Steel sheet and manufacturing method therefor

Abstract

To provide a steel sheet and a manufacturing method therefor, the steel sheet having excellent thermal stability to have high yield ratio and high strength even after heat treatment at a relatively low temperature.SOLUTION: A high strength steel sheet, comprising by wt.%, 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, wherein specific relational expressions determined from the above components satisfy a prescribed range, a microstructure includes ferrite of 95 area% or more and the remaining pearlite, and specific relational expressions determined from the number of precipitates having a diameter of 50 nm or less formed within a grain boundary and a crystal grain of ferrite satisfy a prescribed range.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] Automotive chassis components, wheel rims, connecting members, and construction and mechanical parts Steel plates used in structural members are used for various purposes during the manufacturing process and in use. On the other hand, the strength of the steel may change due to this heating process. However, there is a problem in that it is inferior in durability.

[0003] During the heating process, the carbon in the solid solution state increases, and clustering occurs at the potential, grain boundaries, etc. At the same time, martensite, bainite, and residual iron in the steel are formed. The microstructure of austenite and other materials also changes, causing a sudden change in the strength of the steel, and the formability and durability of the steel also deteriorate. It will also affect durability.

[0004] Thus, the changes in the structure and physical properties of steel during the heating process depend on the initial steel composition and microstructure. This depends heavily on the heat treatment conditions such as heating temperature and maintenance time. Until now, the focus had been solely on suppressing the loss of strength at high temperatures above 600°C.

[0005] For example, Patent Documents 1 and 2 disclose that Cr, Mo, Nb, V, etc. are added as alloy components, We proposed a technology to ensure high-temperature strength by tempering after hot rolling, but this is not suitable for thick steel for construction. This technology is only suitable for the production of steel plates. In addition, steel for construction is inevitably heated by fires, etc. Considering the environmental factors that may affect the steel, large amounts of alloying elements such as Cr, Mo, Nb, and V are added to the steel. By adding this, the strength remains at a constant level even when exposed to high temperatures of over 600°C for a long period of time. However, the above technology requires the use of expensive alloy components to ensure the physical properties. The heat treatment process is required, which increases the manufacturing cost. The drawback is that the thermal stability is excessive when used in short-term exposure to low temperatures. There is.

[0006] Patent Document 3 describes a method for adding Ti, Nb, Cr, Mo, etc. to ensure strength in the weld heat affected zone. This is a technology that uses 6000 times the heat generated by arc welding to melt the welding material and the adjacent area. The invention discloses a process of heating the steel to a high temperature of 100°C or more. When steel is heated above the temperature range, the Cr and Mo in the steel increase the hardening ability of the steel and , forming low-temperature phases such as bainite and martensite to ensure the strength of the heat-affected zone. However, the above technology has the disadvantage that excessive addition of alloy elements results in insufficient formability and is costly. There is also a limit to the amount of demand that is excessive. [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, a material having excellent thermal stability and a high yield ratio even after heat treatment at a relatively low temperature is obtained. The present invention provides a steel plate having high strength and a method for manufacturing the same.

[0009] The subject of the present invention is not limited to the above. A person skilled in the art would understand the entire specification. From the present disclosure, it is not difficult to understand the further object of the present invention. [Means for solving the problem]

[0010] In one aspect of the present invention, the composition contains, in weight percent, C: 0.02 to 0.08%, Si: 0.01 to 0.5% , Mn:0.8~1.8%, Al:0.01~0.1%, P:0.001~0.02%, S:0.001~0.01%, N:0.001~0.01%, Ti:0.0005~0. 13%, Nb: 0.005-0.06%, Mo: 0.001-0.2%, balance and unavoidable Contains 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 weights 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 ) Ferrite grain boundaries and the number of precipitates with a diameter of 50 nm or less that form within the grains.)

[0011] The above steel sheet further contains one or more of Cr, V, Ni and B in a total content of 0.5% or less. That's fine.

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

[0013] The above steel sheets show a change in the yield ratio after heat treatment at 100 to 600°C compared to the yield ratio before heat treatment. The rate may be 10% or less.

