High-strength steel plate with high yield ratio and excellent thermal stability and its manufacturing method

Abstract

The present invention relates to a high-strength steel sheet and a manufacturing method thereof, and more specifically to a steel sheet having excellent thermal stability and a high yield ratio and high strength even after heat treatment at a relatively low temperature, and a manufacturing method thereof.

Description

[Technical Field]

[0001] The present invention relates to a high-strength steel plate and a manufacturing method thereof, and more particularly to a high-strength steel plate with a high yield ratio and excellent thermal stability and a manufacturing method thereof. [Background technology]

[0002] Steel plates used in automobile chassis members, wheel rims, connectors, and structural members of building and machine parts are sometimes heated partially or entirely for various purposes during the manufacturing process and during use. However, this heating process can cause changes in the strength of the steel material, resulting in poor durability.

[0003] During the heating process, the amount of carbon in solid solution increases in steel, forming clusters at the potential, grain boundaries, etc., forming carbides. At the same time, the microstructures of the steel, such as martensite, bainite, and retained austenite, also change, causing a sudden change in the strength of the steel and affecting its formability and durability.

[0004] As such, the changes in the structure and physical properties of steel during the heating process vary depending on the initial steel's composition and microstructure, and are heavily dependent on heat treatment conditions such as heating temperature and maintenance time. However, until now, the focus has been solely on preventing the loss of strength at high temperatures above 600°C.

[0005] For example, Patent Documents 1 and 2 propose a technique for ensuring high-temperature strength by adding alloying elements such as Cr, Mo, Nb, and V and tempering the steel after hot rolling. However, this technique is only suitable for manufacturing thick steel plates for construction. Furthermore, considering the environmental factors that inevitably heat up steel materials for construction due to fires and other factors, adding large amounts of alloying elements such as Cr, Mo, Nb, and V to the steel can ensure a certain level of strength even when exposed to high-temperature environments of 600°C or higher for long periods of time. However, this technique requires the use of expensive alloying elements and a heat treatment process to ensure the necessary physical properties, resulting in excessive manufacturing costs. In particular, when used for short-term exposure to environments below 600°C, the thermal stability is insufficient.

[0006] Patent Document 3 discloses a technology for ensuring the strength of the heat-affected zone (HAZ) by adding elements such as Ti, Nb, Cr, and Mo. The technology involves heating the area adjacent to the weld material melted by the welding heat during arc welding to a high temperature of 600°C or higher. When heated to high temperatures, particularly above the austenite range, the Cr and Mo in the steel increase the hardening ability of the steel, which then forms low-temperature phases such as bainite and martensite upon cooling, ensuring the strength of the HAZ. However, this technology has limitations, such as poor formability and excessive cost due to the addition of excessive alloying elements. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Korean Patent No. 10-0358939 (published on October 16, 2002) [Patent Document 2] Korean Patent No. 10-1290382 (Announced on July 22, 2013) [Patent Document 3] Korean Patent No. 10-0962745 (published on June 3, 2010) Summary of the Invention [Problem to be solved by the invention]

[0008] According to one aspect of the present invention, there are provided a steel sheet having excellent thermal stability and a high yield ratio and high strength even after heat treatment at a relatively low temperature, and a method for manufacturing the same.

[0009] The object of the present invention is not limited to the above-mentioned content, and a person skilled in the art will have no difficulty in understanding further object of the present invention from the entire content of this specification. [Means for solving the problem]

[0010] One aspect of the present invention is a steel sheet containing, by weight percent, C: 0.02 to 0.08%, Si: 0.01 to 0.5%, Mn: 0.8 to 1.8%, Al: 0.01 to 0.1%, P: 0.001 to 0.02%, S: 0.001 to 0.01%, N: 0.001 to 0.01%, Ti: 0.0005 to 0.13%, Nb: 0.005 to 0.06%, Mo: 0.001 to 0.2%, the balance and unavoidable impurities, The |K| value defined by the following relational expression 1 is 0.8 or less, The RA value defined by the following relational expression 2 is 5 to 9.2, The microstructure contains at least 95% by area of ​​ferrite and the remainder pearlite, It is possible to provide a high strength steel sheet having a CN value defined by the following relational expression 3 of 1.5 or more. [Equation 1] |K|=-0.555-1.27[C]+0.043[Si]-0.113[Mn]+0.08[Ti]+0.086[Nb] 2 (Here, [C], [Si], [Mn], [Ti] and [Nb] are the weight percentages of the corresponding alloying elements.) [Equation 2] RA=([Ti] / 48+[Mo] / 96+[V] / 51) / ([Nb] / 93) (Here, [Ti], [Mo], [V] and [Nb] are the weight percentages of the corresponding alloying elements.) [Equation 3] CN=N GB x10 3 xN G -1 (where N GB and N G are the unit area (1 mm 2 ) The number of precipitates with a diameter of 50 nm or less that form at the grain boundaries and within the ferrite grains.

