High-strength steel sheet having excellent thermal stability, and manufacturing method for the same

Abstract

To provide a high-strength steel sheet having properties required for favorable application to automobile chassis parts and the like, particularly excellent formability, bake hardenability, and thermal stability, and a manufacturing method for the same.SOLUTION: A high-strength steel sheet having excellent thermal stability comprises, in weight%, carbon: 0.02 to 0.08%, silicon: 0.01 to 0.5%, manganese: 0.8 to 1.8%, aluminum: 0.01 to 0.1%, phosphorus: 0.001 to 0.02%, sulfur: 0.001 to 0.01%, nitrogen: 0.001 to 0.01%, titanium: 0.01 to 0.12%, niobium: 0.01 to 0.05%, and molybdenum: 0.001 to 0.2%, the balance being iron and other unavoidable impurities, wherein, as a microstructure, the sum of area fractions of ferrite and bainite phases is 90% or more (excluding 100%), and one or more of residual martensite and MA phases are included.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a steel sheet applicable to automobile chassis parts, etc., and more specifically The present invention relates to a high-strength steel plate having excellent thermal stability and a method for manufacturing the same. [Background technology]

[0002] Conventional high-strength hot-rolled steel sheets used for automobile chassis and frames have been used to meet the demand for weight reduction. As a result, high strength and thinness are being promoted, and excellent formability is required in consideration of the shape of the parts. In addition, in order to maximize the durability of the parts, the bake hardening amount ( BH) is required to be at a certain level.

[0003] On the other hand, during the manufacturing process and use of steel, heat is applied to part or all of the steel plate and parts for various purposes. During this heating process, the strength of the steel plate and parts may change, resulting in a decrease in durability. There is a problem.

[0004] Normally, the amount of dissolved carbon in the structure increases during heating, which causes changes in the potential and grain boundaries. Clustering occurs and the final carbides are formed. The structure of martensite, bainite, retained austenite, etc. also changes, and the strength of the steel This changes rapidly and affects moldability and durability.

[0005] In this way, the changes in the structure and physical properties of steel during the heating process are due to the alloy composition and microstructure of the initial steel. It varies depending on the temperature and duration of the heat treatment, and is highly dependent on the heat treatment conditions. Until now, the focus has been on technology to prevent the decrease in strength when heated at high temperatures above 600°C. The reality is that this is the case.

[0006] For example, Patent Documents 1 and 2 disclose a method for adding Cr, Mo, Nb, V, etc. to a steel sheet after hot rolling. This technology is used in the manufacture of thick steel plates for construction. In addition, steel for construction is not suitable for use in environments where it is inevitably heated, such as in a fire. Considering environmental factors, by adding large amounts of elements such as Cr, Mo, Nb, and V to the steel, It is not possible to maintain a certain level of strength even when exposed to high temperature environments of 600°C or higher for long periods of time. However, expensive alloy elements are used and a heat treatment process is required to ensure the desired physical properties. This leads to excessive manufacturing costs. The thermal stability is too high for short-term exposure to a heated environment. There is a place.

[0007] On the other hand, Patent Document 3 describes a method for adding Ti, Nb, Cr, Mo, etc. to ensure the strength of the weld heat affected zone. This technology protects the area adjacent to the welding material melted by the welding heat during arc welding. The patent discloses a process of heating to a high temperature of 600°C or more. When steel is heated to temperatures above the iron oxide region, the addition of Cr and Mo increases the hardening ability of the steel. During subsequent cooling, low-temperature phases such as bainite and martensite are formed, resulting in increased strength. However, the technology to maximize the hardening ability of steel sheets is In order to apply it to automotive steel sheets, high formability must be ensured even after heat treatment as required. There are limits. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Korean Patent Publication No. 1997-0043167 [Patent Document 2] Korean Patent Publication No. 2013-0002176 [Patent Document 3] Korean Patent Publication No. 2005-0085873 Summary of the Invention [Problem to be solved by the invention]

[0009] One aspect of the present invention is to provide a composition having physical properties required for suitable application to automobile chassis parts, etc. and a high-strength steel plate having excellent formability, bake hardenability and thermal stability, and a method for manufacturing the same. The goal is to provide a way to do this.