[0014] Another aspect of the present invention is a composition comprising, in weight percent, C: 0.02 to 0.08%, Si: 0.01 to 0. 5%, Mn:0.8~1.8%, Al:0.01~0.1%, P:0.001~0.02 %, S:0.001~0.01%, N:0.001~0.01%, Ti:0.0005~ 0.13%, Nb: 0.005-0.06%, Mo: 0.001-0.2%, balance and impurities It contains unavoidable impurities, and the |K| value defined by the following relational formula 1 is 0.8 or less, and Steel slabs with an RA value of 5 to 9.2 as defined in 2 were placed in the temperature range of 1100 to 1350°C. Reheating stage; hot rolling the reheated steel slab at a rolling finish temperature of 850 to 1150°C; The above hot-rolled steel sheet is cooled at a rate of 10-70°C / s to a temperature range of 550-700°C. cooling to 100°C and then winding; and The coiled steel sheet is cooled to a temperature of 500°C or less at a cooling rate of 10 to 50°C / h. It is possible to provide a method for manufacturing a high strength steel plate, which includes the steps of: [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 weights 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 above steel sheet further contains one or more of Cr, V, Ni and B in a total content of 0.5% or less. That's fine.

[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] After the above pickling and oiling treatment, the steel sheet is heated to a temperature range of 450 to 740°C, and then molten zinc is added. The method may further include a step of lead plating.

[0019] The hot dip galvanizing described above contains, by weight, Mg: 0.01 to 30%, Al: 0.01 to 50%, The plating bath may contain Zn and the balance Zn and unavoidable impurities. [Effects of the Invention]

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

[0021] According to another aspect of the present invention, heat treatment can be performed at a relatively low temperature in a short time, and the use It is a high strength material that can be easily used in the production of plated sheet materials using molten zinc, etc. The present invention provides a heat-resistant steel sheet and a method for manufacturing the same. [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] The following describes preferred embodiments of the present invention. and the scope of the present invention should not be construed as being limited to the embodiments described below. The present embodiment is not intended to be limiting unless it is specifically stated by a person having ordinary skill in the art to which the present invention pertains. The following is provided to further explain the present invention.

[0024] In order to solve the above-mentioned problems of the prior art, the present inventors have investigated various components and microstructures. The change in room temperature tensile strength after short-term heat treatment in the temperature range of 100 to 600°C was As a result of measuring the temperature change, it was found that the change in tensile strength depends on the slope of the dynamic strength value measured during the temperature rise of the steel. It was confirmed that.

[0025] From the results, the inventors have determined the content of the main components of steel, namely C, Mn, Si, Ti, and Nb. It is possible to derive the relations 1 and 2 that optimize the It was confirmed that excellent thermal stability can be ensured by using the above-mentioned method, and the present invention was completed. This led to the following.

[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] The steel according to one aspect of the present invention contains, in weight percent, C: 0.02 to 0.08%, Si: 0.01 to 0 .5%, Mn:0.8~1.8%, Al:0.01~0.1%, P:0.001~0.0 2%, S:0.001~0.01%, N:0.001~0.01%, Ti:0.0005 ~0.13%, Nb: 0.005~0.06%, Mo: 0.001~0.2%, balance and Contains unavoidable impurities, and contains one or more of Cr, V, Ni and B with a total content of 0.5% or less It can be done.

[0030] Carbon (C): 0.02~0.08% Carbon (C) is the most economical and effective element for strengthening steel, and the more C is added, the stronger the strength of the steel. When carbon (C) is added, the tensile strength increases due to the precipitation strengthening effect or an increase in the bainite fraction. If the content is less than 0.02%, it is difficult to obtain the above-mentioned effects sufficiently. If the content exceeds 0.08%, coarse carbides are formed, which reduces formability and weldability. During heat treatment in the range of 100 to 600°C, the solid solution strengthening effect of carbon (C) decreases, and excess carbon is present in the structure. This can result in a significant decrease in strength after heat treatment.