[0011] The steel sheet may further contain one or more of Cr, V, Ni and B in a total content of 0.5% or less.

[0012] The steel plate may have a tensile strength of 590 MPa or more, an elongation of 19% or more, and a yield ratio of 0.8 or more.

[0013] The steel sheet may have a yield ratio after heat treatment at 100 to 600°C that changes by 10% or less compared to the yield ratio before the heat treatment.

[0014] Another aspect of the present invention is a method for producing a steel slab having a |K| value of 0.8 or less as defined by the following relational expression 1 and an RA value of 5 to 9.2 as defined by the following relational expression 2, the method comprising the steps of: reheating a steel slab, in weight percent, at a temperature range of 1100 to 1350°C, the steel slab containing 0.02 to 0.08% C, 0.01 to 0.5% Si, 0.8 to 1.8% Mn, 0.01 to 0.1% Al, 0.001 to 0.02% P, 0.001 to 0.01% S, 0.001 to 0.01% N, 0.0005 to 0.13% Ti, 0.005 to 0.06% Nb, 0.001 to 0.2% Mo, and the remainder and unavoidable impurities; hot rolling the reheated steel slab at a rolling finish temperature of 850 to 1150°C; cooling the hot-rolled steel sheet to a temperature range of 550 to 700°C at a cooling rate of 10 to 70°C / s, and then coiling the steel sheet; and It is possible to provide a method for producing a high strength steel plate, which includes a step of cooling the coiled steel plate to a temperature of 500° C. or less at a cooling rate of 10 to 50° C. / h. [Equation 1] |K|=-0.555-1.27[C]+0.043[Si]-0.113[Mn]+0.08[Ti]+0.086[Nb] 2 (Here, [C], [Si], [Mn], [Ti] and [Nb] are the weight percentages of the corresponding alloying elements.) [Equation 2] RA=([Ti] / 48+[Mo] / 96+[V] / 51) / ([Nb] / 93) (Here, [Ti], [Mo], [V] and [Nb] are the weight percentages of the corresponding alloying elements.)

[0015] The steel sheet may further contain one or more of Cr, V, Ni and B in a total content of 0.5% or less.

[0016] The cooling step may begin within 30 minutes of winding.

[0017] The method may further include pickling and oiling the cooled steel sheet.

[0018] The method may further include a step of heating the pickled and oiled steel sheet to a temperature range of 450 to 740° C. and then hot-dip galvanizing the steel sheet.

[0019] The hot dip galvanizing may be carried out using a plating bath containing, by weight, 0.01 to 30% Mg, 0.01 to 50% Al, and the balance being Zn and unavoidable impurities. [Effects of the Invention]

[0020] According to one aspect of the present invention, it is possible to provide a steel sheet that has excellent thermal stability and has a high yield ratio and high strength even after heat treatment at a relatively low temperature, and a method for manufacturing the same.

[0021] According to another aspect of the present invention, it is possible to provide a high-strength steel sheet and a method for manufacturing the same, which can be subjected to heat treatment at a relatively low temperature in a short time, can be applied to a wide range of applications, and is easy to use in manufacturing plated sheet materials using molten zinc or the like. [Brief explanation of the drawings]

[0022] [Figure 1]1 is a graph showing the correlation between the |K| value in Relational Formula 1 and the YR change ratio before and after heat treatment in an example of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0023] Hereinafter, preferred embodiments of the present invention will be described. The embodiments of the present invention can be modified in various ways, and the scope of the present invention should not be construed as being limited to the embodiments described below. The present embodiments are provided to further explain the present invention in detail to those skilled in the art to which the invention pertains.

[0024] In order to solve the above-mentioned problems of the conventional art, the present inventors measured the change in room temperature tensile strength of steels having various components and microstructures after short-term heat treatment in the temperature range of 100 to 600°C, and confirmed that the change in tensile strength depends on the slope of the dynamic strength value measured during the temperature rise of the steel material.