[0010] The object of the present invention is not limited to the above. The object of the present invention is to provide a method for manufacturing a semiconductor device according to the present invention. This is understood by those skilled in the art to which the present invention pertains. If so, there is no difficulty in understanding the further object of the present invention. [Means for solving the problem]

[0011] One aspect of the present invention is a composition comprising, in weight percent, carbon (C): 0.02 to 0.08%, silicon (Si): : 0.01 to 0.5%, Manganese (Mn): 0.8 to 1.8%, Aluminum (Al): 0.01-0.1%, Phosphorus (P): 0.001-0.02%, Sulfur (S): 0.001- 0.01%, Nitrogen (N): 0.001-0.01%, Titanium (Ti): 0.01-0.1 2%, Niobium (Nb): 0.01-0.05, Molybdenum (Mo): 0.001-0.2 %, the remainder being iron and other unavoidable impurities, and satisfying the following relations 1 and 2: The microstructure is such that the total area fraction of ferrite and bainite phases is 90% or more (100% The remaining phases are martensite and one or more of the MA phases, and the thermal stability is To provide excellent high-strength steel plates.

[0012] [Equation 1] |K|≦0.85 (Here, K=-0.6-0.87[C]+0.03[Si]-0.14[Mn]+0. 09[Ti]+0.01[Nb] 2 (where each element represents its weight content.)

[0013] [Equation 2] 5≦A≦20 (Here, A = ([Ti] / 48 + [Mo] / 96) × ([Nb] / 93) -1 indicates, Each element means its weight content.)

[0014] Another aspect of the present invention is to use a steel slab having the above-mentioned alloy composition and satisfying the relational formulas 1 and 2. a step of heating the steel slab to a temperature range of 1100 to 1350°C; The steel slab is hot-rolled at a temperature range of 850 to 1150°C to produce a hot-rolled steel sheet. and the above hot-rolled steel sheet is cooled to a temperature range of 400 to 550°C at an average cooling rate of 10 to 100°C / s. The present invention provides a method for producing a high-strength steel sheet having excellent thermal stability, which includes the steps of cooling the steel sheet at 1000 K and coiling the steel sheet. [Effects of the Invention]

[0015] According to the present invention, since the material has high strength and excellent thermal stability, it can be used at a relatively low temperature. It is possible to provide a steel sheet that is excellent in strength and bake hardenability even after heat treatment at .

[0016] The above steel sheet is heat treated at a lower temperature compared to the relatively high temperature required for conventional steel sheets. Since the heat treatment can be performed at room temperature, the range of applicable uses can be expanded. [Brief explanation of the drawings]

[0017] [Figure 1] 1 is a graph showing the correlation between the |K| value depending on the alloying elements and the relationship between the change in strength before and after heat treatment (ΔTS) and the bake hardenability [ΔTS × BHh −1] in an example of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0018] The inventors of the present invention have developed a method for producing steels having various alloy compositions but different microstructures. The change in room temperature tensile strength after heat treatment in the temperature range of 100 to 600°C was measured. It was confirmed that the change in the temperature-induced tensile strength of the steel depends on the slope of the dynamic strength measured during the temperature rise. Ta.

[0019] Based on this, the present inventors have proposed the following method for ensuring excellent thermal stability of a steel sheet: By optimizing the relationship between the content of specific elements and controlling the conditions of the steel sheet manufacturing process, It was confirmed that a steel sheet with excellent thermal stability could be provided, and the present invention was completed.

[0020] The present invention will be described in detail below.

[0021] A high-strength steel plate with excellent thermal stability according to one aspect of the present invention has, by weight %, carbon (C): 0. 02-0.08%, Silicon (Si): 0.01-0.5%, Manganese (Mn): 0.8 ~1.8%, Aluminum (Al): 0.01~0.1%, Phosphorus (P): 0.001~0 0.02%, Sulfur (S): 0.001-0.01%, Nitrogen (N): 0.001-0.01% , titanium (Ti): 0.01 to 0.12%, niobium (Nb): 0.01 to 0.05%, molybdenum (Mo): 0.01 to 0.05%, It may contain 0.001 to 0.2% of lysine (Mo).

[0022] The reasons for limiting the alloy composition of the steel sheet provided by the present invention as described above will be explained below. This will be explained in detail.

[0023] On the other hand, unless otherwise specified in the present invention, the content of each element is based on weight, and the proportion of the structure is based on surface area. The product is the basis.

