was annealed at a cold rolling annealing temperature of 770°C according to Table 2 above for a cold rolling annealing time of 50 seconds.

[0163] Comparative Example 4

[0164] It was manufactured in the same manner as Example 1, except that it was annealed at a cold rolling annealing temperature of 770°C according to Table 2 above for a cold rolling annealing time of 300 seconds.

[0165] Comparative Example 5

[0166] It was manufactured in the same manner as Example 1, except that it was annealed at a cold rolling annealing temperature of 800°C according to Table 2 above for a cold rolling annealing time of 10 seconds.

[0167] Comparative Example 6

[0168] It was manufactured in the same manner as Example 1, except that it was annealed at a cold rolling annealing temperature of 800°C according to Table 2 above for a cold rolling annealing time of 50 seconds.

[0169] Comparative Example 7

[0170] A slab composed of component B according to Table 1 above, the remainder iron (Fe) and unavoidable impurities was used, and the same process as Example 1 was performed except that the slab was annealed at a cold rolling annealing temperature of 800°C according to Table 2 above for a cold rolling annealing time of 30 seconds.

[0171] Comparative Example 8

[0172] A slab composed of component B according to Table 1 above, the remainder iron (Fe) and unavoidable impurities was used, and the same process as Example 1 was performed except that the slab was annealed at a cold rolling annealing temperature of 770°C according to Table 2 above for a cold rolling annealing time of 100 seconds.

[0173] Comparative Example 9

[0174] A slab composed of component B according to Table 1 above, the remainder iron (Fe) and unavoidable impurities was used, and the same process as Example 1 was performed except that the slab was annealed at a cold rolling annealing temperature of 740°C according to Table 2 above for a cold rolling annealing time of 300 seconds.

[0175] Comparative Example 10

[0176] A slab composed of component C according to Table 1 above, the remainder iron (Fe) and unavoidable impurities was used, and the same process as Example 1 was performed except that the slab was annealed at a cold rolling annealing temperature of 800°C according to Table 2 above for a cold rolling annealing time of 30 seconds.

[0177] Comparative Example 11

[0178] A slab composed of component C according to Table 1 above, the remainder iron (Fe) and unavoidable impurities was used, and the same process as Example 1 was performed except that the slab was annealed at a cold rolling annealing temperature of 770°C according to Table 2 above for a cold rolling annealing time of 100 seconds.

[0179] Comparative Example 12

[0180] A slab composed of component C according to Table 1 above, the remainder iron (Fe) and unavoidable impurities was used, and the same process as Example 1 was performed except that the slab was annealed at a cold rolling annealing temperature of 740°C according to Table 2 above for a cold rolling annealing time of 300 seconds.

[0181]

[0182]

[0183]

[0184]

[0185] Table 3 above is a table showing the grain size, recrystallization fraction, and dislocation density according to examples and comparative examples, and Table 4 above is a table showing the properties (yield strength, yield ratio, and elongation) of cold-rolled precipitation hardening steel according to the grain size, recrystallization fraction, and dislocation density shown in Table 3.

[0186] In the present invention, the yield ratio (YR) can be expressed as the ratio of the yield strength to the tensile strength (yield strength / tensile strength).

[0187] Referring to Table 3 and Table 4, Examples 1 to 3 contain the element contents as described above, particularly titanium (Ti) of 0.17 wt% or more and 0.30 wt% or less, and satisfy the cold rolling annealing temperature of 740 ℃ or more and 800 ℃ or less and the cold rolling annealing time of 30 seconds or more and 300 seconds or less, and thus achieve the grain size of 2.7 ㎛ or more and 4.2 ㎛ or less, the recrystallization fraction of 15% or more and 22% or less, and the crystallinity of 6.5*10 14 / m 2 Ideal 7.5*10 14 / m 2 It satisfies the dislocation density below, and thus has a high yield strength of 800 MPa or more and a high yield ratio of 95% or more, while having a high elongation of 8% or more.

[0188] FIGS. 2 to 4 are graphs showing the results of potential density analysis according to one embodiment of the present invention, and FIGS. 5 to 7 are graphs showing the results of precipitate distribution analysis according to one embodiment of the present invention.

[0189] Specifically, FIGS. 2 and 5 are graphs showing the results of dislocation density analysis and precipitate distribution analysis of Example 1, FIGS. 3 and 6 are graphs showing the results of dislocation density analysis and precipitate distribution analysis of Example 2, and FIGS. 6 and 7 are graphs showing the results of dislocation density analysis and precipitate distribution analysis of Example 3.