[0025] From the results, the inventors were able to derive Relational Equations 1 and 2 for optimizing the contents of the main components of steel, namely C, Mn, Si, Ti, and Nb. In addition, they confirmed that excellent thermal stability can be ensured by controlling the manufacturing process conditions, and thus completed the present invention.

[0026] The present invention will be described in detail below.

[0027] The steel composition of the present invention will be described in detail below.

[0028] In the present invention, unless otherwise specified, the percentage representing the content of each element is based on weight.

[0029] A steel according to one aspect of the present invention contains, by weight percent, C: 0.02 to 0.08%, Si: 0.01 to 0.5%, Mn: 0.8 to 1.8%, Al: 0.01 to 0.1%, P: 0.001 to 0.02%, S: 0.001 to 0.01%, N: 0.001 to 0.01%, Ti: 0.0005 to 0.13%, Nb: 0.005 to 0.06%, Mo: 0.001 to 0.2%, the balance and unavoidable impurities, and may contain one or more of Cr, V, Ni, and B in a total content of 0.5% or less.

[0030] Carbon (C): 0.02~0.08% Carbon (C) is the most economical and effective element for strengthening steel, and increasing its content increases tensile strength through its precipitation strengthening effect or an increase in the bainite fraction. If the carbon (C) content is less than 0.02%, it is difficult to fully achieve this effect. If the carbon (C) content exceeds 0.08%, coarse carbides are formed, resulting in reduced formability and weldability. Furthermore, during heat treatment at temperatures between 100 and 600°C, the solid-solution strengthening effect of carbon (C) is reduced, and excess carbides are formed in the structure, significantly reducing strength after heat treatment.

[0031] Therefore, the carbon (C) content can be set to 0.02 to 0.08%, more preferably 0.025% or more, even more preferably 0.03% or more, and more preferably 0.075% or less.

[0032] Silicon (Si): 0.001 to 0.5% Silicon (Si) is an element that is advantageous in that it deoxidizes molten steel, strengthens the solid solution, and retards the formation of coarse carbides, improving formability. It also has the effect of suppressing carbide formation during heat treatment in the 100-600°C range. If the silicon (Si) content is less than 0.001%, it is difficult to achieve these effects and thermal stability may also be impaired. On the other hand, if the silicon (Si) content exceeds 0.5%, red scale formed on the steel sheet surface during hot rolling not only significantly deteriorates the surface quality of the steel sheet, but also reduces ductility and weldability.

[0033] Therefore, the silicon (Si) content can be set to 0.001 to 0.5%, and more preferably 0.001 to 0.45%.

[0034] Manganese (Mn): 0.8-1.8% Manganese (Mn), like Si, is an effective element for solid-solution strengthening of steel. If the manganese (Mn) content is less than 0.8%, the above effect of addition cannot be obtained. If the Mn content exceeds 1.8%, segregation will develop significantly in the center of the slab during casting, and microstructures will be unevenly formed in the thickness direction during cooling after hot rolling, which may reduce ductility and formability. Furthermore, there is a problem that carbides easily grow during heat treatment in the 100-600°C range, resulting in poor thermal stability.

[0035] Therefore, the manganese (Mn) content can be set to 0.8 to 1.8%, more preferably 0.9% or more, and more preferably 1.7% or less.

[0036] Aluminum (Al): 0.01 to 0.1% Aluminum (Al) is added mainly for deoxidation purposes. If the content of aluminum (Al) is less than 0.01%, the above-mentioned effect of addition is insufficient. If the content of aluminum (Al) exceeds 0.1%, Al combines with N to form AlN, which may cause corner cracks in slabs during continuous casting and casting, and defects due to the formation of inclusions may occur.

[0037] Therefore, the aluminum (Al) content can be set to 0.01 to 0.1%, more preferably 0.02% or more, and more preferably 0.08% or less.

[0038] Phosphorus (P): 0.001 to 0.02% Like Si, phosphorus (P) simultaneously strengthens the solid solution and promotes ferrite transformation. If the phosphorus (P) content exceeds 0.02%, embrittlement occurs due to grain boundary segregation, and fine cracks are likely to occur during molding. However, reducing the P content to less than 0.001% is economically disadvantageous because of the excessive manufacturing costs, and may not be sufficient to obtain sufficient strength.

[0039] Therefore, the phosphorus (P) content can be set to 0.001 to 0.02%.