[0024] Carbon (C): 0.02~0.08% Carbon (C) is the most economical and effective element for strengthening steel, and its content The higher the temperature, the greater the effect of precipitation strengthening or the higher the fraction of the low-temperature phase, resulting in an increase in tensile strength. It can induce improvement in the degree of

[0025] If the C content is less than 0.02%, the effect of precipitation strengthening and the formation of low-temperature phases are reduced. On the other hand, when the content is 0.0 If it exceeds 8%, low-temperature phases are formed excessively, and the formability and weldability are reduced due to the formation of carbides. In addition, excessive addition of C can cause problems during heat treatment in the range of 100 to 600°C. This induces the reduction of low-temperature phases and the formation of additional excess carbides, resulting in improved strength and bake hardenability after heat treatment. The problem is that the strength is significantly reduced and the moldability is further reduced.

[0026] Therefore, the C content can be 0.02 to 0.08%, and more advantageously 0.03% or more. It can contain up to 0.07% of zinc.

[0027] Silicon (Si): 0.01 to 0.5% Silicon (Si) has the effect of deoxidizing molten steel and strengthening the solid solution, and prevents the formation of coarse carbides. This is advantageous in delaying the heat treatment and improving formability. It also has the effect of suppressing the formation of carbides during processing.

[0028] If the Si content is less than 0.01%, the effect of delaying the formation of carbides is lost. The content is low, making it difficult to improve moldability and reducing thermal stability. If the amount exceeds 0.5%, red scale due to Si will form on the steel sheet surface during hot rolling, Not only does this significantly reduce the surface quality of the steel plate, but it also reduces its ductility and weldability. .

[0029] Therefore, the Si content can be 0.01 to 0.5%, and more advantageously 0.05% or more. The above may include:

[0030] Manganese (Mn): 0.8-1.8% Manganese (Mn), like Si, is an effective element for solid solution strengthening of steel. , which increases the hardenability of the steel and facilitates the formation of low-temperature phases.

[0031] If the Mn content is less than 0.8%, the above-mentioned effects are difficult to obtain sufficiently. On the other hand, if its content exceeds 1.8%, the hardenability increases excessively and the fraction of martensite phase increases. When the slab is cast in the continuous casting process, segregation develops in the center of the thickness, resulting in poor formability. In addition, carbides are easily generated during heat treatment in the range of 100 to 600°C, which reduces strength and The amount of bake hardening may vary significantly.

[0032] Therefore, the Mn content can be 0.8 to 1.8%.

[0033] Aluminum (Al): 0.01 to 0.1% Aluminum (Al) is an element added mainly for deoxidation, and its content is 0. If the content is less than 0.1%, the deoxidizing effect is insufficient. When this occurs, it combines with nitrogen (N) in the steel to precipitate AlN, which is then deposited in the slab during continuous casting and casting. This increases the risk of core cracks and makes it more likely that defects will occur due to the formation of inclusions.

[0034] Therefore, the Al content can be 0.01 to 0.1%, and more advantageously 0.013%. It can include the above.

[0035] In the present invention, it is clarified that the above Al is soluble aluminum (Sol. Al).

[0036] Phosphorus (P): 0.001 to 0.02% Phosphorus (P), like Si, has the effect of solid solution strengthening and promoting ferrite transformation. However, if its content exceeds 0.02%, it may cause embrittlement due to grain boundary segregation. This causes microcracks to form during molding, significantly reducing ductility and impact resistance. .

[0037] On the other hand, controlling the P content to less than 0.001% would require excessive manufacturing costs. This is economically disadvantageous because it is essential, and is also disadvantageous in ensuring the target level of strength.

[0038] Therefore, the P content can be 0.001 to 0.02%.

[0039] Sulfur (S): 0.001 to 0.01% Sulfur (S) is an impurity present in steel, and if its content exceeds 0.01%, It combines with Mn and other elements in the steel to form non-metallic inclusions, which cause minute cracks during cutting of the steel. On the other hand, in order to control the S content to less than 0.001%, However, this requires an excessive amount of time during steelmaking operation, resulting in a problem of reduced productivity.

[0040] Therefore, the S content can be 0.001 to 0.01%.

[0041] Nitrogen (N): 0.001-0.01% Nitrogen (N), along with the above-mentioned C, is a typical solid solution strengthening element, and works well with Ti, Al, etc. in steel. Generally, the effect of solid solution strengthening of N is superior to that of C, but However, there is a problem that the higher the N content in steel, the greater the decrease in toughness. Taking this into consideration, the N content can be set to 0.01% or less. Controlling the temperature to less than 0.01% requires excessive time during steelmaking operations, which reduces productivity. There is a problem that...

[0042] Therefore, the N content can be 0.001 to 0.01%.