[0190] Referring to FIG. 2 and FIG. 5, Example 1 has an average potential density of 6.86*10 as in Table 3. 14 / m 2 It represents.

[0191] Referring to FIG. 3 and FIG. 6, Example 2 has an average dislocation density of 5.98*10 as in Table 3. 14 / m 2 It represents.

[0192] Referring to FIG. 4 and FIG. 7, Example 3 has an average dislocation density of 7.15*10 as in Table 3. 14 / m 2 It represents.

[0193] Figure 8 is a graph showing the annealing temperature and annealing time conditions of an embodiment of the present invention. Specifically, Figure 2 is a graph showing the annealing temperature and annealing time conditions of an embodiment for obtaining the target properties of the present invention, and data from Examples 1 to 3 are shown.

[0194] The correlation equation derived through the conditions is as shown in Equation 2 below.

[0195] <Formula 2>

[0196] y = 898.26*x -0.034

[0197] (Correlation coefficient R considering x, y errors 2= 0.9986±0.0424)

[0198] (Here, x represents the cold rolling annealing time (s), and y represents the cold rolling annealing temperature (℃))

[0199] The above correlation equation is included in the range of Equation 1. Therefore, when the cold rolling annealing step is performed under temperature and time conditions satisfying Equation 2 within a cold rolling annealing temperature of 740°C or more and 800°C or less and a cold rolling annealing time of 30 seconds or more and 300 seconds or less, a cold rolling precipitation hardening steel having the desired properties can be obtained.

[0200] On the other hand, Comparative Example 1 contains the same components as the Example, but does not satisfy the range of Formula 1, so the strength is high but the elongation is low.

[0201] Comparative Example 2 contains the same components as the examples, but the cold rolling annealing time is too long to secure sufficient yield strength, resulting in a yield strength of 773 MPa. Comparative Example 3 exhibits an elongation of 3.9% because the cold rolling annealing time is too short to secure sufficient elongation.

[0202] Comparative Example 4 contains the same components as the example, but does not satisfy the range of Equation 1, resulting in an excessively large recrystallized fraction.

[0203] Comparative Example 5 contains the same components as the examples, but the cold rolling annealing time is too short, so it is not sufficiently recrystallized and the dislocation density is too high, showing a low elongation of 5.9%.

[0204] Comparative Example 6 contains the same components as the example, but does not satisfy the range of Equation 1, resulting in an excessively large recrystallized fraction.

[0205] Comparative examples 7 to 9 satisfy the cold rolling annealing temperature of 740°C or more and 800°C or less and the cold rolling annealing time of 30 seconds or more and 300 seconds or less, but the titanium content was too low at 0.15 wt% to obtain sufficient yield strength and yield ratio.

[0206] Comparative examples 10 to 12 satisfy the cold rolling annealing temperature of 740°C or more and 800°C or less and the cold rolling annealing time of 30 seconds or more and 300 seconds or less, but the titanium content was too low at 0.11 wt% to obtain sufficient yield strength and yield ratio.

[0207] The embodiments of the present invention are merely illustrative, and those skilled in the art will understand that various modifications and equivalent alternative embodiments are possible. Therefore, the true scope of technical protection of the present invention should be determined by the technical spirit of the appended claims.