[0040] Sulfur (S): 0.001 to 0.01% Sulfur (S) is an impurity present in steel, and if the sulfur (S) content exceeds 0.01%, it combines with Mn and other elements to form non-metallic inclusions, which can cause microcracks during steel cutting. However, reducing the sulfur content to less than 0.001% can take an excessive amount of time during steelmaking operations, resulting in reduced productivity.

[0041] Therefore, the sulfur (S) content can be set to 0.001 to 0.01%.

[0042] Nitrogen (N): 0.001-0.01% Nitrogen (N), along with C, is a representative solid solution strengthening element that forms coarse precipitates along with Ti, Al, etc. Generally, the solid solution strengthening effect of nitrogen (N) is superior to that of C. However, as the amount of nitrogen (N) in steel increases, the toughness decreases significantly, which is why the upper limit is set at 0.01%. On the other hand, reducing the N content to less than 0.001% requires excessive time during steelmaking operations, resulting in reduced productivity.

[0043] Therefore, the nitrogen (N) content can be set to 0.001 to 0.01%.

[0044] Titanium (Ti): 0.005 to 0.13% Titanium (Ti), along with Nb and V, is a representative precipitation strengthening element, forming coarse TiN due to its strong affinity with N. TiN has the effect of suppressing grain growth during the heating process for hot rolling. In addition, the titanium (Ti) that remains after reacting with N dissolves in the steel and bonds with C to form TiC precipitates, improving the strength of the steel. If the titanium (Ti) content is less than 0.005%, it is difficult to achieve the above effect, and if the content exceeds 0.13%, the coarsening of TiN and TiC precipitates can reduce formability.

[0045] Therefore, the titanium (Ti) content can be set to 0.005 to 0.13%, more preferably 0.01% or more, and more preferably 0.12% or less.

[0046] Niobium (Nb): 0.005-0.06% Niobium (Nb), along with Ti and V, is a typical precipitation strengthening element that precipitates during hot rolling and has a grain refinement effect due to delayed recrystallization, thereby effectively improving the strength and impact toughness of steel. If the niobium (Nb) content is less than 0.005%, the above effect cannot be obtained, and if the Nb content exceeds 0.06%, excessive delayed recrystallization during hot rolling can lead to the formation of elongated grains and coarse complex precipitates, which can reduce formability.

[0047] Therefore, the niobium (Nb) content can be set to 0.005 to 0.06%, more preferably 0.01% or more, and more preferably 0.05% or less.

[0048] Molybdenum (Mo): 0.001 to 0.2% Molybdenum (Mo) is an element that increases the hardenability of steel, facilitates the formation of bainite, and refines precipitates within ferrite grains, making it effective in improving the strength and thermal stability of steel. To achieve these effects, it is advantageous to include 0.001% or more of Mo. However, if the Mo content exceeds 0.2%, the hardenability increases, martensite is formed, and thermal stability decreases sharply, which can be disadvantageous in terms of economy and weldability.

[0049] Therefore, the molybdenum (Mo) content can be set to 0.001 to 0.2%, more preferably 0.002% or more, and more preferably 0.19% or less.

[0050] In addition to the above-described composition, the steel of the present invention may contain the remaining iron (Fe) and inevitable impurities. Since inevitable impurities may be unintentionally mixed in during normal manufacturing processes, they cannot be excluded. Since such impurities are known to anyone skilled in the field of normal steel manufacturing, the details of all of them will not be specifically mentioned in this specification.

[0051] The steel according to one aspect of the present invention may contain one or more of chromium (Cr), vanadium (V), nickel (Ni) and boron (B) in a total content of 0.5% or less.

[0052] Adding one or more of chromium (Cr), vanadium (V), nickel (Ni), and boron (B) can further improve solid solution strengthening and precipitation strengthening. However, if the total content of these elements exceeds 0.5%, the ductility and formability of the steel may be insufficient, and it may be disadvantageous from a cost perspective.

[0053] The steel of the present invention can have a |K| value defined by the following relational expression 1 of 0.8 or less.

[0054] The thermal stability of steel, as related to the |K| value in Equation 1, is based on the steel's resistance to deformation when an external force is applied to the steel at a given temperature. For example, a high-temperature compression test or high-temperature tensile test is performed on steel. During the test, the material is heated at a constant heating rate, and an external force is applied at a constant deformation rate, measuring the force acting on the material per unit area. The stress-temperature curve obtained from this test indicates the steel's sensitivity to temperature, and the slope of the curve, the |K| value, can be considered an intrinsic physical property of steel.