[0043] Titanium (Ti): 0.01 to 0.12% Titanium (Ti) is a typical precipitation strengthening element along with Nb and V, and has a strong affinity with N. Coarse TiN is formed in the steel by the heating process for hot rolling, which causes the grains to grow. In addition, the Ti that remains after reacting with N dissolves in the steel and bonds with C. This forms TiC precipitates, which contributes to improving the strength of the steel.

[0044] If the Ti content is less than 0.01%, it is difficult to obtain the above-mentioned effects sufficiently. On the other hand, if its content exceeds 0.12%, coarse TiN and TiC precipitates, which impair formability. There is a problem of decline.

[0045] Therefore, the Ti content can be 0.01 to 0.12%, and more advantageously 0.115 It can contain up to %.

[0046] Niobium (Nb): 0.01-0.05% Niobium (Nb) is a typical precipitation strengthening element along with Ti and V, and precipitates during hot rolling. However, due to the grain refinement effect caused by delayed recrystallization, it is effective in improving the strength and impact toughness of steel. be.

[0047] To obtain the above-mentioned effects, it is advantageous to contain 0.01% or more of Nb. If the content exceeds 0.05%, excessive recrystallization delay during hot rolling can cause the elongated crystals to shrink. The problem is that grains are formed and coarse composite precipitates are formed, which reduces formability.

[0048] Therefore, the Nb content can be 0.01 to 0.05%, and more advantageously 0.0 It may contain at least 11% and at most 0.049%.

[0049] Molybdenum (Mo): 0.001 to 0.2% Molybdenum (Mo) increases the hardenability of steel and facilitates the formation of bainite in steel. It has the effect of refining precipitates within ferrite grains, and is effective in improving the strength and thermal stability of steel. It is target.

[0050] To obtain the above-mentioned effects, it is advantageous to include Mo at a content of 0.001% or more. If the content exceeds 0.2%, the hardenability increases and martensite is formed, resulting in thermal stability. This is disadvantageous in terms of economy and ensuring weldability.

[0051] Therefore, the Mo content can be 0.001 to 0.2%, and more preferably 0.0 It may contain more than 0.02% and less than 0.19%.

[0052] The steel sheet of the present invention contains, in addition to the above-mentioned alloy composition, chromium (Cr), vanadium (V), nickel (Ni), and the like. Further contain one or more of nickel (Ni) and boron (B) in a total content of 1.5% or less. can be done.

[0053] One of the above chromium (Cr), vanadium (V), nickel (Ni) and boron (B) By adding more than one species, further precipitation effect can be achieved, and the appropriate fraction of bainite can be obtained. The formation of can also be advantageously achieved.

[0054] On the other hand, among the above elements, chromium (Cr) can be contained up to 1.0%. However, if its content exceeds 1.0%, the hardening ability becomes excessive and the martensite content in the structure increases. The rapid increase in the rate not only reduces the thermal stability of the steel but also increases the cost of ferroalloys. Therefore, the Cr content is set to 1.0% or less. It can be contained in an amount of 0.8% or less, more preferably 0.8% or less.

[0055] The remaining component of the present invention is iron (Fe). However, in the normal manufacturing process, iron is present in the raw materials or the surrounding environment. Unintended impurities may inevitably be introduced from the These impurities are obvious to any engineer working in the normal manufacturing process. The entire contents of which will not be specifically mentioned in this specification.

[0056] The steel sheet of the present invention having the above-mentioned alloy composition has the following relationship between the contents of specific elements in the steel: It is preferable that the relationship 1 and the relationship 2 are satisfied.

[0057] [Equation 1] |K|≦0.85 (Here, K=-0.6-0.87[C]+0.03[Si]-0.14[Mn]+0. 09[Ti]+0.01[Nb] 2 (where each element represents its weight content.)

[0058] [Equation 2] 5≦A≦20 (Here, A = ([Ti] / 48 + [Mo] / 96) × ([Nb] / 93) -1 indicates, Each element means its weight content.)

[0059] In the present invention, |K| shown in the above relational expression 1 is measured during the temperature rise for heating the steel to a specific temperature. The slope of the dynamic strength value is the deformation of the steel material against the external force applied to the steel material at a specified temperature. Based on resistance.

[0060] For example, when testing steel with a high-temperature compression test or a high-temperature tensile test, the material is At the same time, an external force is applied at a constant deformation rate to the material. The stress-temperature curve obtained from this can be used to measure the applied force. It means the sensitivity to temperature, and in particular the slope of the K value is judged as an inherent physical property of steel. This can be done.

[0061] In the present invention, if the |K| value exceeds 0.85, the thermal stability of the steel sheet becomes insufficient, and The change in yield strength before and after heat treatment in the 0 to 600°C range becomes large.