[0031] Therefore, the carbon (C) content can be set to 0.02 to 0.08%. The content of Si is preferably 0.025% or more, and more preferably 0.03% or more. Preferably, it can be set to 0.075% or less.

[0032] Silicon (Si): 0.001 to 0.5% Silicon (Si) deoxidizes molten steel and has a solid solution strengthening effect, delaying the formation of coarse carbides. It is an element that is advantageous in improving formability by increasing the temperature. When the alloy is subjected to high-temperature treatment, it has the effect of suppressing the formation of carbides. The silicon (Si) content is 0.001%. If the content is less than 100%, it is difficult to obtain the above-mentioned effects, and the thermal stability may also be poor. If it exceeds 0.5%, red scale due to silicon (Si) will appear on the surface of the steel sheet during hot rolling. Not only does this result in a very poor surface quality of the steel sheet, but it also reduces ductility and weldability. There are some problems.

[0033] Therefore, the silicon (Si) content can be set to 0.001 to 0.5%. More preferably, it can be set to 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 content of manganese (Mn) is less than 0.8%, the above effects of the addition cannot be obtained, and If the content exceeds 1.8%, segregation will develop significantly at the center of the thickness during slab casting, and hot rolling will be difficult. During subsequent cooling, the microstructure may be formed unevenly in the thickness direction, which may reduce ductility and formability. In addition, carbides grow easily during heat treatment in the range of 100 to 600°C, and the heat There is also the problem of poor stability.

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

[0036] Aluminum (Al): 0.01 to 0.1% Aluminum (Al) is added mainly for deoxidation, and the content of aluminum (Al) If the content is less than 0.01%, the above-mentioned effect of addition will be insufficient, and if the content exceeds 0.1%, N This bonds with the slab to form AlN, which makes corner cracks less likely to occur in the slab during continuous casting and casting. This may result in defects due to inclusion formation.

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

[0038] Phosphorus (P): 0.001 to 0.02% Phosphorus (P), like Si, simultaneously has the effects of solid solution strengthening and ferrite transformation promotion. If the phosphorus (P) content exceeds 0.02%, embrittlement occurs due to grain boundary segregation, and during molding To manufacture alloys with a content of less than 0.001%, fine cracks are likely to occur. The manufacturing costs are excessive, which is economically disadvantageous, and the strength may not be sufficient. There is a gender.

[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%, This causes the steel to bond with Mn and other elements to form non-metallic inclusions, which can cause minute cracks during cutting. On the other hand, in order to manufacture steel with a content of less than 0.001%, Operational times can be excessive, reducing 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 typical solid solution strengthening element, and together with Ti and Al, it Generally, the solid solution strengthening effect of nitrogen (N) is better than that of carbon, but the presence of nitrogen ( The problem is that the toughness decreases significantly as the amount of N increases. On the other hand, in order to manufacture steel with a content of less than 0.001%, It takes too much time and reduces 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) is a typical precipitation strengthening element along with Nb and V, and has a strong affinity with N. Coarse TiN is formed. TiN inhibits the growth of crystal grains 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. This forms TiC precipitates, which improves the strength of the steel. If the content is less than 0.005%, it is difficult to obtain the above effects, and if the content exceeds 0.13%, If the temperature is too high, TiN and TiC precipitates may become coarse, which may reduce formability.

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

[0046] Niobium (Nb): 0.005-0.06% Niobium (Nb) is a typical precipitation strengthening element along with Ti and V. It precipitates during hot rolling. The grain refinement effect due to delayed recrystallization is effective in improving the strength and impact toughness of steel. If the niobium (Nb) content is less than 0.005%, the above effects cannot be obtained. If the content of Cu exceeds 0.06%, excessive recrystallization delay during hot rolling may cause elongated grain formation and The formation of coarse complex precipitates can reduce formability.

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

[0048] Molybdenum (Mo): 0.001 to 0.2% Molybdenum (Mo) increases the hardenability of steel, facilitating the formation of bainite, and It has the effect of refining precipitates within steel grains, and is an element that is effective in improving the strength and thermal stability of steel. To obtain the above-mentioned effects, it is advantageous to include 0.001% or more of Nb. If the content exceeds 0.2%, the hardenability increases, martensite is formed, and thermal stability decreases. This can be disadvantageous from the economical standpoint and the standpoint of ensuring weldability.