Claims

As a method for manufacturing cold-rolled precipitation hardening steel, Carbon (C) 0.04 wt% or more and 0.07 wt% or less, silicon (Si) 0.1 wt% or more and 0.3 wt% or less, manganese (Mn) 1.1 wt% or more and 1.5 wt% or less, phosphorus (P) 0 to 0.02 wt% or less, sulfur (S) 0 to 0.02 wt% or less, nitrogen (N) 0 to 0.005 wt% or less, aluminum (Al) 0.015 wt% or more and 0.060 wt% or less, niobium (Nb) 0.02 wt% or more and 0.04 wt% or less, titanium (Ti) 0.17 wt% or more and 0.30 wt% or less, chromium (Cr) 0 to 0.1 wt% or less, boron (B) 0 to 0.001 wt% or less, molybdenum (Mo) 0 to 0.06 wt% or less, the remainder iron (Fe) and A hot rolling step for manufacturing hot-rolled sheets by hot-rolling slabs containing other unavoidable impurities; A cold rolling step of manufacturing a cold rolled sheet by rolling the hot rolled sheet; and A cold rolling annealing step of manufacturing a cold rolled annealed plate by annealing the cold rolled plate; Including, The above cold-rolled annealing step is a method for manufacturing cold-rolled precipitation hardening steel, wherein the cold-rolled sheet is annealed at a temperature of 740°C or higher and 800°C or lower, and the cold-rolled sheet is annealed for a time of 30 seconds or higher and 300 seconds or lower. In the first paragraph, A method for manufacturing cold-rolled precipitation hardening steel, wherein the volume fraction of ferrite having an average crystal size of 2.7 ㎛ or more and 4.2 ㎛ or more contained in the above cold-rolled annealed plate is 98% or more. In the first paragraph, The above cold rolling annealing step is a method for manufacturing cold rolled precipitation hardening steel, wherein the annealing temperature and annealing time satisfy the following formula 1. <Formula 1> ((898.03)*(x-5) (-0.0338) )-5 ≤ y ≤ ((898.03)*(x+5) (-0.0338) )+5 (At this time, 30 ≤ x ≤ 300, 740 ≤ y ≤ 800) (Here, x represents the cold rolling annealing time (s), and y represents the cold rolling annealing temperature (℃)) In the first paragraph, A method for manufacturing cold-rolled precipitation hardening steel, wherein the reduction ratio in the above cold rolling step is 45% or more and 60% or less. In the first paragraph, The above hot rolling step is, Step of reheating the above slab; A step of final rolling the reheated slab; and A step of coiling the above-mentioned slab after finishing rolling at a temperature of 500°C or higher and 550°C or lower; A method for manufacturing cold-rolled precipitation hardening steel, comprising: In the first paragraph, A method for manufacturing cold-rolled precipitation hardening steel, wherein the recrystallization fraction of the above cold-rolled annealed plate is 15% or more and 22% or less. In the first paragraph, The dislocation density of the above cold-rolled annealed plate is 6.5*10 14 / m 2 Ideal 7.5*10 14 / m 2 Below, manufacturing method of cold rolled precipitation hardening steel. In the first paragraph, A method for manufacturing cold-rolled precipitation hardening steel, wherein the average size of the precipitates contained in the above cold-rolled annealed sheet is 6 nm or more and 10 nm or less. In the first paragraph, A method for manufacturing cold-rolled precipitation hardening steel, wherein the yield strength of the cold-rolled precipitation hardening steel is 800 MPa or more, the yield ratio is 95% or more, and the elongation is 8% or more. As a cold-rolled precipitation hardening steel, Carbon (C) 0.04 wt% or more and 0.07 wt% or less, silicon (Si) 0.1 wt% or more and 0.3 wt% or less, manganese (Mn) 1.1 wt% or more and 1.5 wt% or less, phosphorus (P) 0 to 0.02 wt% or less, sulfur (S) 0 to 0.02 wt% or less, nitrogen (N) 0 to 0.005 wt% or less, aluminum (Al) 0.015 wt% or more and 0.060 wt% or less, niobium (Nb) 0.02 wt% or more and 0.04 wt% or less, titanium (Ti) 0.17 wt% or more and 0.30 wt% or less, chromium (Cr) 0 to 0.1 wt% or less, boron (B) 0 to 0.001 wt% or less, molybdenum (Mo) 0 to 0.06 wt% or less, the remainder iron (Fe) and Contains other unavoidable impurities, Cold-rolled precipitation hardening steel, wherein the volume fraction of ferrite having an average crystal size of 2.7 ㎛ or more and 4.2 ㎛ or more is 98% or more. In Article 10, Cold rolled precipitation hardening steel having a recrystallization fraction of 15% or more and 22% or less. In Article 10, The dislocation density of the above cold-rolled precipitation hardening steel is 6.5*10 14 / m 2 Ideal 7.5*10 14 / m 2 Below, cold rolled precipitation hardening steel. In Article 10, Cold-rolled precipitation hardening steel, wherein the average size of the precipitate contained in the above cold-rolled precipitation hardening steel is 6 nm or more and 10 nm or less. In Article 10, Cold-rolled precipitation hardening steel having a yield strength of 800 MPa or more and a tensile strength of 800 MPa or more. In Article 10, Cold-rolled precipitation hardening steel having a yield ratio of 95% or more and an elongation of 8% or more.

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