[0062] On the other hand, the change in yield strength before and after heat treatment in a specific temperature range is such that the above relational expression 2 is satisfied simultaneously. This allows for a more stable trend to be demonstrated.

[0063] If the value A in the above relational expression 2 is less than 5, precipitates with a diameter of 50 nm or more will not form in the microstructure of the steel sheet. The fraction of precipitates in the grains decreases, and the thermal stability decreases. The reduction of precipitates with coherent interfaces with the matrix structure results in a thermal Furthermore, when the A value exceeds 20, the thermal stability becomes poor. The improvement effect of the alloying elements is reduced and a large amount of expensive alloying elements must be added, which is economical. It will be disadvantageous.

[0064] The steel sheet of the present invention, which satisfies both the above-mentioned alloy composition and the relational formula 1 and the relational formula 2, can be used in practice. When used as an actual component, it is recommended to heat the component for a short period at a relatively low temperature (for example, below 600°C). The intended properties can be obtained even after processing, and the range of applications can be expanded. It is also useful for obtaining plated steel sheets.

[0065] The steel sheet of the present invention, which satisfies the above-mentioned alloy composition and both Relational Formula 1 and Relational Formula 2, has a fine structure and The main phases are ferrite and bainite, and the total fraction of these phases is 90%. % or more (excluding 100%). If the total is less than 90%, the formability will be poor due to the excessive formation of martensite and MA phases in the structure. Not only does this decrease the thermal stability, but it also becomes difficult to ensure the desired thermal stability.

[0066] The ferrite phase among the above-mentioned main phases advantageously has an area fraction of 30 to 80%. It is advantageous to also include 10 to 60% of the night phase.

[0067] The steel sheet of the present invention has a structure consisting of martensite and MA (Matrix-Austenite) as the remaining structure excluding the main phase. It may contain one or more of the following phases: a mixed structure of austenite and austenite; The thermal stability of the steel sheet is improved by containing each of these elements at an area fraction of 5% or less (excluding 0%). This can be advantageous in securing the

[0068] However, if the fraction of each of the martensite and MA phases exceeds 5%, the thermal stability of the steel sheet decreases. The quality of the material decreases, and localized stress concentration occurs easily during deformation, increasing the risk of cracks. There is a risk.

[0069] In addition, the steel sheet of the present invention may further contain a pearlite phase, and the pearlite phase can be contained at an area fraction of 5% or less (including 0%).

[0070] As described above, the steel sheet of the present invention contains ferrite and bainite phases as the main phases. It has the characteristics of being high in strength while also having excellent bake hardenability and hole expandability.

[0071] Specifically, it has a tensile strength of 590 MPa or more, a yield ratio of 0.7 or more, and is capable of hole expansion. It is characterized by a heat resistance (HER) of 40% or more and a bake hardening amount (BH) of 30 MPa or more. do.

[0072] In particular, the steel sheet of the present invention has excellent thermal stability, and is heat-treated at high temperatures of 600°C or higher. Unlike steel sheets that require high temperature treatment, heat treatment can be performed in the temperature range of 100 to 600°C. , the amount of bake hardening after such heat treatment (BH h ) is maintained at 30 MPa or more. There is fruit.

[0073] Furthermore, the change in strength before and after heat treatment in the above temperature range is minimized, and The change in strength (ΔTS) and the amount of bake hardening (BHh ) relationship [ΔTS×BH h -1 ] has the effect of being 0.7 or less in absolute value.

[0074] Hereinafter, another aspect of the present invention, a high-strength steel sheet having excellent thermal stability provided by the present invention, will be described. The manufacturing method will be described in detail.

[0075] The high strength steel plate according to the present invention is a steel plate that satisfies the alloy composition and chemical relationship proposed in the present invention. The roll can be manufactured by a series of processes: heating, hot rolling, cooling, and coiling. Cut.

[0076] The conditions for each of the above steps will be explained in detail below.

[0077] [Heating steel slabs] In the present invention, the steel slab is heated and homogenized before hot rolling. It is preferable to carry out the heating step at 1100 to 1350°C.

[0078] If the heating temperature is less than 1100°C, the precipitates will not be sufficiently redissolved, and the resulting alloy will not be redissolved in the process after hot rolling. On the other hand, when the temperature exceeds 1350°C, the formation of fine precipitates decreases. However, there is a problem in that the strength decreases due to the coarsening of the austenite grains.