[0055] The inventors conducted experiments and measurements of the |K| value, which corresponds to the gradient of the obtained stress-temperature curve, for various steel materials, and were able to derive Relational Formula 1. [Equation 1] |K|=-0.555-1.27[C]+0.043[Si]-0.113[Mn]+0.08[Ti]+0.086[Nb] 2 (Here, [C], [Si], [Mn], [Ti] and [Nb] are the weight percentages of the corresponding alloying elements.)

[0056] The |K| value in Relational Equation 1 of the present invention is the gradient of the dynamic strength value measured during the heat treatment temperature rise of the steel material, and is an inherent physical property of the steel. In the present invention, if the |K| value exceeds 0.8, thermal stability may be insufficient and the change in yield ratio before and after the heat treatment at 100 to 600°C may increase. However, if the value is less than 0.6, it may be difficult to obtain high strength properties, so the |K| value is preferably 0.6 or more.

[0057] The steel of the present invention can have an RA value defined by the following relational expression 2 of 5 to 9.2.

[0058] The change in yield strength before and after heat treatment tends to be more stable when the above-mentioned relational expression 1 and the following relational expression 2 are simultaneously satisfied. If the RA value in relational expression 2 is less than 5, precipitates with a diameter of 50 nm or more increase in the microstructure of the steel sheet, resulting in insufficient thermal stability. If the value exceeds 9.2, the effect of improving thermal stability decreases and there are economical disadvantages due to the addition of large amounts of expensive alloy elements. More preferably, the upper limit can be set to 9.0. [Equation 2] RA=([Ti] / 48+[Mo] / 96+[V] / 51) / ([Nb] / 93) (Here, [Ti], [Mo], [V] and [Nb] are the weight percentages of the corresponding alloying elements.)

[0059] The steel microstructure of the present invention will now be described in detail.

[0060] In the present invention, unless otherwise specified, the percentages indicating the fractions of the microstructures are based on the area.

[0061] The microstructure of the steel according to one aspect of the present invention may contain 95% by area or more of ferrite and the remainder pearlite, and may have a CN value defined by the following relational expression 3 of 1.5 or more.

[0062] If the area fraction of ferrite is less than 95%, there are problems such as reduced formability due to the formation of excessive pearlite and other structures, and reduced thermal stability due to a significant increase in the pearlite fraction in the structure during heat treatment at 600°C or less. More preferably, the ferrite fraction can be 97% or more.

[0063] The following relational expression 3 represents the distribution characteristics of fine precipitates at the ferrite grain boundaries and within the grains, and the present invention is characterized by mainly utilizing coherent precipitates formed within the ferrite grains to ensure strength and thermal stability.

[0064] Setting the standard at precipitates with an average diameter of 50 nm or less can be a factor in reducing the impact resistance and formability of the steel sheet in the case of relatively coarse precipitates exceeding 50 nm. Therefore, in the present invention, the formation of coarse precipitates exceeding 50 nm is suppressed, and fine precipitates with an average diameter of 50 nm or less are formed.

[0065] If the CN value in the following relational expression 3 is less than 1.5, this means that fine coherent precipitates are not sufficiently formed at the grain boundaries relative to the grain interiors, and coarse carbides are likely to form and grow at the grain boundaries where the solid solubility of carbon is relatively high relative to the grain interiors. As a result, grain boundary embrittlement may occur, potentially resulting in problems such as reduced impact resistance, formability, and thermal stability. The CN value is more preferably 3 or more, even more preferably 5 or more, and particularly preferably 10 or more. [Equation 3] CN=N GB x10 3 xN G -1 (where N GB and N G are the unit area (1 mm 2 ) The number of precipitates with a diameter of 50 nm or less that form at the grain boundaries and within the ferrite grains.

[0066] The steel production method of the present invention will be described in detail below.

[0067] The steel according to one aspect of the present invention can be produced by reheating, hot rolling, cooling, coiling, and cooling a steel slab satisfying the alloy composition described above.

[0068] Slab Reheat A steel slab satisfying the above-mentioned alloy composition can be reheated in the temperature range of 1100 to 1350°C.

[0069] If the reheating temperature is less than 1100°C, precipitates containing Ti, Nb, Mo, and V are not fully redissolved, which reduces the formation of fine precipitates in the process after hot rolling, and coarse TiN may remain.On the other hand, if the temperature exceeds 1350°C, the strength may decrease due to austenite grain growth.

[0070] hot rolling The reheated steel slab can be hot rolled at a rolling finish temperature of 850 to 1150°C.