[0079] Therefore, the steel slab can be heated in the temperature range of 1100 to 1350°C.

[0080] [Hot rolling] The steel slab heated as described above can be hot-rolled to produce a hot-rolled steel sheet, Hot rolling can be carried out in the temperature range of 850 to 1150°C.

[0081] If the final temperature during the hot rolling, i.e., the finishing temperature, is less than 850°C, excessive recrystallization occurs. Delay causes the development of elongated grains, which leads to severe anisotropy and reduced formability. If the hot rolling is started at a temperature exceeding 1150°C during the hot rolling, the temperature of the hot-rolled steel sheet As a result, the grain size becomes coarse and the surface quality of the hot-rolled steel sheet deteriorates.

[0082] [Cooling and winding] The hot-rolled steel sheet produced as described above can be cooled to a specific temperature and then coiled. Specifically, the cooling is performed at a rate of 10 to 100°C / s to a temperature range of 400 to 550°C. After that, the winding step can be carried out within that temperature range.

[0083] If the temperature at which the cooling is completed, i.e., the coiling temperature, is less than 400°C, martensite will form in the steel. The unnecessary formation of low-temperature phases such as thiophene and MA phases reduces the thermal stability of the structure, which leads to thermal The formability decreases both before and after heat treatment, and the decrease in strength after heat treatment increases. On the other hand, when the temperature exceeds 550°C, bainite, martensite, and MA The phase is not secured at the proper fraction, and the target level of bake hardening (BH) is not achieved both before and after heat treatment. ) becomes difficult to secure.

[0084] On the other hand, when cooling within the above temperature range, if the cooling rate is less than 10°C / s, the base The crystal grains of the base structure become coarse, which causes the structure to become non-uniform. If the melting point exceeds 1 / s, the fraction of the low-temperature phase increases, causing problems such as a decrease in thermal stability.

[0085] [Final cooling] As described above, the cooled and coiled hot-rolled steel sheet can be cooled to approximately room temperature. The cooling can be performed from room temperature to 200°C at a cooling rate of 10 to 50°C / hour.

[0086] If the cooling rate exceeds 50°C / hour, some of the untransformed phases in the steel will be martensite. The thermal stability is reduced due to the above cooling rate. If the temperature is controlled to less than 10°C / hour, excessive ferrite phase is formed in the structure, causing seizure. Not only is it difficult to ensure the required hardening amount (BH), but separate heating equipment is required to control the slow cooling. This is economically disadvantageous because it requires

[0087] The present invention further comprises the steps of pickling and oiling the steel sheet obtained after the completion of the final cooling. In addition, the pickled and oiled steel sheet may be heated at a temperature of 450 to 740°C. The method may further include a step of hot dip galvanizing by heating in a range.

[0088] The hot dip galvanizing can be carried out using a zinc-based plating bath. The alloy composition is not particularly limited, but an example is magnesium (Mg): 0.01 ~30 wt%, aluminum (Al): 0.01~50 wt%, and the remaining Zn and unavoidable The plating bath may also contain pure materials.

[0089] The present invention will be described in more detail below with reference to examples. However, the following examples are merely examples of the present invention. The present invention is not intended to limit the scope of the present invention. It should be noted that the scope of the present invention is not limited to the scope of the claims. It is determined by the matters stated and the matters that can be reasonably inferred from them. [Example]

[0090] (Example) A steel slab having the alloy composition shown in Table 1 below was prepared and heat-treated at 1100 to 1350°C. Thereafter, the steel sheets were subjected to finish hot rolling under the temperature conditions shown in Table 2 below to produce hot-rolled steel sheets. The hot rolling was started at a temperature lower than the heating temperature. The film was cooled to the temperature shown in Table 2 below at the cooling rate shown in Table 2 below, and then wound up at that temperature. After that, the steel sheets were finally cooled to room temperature at a cooling rate of 10 to 50°C / hour. Ta.

[0091] The mechanical properties and microstructure of each steel sheet were measured, and the results are shown in Table 3 below.

[0092] First, DIN standard test pieces were taken in the rolling direction, and then the tensile strength ( The tensile strength (TS) and elongation at break (El) were measured at room temperature.

[0093] In addition, the hole expandability and bake hardenability were evaluated for the same test pieces. is shown as the average value after three measurements at room temperature.

[0094] The hole expandability was measured by preparing a square test piece measuring 120 mm on each side and punching it. A hole with a diameter of 10 mm was punched in the center of the test piece, and then the test piece was placed with the burr facing up. The diameter of the hole expanded until just before a crack appeared on the circumference was compared with the diameter of the initial hole. The calculation was made as a percentage of the diameter (10 mm).

[0095] The bake hardening capacity is shown by measuring the amount of bake hardening. After 2% pre-strain, the specimen was heat treated at 170°C for 20 minutes and then cooled to room temperature. Strength value after heating (MPa, BH h ) was measured and the difference in intensity was calculated.

[0096] Then, to measure the change in strength before and after heat treatment, the tensile strength (TS) before heat treatment and After heat treatment at 500°C for 10 minutes, the tensile strength (TS) was measured after air cooling to room temperature. h ) is measured It was determined.

[0097] Then, the difference in strength before and after heat treatment at 500°C (ΔTS = TS h -TS) and after heat treatment The relationship between the bake hardening ability of [ΔTS × BH h -1 ] was calculated and then expressed as an absolute value.

[0098] On the other hand, to analyze the microstructure of each steel plate, an SEM was used at magnifications of 3000 and 5000. After observing the fraction, the fraction of each phase was measured.

[0099] At this time, the martensite and MA phases were separated by Nital and Repela. After etching, the specimen was analyzed using an optical microscope and an image analyzer. The measurements were analyzed using a microscope at ×1000 magnification.

[0100] [Table 1] (In Table 1, comparative steels 8 and 9 satisfy the alloy composition of the present invention, but are manufactured as follows. It was classified as a comparative steel because it fell outside the manufacturing conditions.)

[0101] [Table 2]

[0102] [Table 3]

[0103] As shown in Tables 1 to 3 above, the alloy composition system and manufacturing conditions proposed in the present invention were all satisfied. In addition, the steels 1 to 5 of the present invention have the intended structure, ensuring the desired physical properties. You can check that this has been done.

[0104] On the other hand, comparative steels 1 to 7 are examples that do not satisfy the alloy composition proposed in the present invention. Comparative steels 1, 3 and 4 each contained excessive amounts of C, Si and Mn, and satisfied relational expression 1. Therefore, the martensite phase and the MA phase are unnecessarily formed in the steel structure, and the steel sheet It was confirmed that the hole expandability of the steel sheet deteriorated and the decrease in tensile strength after heat treatment increased. can.

[0105] Comparative steels 2 and 5 have insufficient C and Mn contents, respectively, and the hardenability of the steel sheet is poor. The strength is maintained at less than 590 MPa because the low-temperature phase fraction is not sufficiently formed. The bake hardening ability decreased both before and after the heat treatment.

[0106] Comparative steels 6 and 7 contain excessive amounts of Ti and Nb, respectively, and carbides are excessive. Therefore, a constant fraction of the low-temperature phase cannot be secured, and therefore, It was also confirmed that the amount of bake hardening decreased and the hole expandability decreased due to an increase in coarse precipitates. can be done.

[0107] Comparative steels 8 and 9 satisfy the total alloying composition of the present invention, but the coiling temperature is lower than that of the present invention. Comparative steel 8 is an example that deviates from the above-mentioned range. The warm phase was not sufficiently formed, making it difficult to ensure the bake hardening ability before and after heat treatment. When the coiling temperature is very low, as in 9, the low-temperature phase is unnecessarily formed, resulting in a low yield ratio. The strength and the amount of bake hardening before and after the heat treatment were significantly different.

[0108] Figure 1 shows the relationship between the |K| value and the change in strength (ΔTS) before and after heat treatment and the bake hardness depending on the alloying elements. The relationship between the chemical potential [ΔTS × BH h -1 ] is shown in the graph. As shown in FIG. 1, only the inventive steel produced by the present invention had a |K| value of 0.85 or less. and that there is little change in strength before and after heat treatment, i.e., that the material has excellent thermal stability. On the other hand, in the case of comparative steels that fall outside the range of the |K| value proposed in this invention, It can be seen that a significant change in the physical properties of the Comparative steels 8 and 9 shown below were manufactured under conditions (coiling temperature) that were outside the scope of the present invention, resulting in the intended results. It is impossible to ensure the required physical properties.