[0071] If the rolling end temperature exceeds 1150°C, the temperature of the steel sheet becomes too high, which may cause the grain size to become coarse and deteriorate the surface quality of the steel sheet.On the other hand, if the temperature is less than 850°C, the recrystallization may be delayed too much, which may cause the development of elongated grains, resulting in severe anisotropy and poor formability.

[0072] Cooling and winding The hot-rolled steel sheet can be cooled to a temperature range of 550 to 700°C at a cooling rate of 10 to 70°C / s, and then coiled.

[0073] If the cooling end and coiling temperature are less than 550°C, bainite in the steel will be unnecessarily formed, significantly reducing the precipitation strengthening effect of the steel, and an MA (Martensite & Austenite) phase will be formed, which may reduce formability. On the other hand, if the temperature exceeds 700°C, the ferrite grains will become coarse, which will make it difficult to ensure strength and may also result in poor formability. More preferably, the steel can be cooled to a temperature range of 600 to 700°C before coiling.

[0074] If the cooling rate is less than 10°C / s, the crystal grains of the matrix structure will become coarse and the microstructure will become non-uniform, whereas if the rate exceeds 70°C / s, bainite and martensite will be more likely to form, which may result in severe deviation in the strength of the steel and reduced formability.

[0075] cooling The coiled steel sheet can be cooled to a temperature of 500°C or less at a cooling rate of 10 to 50°C / h, and the cooling may be started within 30 minutes after coiling.

[0076] If a steel sheet coiled at a high coiling temperature in the range of 550 to 700°C is maintained at a high temperature for a long period of time, the precipitates may become coarse, so in the present invention, to prevent this, cooling can be started within 30 minutes after coiling. If the cooling rate is less than 10°C / h, the time maintained at high temperature is too long and the above effects cannot be obtained, while if the cooling rate exceeds 50°C / h, bainite or martensite is formed locally, which may result in significant deviation in strength of the steel and reduced formability.

[0077] The present invention may further include a step of pickling and oiling the cooled steel sheet, and may further include a step of heating the pickled and oiled steel sheet to a temperature range of 450 to 740°C and hot-dip galvanizing it.

[0078] The hot dip galvanizing can use a zinc-based plating bath, and the alloy composition of the plating bath is not particularly limited. As an example, the plating bath may contain, by weight, 0.01 to 30% Mg, 0.01 to 50% Al, and the balance Zn and unavoidable impurities.

[0079] The steel sheet of the present invention manufactured as described above has a tensile strength of 590 MPa or more, an elongation of 19% or more, a yield ratio of 0.8 or more, and a rate of change in the yield ratio after heat treatment at 100 to 600°C of 10% or less compared to the yield ratio before the heat treatment, so that the steel sheet has excellent thermal stability, a high yield ratio, and high strength properties.

[0080] The present invention will be described in more detail below with reference to examples. However, it should be noted that the following examples are intended to illustrate and explain the present invention in more detail, and are not intended to limit the scope of the present invention. [Example]

[0081] Table 1 below shows the alloying elements for each steel type, the |K| value of Relational Formula 1, and the RA value of Relational Formula 2. The |K| value of Relational Formula 1 in the present invention was measured by applying a high-temperature compression test. Specifically, a rod-shaped sample with a diameter of 10 mm and a length of 15 mm was heated to 600°C at a heating rate of 1°C / s, and simultaneously subjected to a 30% deformation at a deformation rate of 0.005°C / s. The |K| value corresponding to the slope of the 400 to 600°C section of the obtained stress-temperature curve was measured.

[0082] [Table 1] [Equation 1] |K|=-0.555-1.27[C]+0.043[Si]-0.113[Mn]+0.08[Ti]+0.086[Nb] 2 (Here, [C], [Si], [Mn], [Ti] and [Nb] are the weight percentages of the corresponding alloying elements.) [Equation 2] RA=([Ti] / 48+[Mo] / 96+[V] / 51) / ([Nb] / 93) (Here, [Ti], [Mo], [V] and [Nb] are the weight percentages of the alloying elements.)

[0083] Steel sheets were manufactured using the steel types in Table 1 above, applying the rolling finish temperatures and coiling temperatures shown in Table 2 below. Reheating not shown in Table 2 was performed in the temperature range of 1100 to 1350°C, the cooling rate immediately after hot rolling was on the order of 10 to 70°C / s, and the cooling rate of the steel sheet after coiling was also performed at a rate of 10 to 50°C / h, and cooling began within 30 minutes after coiling.