Claims

1. By weight, carbon (C): 0.02 to 0.08%, silicon (Si): 0.01 to 0.5 %, manganese (Mn): 0.8 to 1.8%, aluminum (Al): 0.01 to 0.1% , Phosphorus (P): 0.001-0.02%, Sulfur (S): 0.001-0.01%, Nitrogen ( N): 0.001 to 0.01%, titanium (Ti): 0.01 to 0.12%, niobium (Nb ): 0.01-0.05%, molybdenum (Mo): 0.001-0.2%, the remainder iron and Contains other unavoidable impurities The following relational formula 1 and relational formula 2 are satisfied, As for the microstructure, the total area fraction of ferrite and bainite phases is 90% or more (100% The remaining phases are martensite and / or MA phases, and the thermal stability is improved. Excellent high strength steel plate. [Relationship 1] |K|≦0.85 (Here, K = -0.6 - 0.87 [C] + 0.03 [Si] - 0.14 [Mn] + 0. 09[Ti]+0.01[Nb] ２ (where each element represents its weight content.) [Relationship 2] 5≦A≦20 (where A = ([Ti] / 48 + [Mo] / 96) x ([Nb] / 93) －１ indicates, The weight content of each element is indicated.)

2. The steel plate is made of chromium (Cr), vanadium (V), nickel (N) and boron (B). The thermally stable polymer composition according to claim 1, further comprising one or more of the following in a total content of 1.5% or less. High strength steel plate.

3. The steel sheet has martensite and MA phases each with an area fraction of 5% or less (excluding 0%). The high strength steel plate having excellent thermal stability according to claim 1, comprising:

4. The steel sheet has a tensile strength of 590 MPa or more, a yield ratio of 0.7 or more, and hole expandability (HE 2. The thermal quenching method according to claim 1, wherein the heat resistance (H) is 40% or more and the bake hardening amount (BH) is 30 MPa or more. High-strength steel plate with excellent stability.

5. The steel sheet has a bake hardening amount (BH) after heat treatment at 100 to 600°C. ｈ ) is 30 MPa or more can be, The change in strength before and after the heat treatment (ΔTS) and the amount of bake hardening after the heat treatment (B H ｈ ) relationship [ΔTS × BH ｈ －１ ] is 0.7 or less absolute value. High strength steel plate with excellent mechanical stability.

6. By weight, carbon (C): 0.02 to 0.08%, silicon (Si): 0.01 to 0.5 %, manganese (Mn): 0.8 to 1.8%, aluminum (Al): 0.01 to 0.1% , Phosphorus (P): 0.001-0.02%, Sulfur (S): 0.001-0.01%, Nitrogen ( N): 0.001 to 0.01%, titanium (Ti): 0.01 to 0.12%, niobium (Nb ): 0.01-0.05%, molybdenum (Mo): 0.001-0.2%, the remainder iron and preparing a steel slab containing other unavoidable impurities and satisfying the following Relational Formulas 1 and 2: Floor; heating the steel slab to a temperature range of 1100 to 1350°C; The heated steel slab is hot-rolled in the temperature range of 850 to 1150°C to produce a hot-rolled steel sheet. manufacturing the The hot-rolled steel sheet is cooled to a temperature range of 400 to 550°C at an average cooling rate of 10 to 100°C / s. A method for producing a high strength steel sheet having excellent thermal stability, comprising the steps of cooling and coiling. [Relationship 1] |K|≦0.85 (Here, K = -0.6 - 0.87 [C] + 0.03 [Si] - 0.14 [Mn] + 0. 09[Ti]+0.01[Nb] ２ (where each element represents its weight content.) [Relationship 2] 5≦A≦20 (where A = ([Ti] / 48 + [Mo] / 96) x ([Nb] / 93) －１ indicates, The weight content of each element is indicated.)

7. 7. The method of claim 6, further comprising the step of cooling the coiled hot-rolled steel sheet to room temperature to 200°C. A method for producing a high-strength steel plate having excellent thermal stability according to the above.

8. The method according to claim 7, further comprising pickling and oiling the hot-rolled steel sheet after the cooling. A method for manufacturing high-strength steel plates with excellent thermal stability.

9. After the pickling and oiling, the hot-rolled steel sheet is heated in the temperature range of 450 to 740°C, and then melted. The method for manufacturing the high strength steel plate having excellent thermal stability according to claim 8, further comprising a step of galvanizing. Construction method.

10. The hot dip galvanizing is performed using a mixture of magnesium (Mg): 0.01 to 30 wt % and aluminum (Al): 0.01 to 50 wt % and the rest is a plating bath containing Zn and unavoidable impurities. The method for producing a high strength steel plate having excellent thermal stability according to claim 9,

11. The steel slab contains chromium (Cr), vanadium (V), nickel (N) and boron (B 7. The thermally stable composition according to claim 6, further comprising one or more of the following in a total content of 1.5% or less: A manufacturing method for high-strength steel plates with excellent properties.

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