[0084] Table 2 below shows the phase fractions of the microstructure of the steel sheets after cooling and before heat treatment, and the CN values ​​of Relational Formula 3. The fractions of ferrite (F), bainite (B), martensite (M) and pearlite (P) were measured at a point 1 / 4 of the thickness of each steel type and analyzed using an SEM at magnifications of x3000 and x5000. Here, bainite includes low-temperature ferrite, and pearlite includes carbides with a diameter of 0.1 μm or more. The value of Relational Formula 3 for the distribution of precipitates with a diameter of 50 nm or less formed within ferrite grains and at grain boundaries is calculated based on the area per unit area (1 mm 2 ) was calculated using TEM analysis.

[0085] [Table 2] F: Ferrite, B: Bainite, M: Martensite, P: Pearlite [Equation 3] CN=N GB x10 3 xN G -1 (where N GB and N G are the unit area (1 mm 2 ) The number of precipitates with a diameter of 50 nm or less that form at the grain boundaries and within the ferrite grains.

[0086] Table 3 below shows the mechanical properties before and after heat treatment. The tensile strength (TS), elongation at break (El), and yield ratio (yield strength / tensile strength) are shown, and the change in yield ratio before and after heat treatment was calculated and shown. The tensile test was performed using JIS No. 5 standard test pieces taken perpendicular to the rolling direction. The properties in Table 3 were evaluated at room temperature both before and after heat treatment, and the heat treatment consisted of maintaining the specimen at 500°C for 60 minutes and then air-cooling it to room temperature.

[0087] [Table 3] TS0: Tensile strength before heat treatment (MPa), El0: Elongation before heat treatment (%), YR0: Yield ratio before heat treatment TS h : Tensile strength after heat treatment (MPa), El h : Elongation rate after heat treatment (%), YR h : Yield ratio after heat treatment

[0088] As shown in Tables 2 and 3, invention steels 1 to 7, which have the alloy composition and manufacturing method proposed in the present invention, have all the mechanical properties targeted by the present invention.

[0089] On the other hand, Comparative Steels 1 to 3 did not satisfy Relation 1 of the present invention. Comparative Steels 1 and 2 contained excessive C or Mn, which was outside the compositional range of the present invention, resulting in the formation of coarse carbides and an increase in pearlite. After heat treatment, the carbides further coarsened and grain growth occurred, resulting in a significant decrease in tensile strength, a yield point phenomenon, a slight increase in yield strength, and a YR change ratio of over 10% before and after heat treatment, indicating reduced thermal stability. Comparative Steel 3 satisfied the alloy composition range of the present invention but did not satisfy Relation 1. It had excessive carbides and pearlite structures, the ferrite fraction did not satisfy the range of the present invention, and fine precipitates at grain boundaries and intragranular regions were reduced, thereby not satisfying Relation 3, resulting in reduced thermal stability.

[0090] Comparative steels 4 and 5 did not satisfy the range of relational expression 2 of the present invention. Comparative steel 4 did not satisfy the range of relational expression 2, and the precipitate fraction increased. However, many precipitates of 50 nm or larger formed at the grain boundaries, and relational expression 3 was not satisfied. As a result, uneven grain growth occurred during heat treatment, and thermal stability characteristics deteriorated. Comparative steel 5 did not satisfy the range of relational expression 2, and fine precipitates at the grain boundaries were insufficient compared to within the grains, and relational expression 3 was also not satisfied. In addition, the hardenability increased, and the ferrite fraction was insufficient due to the formation of bainite. The ratio of the YR change before and after heat treatment for this steel exceeded 10%, which was due to insufficient grain boundary stability during heat treatment, which led to grain growth and deterioration of the formed bainite.

[0091] Comparative Steels 6 and 7 satisfied the alloy composition proposed by the present invention, but the coiling temperature was outside the range of the present invention. In the case of Comparative Steel 6, the cooling end temperature was too high, outside the range proposed by the present invention, resulting in the formation of pearlite in the initial fine structure and coarse precipitates. This fine structure is prone to further coarsening of precipitates after heat treatment, and it can be seen that the tensile strength was reduced by heat treatment. In the case of Comparative Steel 7, the cooling end temperature was too low, outside the range proposed by the present invention, resulting in the formation of bainite and martensite. As a result, the yield ratio before heat treatment did not satisfy the range of the present invention, and the YR change ratio before and after heat treatment also exceeded the range of the present invention.

[0092] Although the present invention has been described in detail with reference to the above embodiments, other embodiments are possible, and the spirit and scope of the claims set forth below should not be limited to the embodiments.

Claims

1. The alloy contains, by weight, C: 0.02 to 0.08%, Si: 0.01 to 0.5%, Mn: 0.8 to 1.8%, Al: 0.01 to 0.1%, P: 0.001 to 0.02%, S: 0.001 to 0.01%, N: 0.001 to 0.01%, Ti: 0.0005 to 0.13%, Nb: 0.005 to 0.06%, and Mo: 0.001 to 0.2%, with the balance being Fe and inevitable impurities; The |K| value defined by the following relational expression 1 is 0.8 or less, The RA value defined by the following relational expression 2 is 5 to 9.2, the microstructure comprises at least 95% by area of ​​ferrite and the remainder pearlite; A steel plate having a CN value defined by the following relational expression 3 of 1.5 or more. [Relationship 1] |K|=|-0.555-127[C]+00433[E]-0113[MO]+008[T� ２ | (Here, [C], [Si], [Mn], [Ti] and [Nb] are the weight percentages of the corresponding alloying elements.) [Relationship 2] RA=([Ti] / 48+[Mo] / 96+[V] / 51) / ([Nb] / 93) (Here, [Ti], [Mo], [V] and [Nb] are the weight percentages of the corresponding alloying elements.) [Relationship 3] CN=N ＧＢ x10 ３ 8N Ｇ －１ (where N ＧＢ and N Ｇ are the unit area (1 mm ２ ) The number of precipitates with a diameter of 50 nm or less formed at the grain boundaries and within the ferrite grains.

2. The steel plate according to claim 1 , further comprising one or more of Cr, V, Ni and B in a total content of 0.5% or less.

3. The steel plate according to claim 1, wherein the steel plate has a tensile strength of 590 MPa or more, an elongation of 19% or more, and a yield ratio of 0.8 or more.

4. 2. The steel sheet according to claim 1, wherein the rate of change in yield ratio after heat treatment at 100 to 600°C is 10% or less compared to the yield ratio before the heat treatment.

5. a step of reheating a steel slab containing, by weight %, C: 0.02 to 0.08%, Si: 0.01 to 0.5%, Mn: 0.8 to 1.8%, Al: 0.01 to 0.1%, P: 0.001 to 0.02%, S: 0.001 to 0.01%, N: 0.001 to 0.01%, Ti: 0.0005 to 0.13%, Nb: 0.005 to 0.06%, and Mo: 0.001 to 0.2%, with the balance being Fe and inevitable impurities, and having a |K| value defined by the following relational expression 1 of 0.8 or less and an RA value defined by the following relational expression 2 of 5 to 9.2, in a temperature range of 1100 to 1350°C; hot rolling the reheated steel slab at a rolling finish temperature of 850 to 1150°C; Cooling the hot-rolled steel sheet to a temperature range of 550 to 700°C at a cooling rate of 10 to 70°C / s, and then coiling it; and and cooling the coiled steel sheet at a cooling rate of 10 to 50°C / h to a temperature of 500°C or less. [Relationship 1] |K|=|-0.555-127[C]+00433[E]-0113[MO]+008[T� ２ | (Here, [C], [Si], [Mn], [Ti] and [Nb] are the weight percentages of the corresponding alloying elements.) [Relationship 2] RA=([Ti] / 48+[Mo] / 96+[V] / 51) / ([Nb] / 93) (Here, [Ti], [Mo], [V] and [Nb] are the weight percentages of the corresponding alloying elements.)

6. The method for producing a steel sheet according to claim 5, wherein the steel sheet further contains one or more of Cr, V, Ni, and B in a total content of 0.5% or less.

7. The method for manufacturing a steel sheet according to claim 5, wherein the cooling step is initiated within 30 minutes after coiling.

8. The method for manufacturing a steel sheet according to claim 5, further comprising the steps of pickling and oiling the cooled steel sheet.

9. The method of claim 8, further comprising the step of heating the pickled and oiled steel sheet to a temperature range of 450 to 740°C and then hot-dip galvanizing the steel sheet.

10. The method for producing a steel sheet according to claim 9, wherein the hot-dip galvanizing uses a plating bath containing, by weight%, Mg: 0.01 to 30%, Al: 0.01 to 50%, and the balance being Zn and inevitable impurities.

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