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

[0050] The steel of the present invention may contain the remaining iron (Fe) and unavoidable impurities in addition to the above-mentioned composition. As unavoidable impurities may be unintentionally mixed in during normal manufacturing processes, It is impossible to eliminate such impurities. Since the details of the above are readily apparent to anyone, the entire contents of the above will not be specifically mentioned in this specification.

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

[0052] One or more of chromium (Cr), vanadium (V), nickel (Ni) and boron (B) By adding the above, solid solution strengthening and precipitation strengthening can be further achieved. If the total content of the above elements exceeds 0.5%, the ductility and formability of the steel will be insufficient, which will be disadvantageous from the cost perspective. This may be the case.

[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 in terms of the |K| value in relation 1 depends on the external stress applied to the steel at a given temperature. It is based on the deformation resistance of steel against force. For example, in steel, high temperature compression test or During the high-temperature tensile test, the material is heated at a constant rate and at the same time, a constant deformation is applied. Apply an external force at a constant speed and measure the force acting on the material per unit area. The stress-temperature curve indicates the temperature sensitivity of the steel, and in particular, its slope |K| The value can be judged as an inherent physical property of steel.

[0055] The inventors have calculated the |K| value corresponding to the slope of the obtained stress-temperature curve for various steel materials. As a result of experiments and measurements, we were able to derive the following relational expression 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 weights of the corresponding alloying elements. %.)

[0056] The |K| value in the relational expression 1 of the present invention is the gradient of the dynamic strength value measured during the heat treatment temperature rise of the steel material. Therefore, it is an inherent physical property of steel, and in the present invention, if the |K| value exceeds 0.8, thermal stability is poor. This may result in an insufficient strength, which may increase the change in yield ratio before and after heat treatment at 100 to 600°C. If the value is less than 0.6, it may be difficult to obtain high strength properties, so it is more preferable. It can be set to 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 is expressed as follows when the above relational expression 1 and the following relational expression 2 are simultaneously satisfied: If the RA value of Relational Formula 2 is less than 5, the microstructure of the steel sheet may be deteriorated. The fine structure contains an increased number of precipitates with a diameter of 50 nm or more, which leads to insufficient thermal stability, and the value is 9.2 If the content exceeds 100%, the effect of improving thermal stability decreases, and the addition of a large amount of expensive alloying 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 fraction of the microstructure are based on the area. .

[0061] The microstructure of the steel according to one aspect of the present invention is at least 95% by area ferrite and the remainder perlite. The CN value defined by the following relational formula 3 can be 1.5 or more. do.

[0062] If the area fraction of ferrite is less than 95%, excessive pearlite and other structural forms will occur. The formation of the granular structure reduces the formability, and the pearlite fraction in the structure decreases during heat treatment at 600°C or less. However, there is a problem that the thermal stability is reduced, such as a significant increase in the temperature. The solubility can be increased to 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. The present invention is directed to the formation of a ferrite grain with a crystalline structure to ensure strength and thermal stability. It is characterized by mainly utilizing the coherent precipitates that are formed.

[0064] The standard for precipitates with an average diameter of 50 nm or less is relatively coarse, exceeding 50 nm. In the case of precipitates, they can be a factor that reduces the impact resistance and formability of the steel sheet. The formation of coarse precipitates exceeding 50 nm is suppressed, and fine precipitates of 50 nm or less are formed.

[0065] If the CN value of the following relational expression 3 is less than 1.5, there is a fine alignment at the grain boundary relative to the grain interior. This means that precipitates are not formed sufficiently, and the solid solubility of carbon in the grains is relatively high. The formation and growth of coarse carbides at grain boundaries becomes easier, resulting in the occurrence of grain boundary embrittlement. However, problems such as reduced impact resistance, moldability, and thermal stability may occur. More preferably, it is 3 or more, further preferably 5 or more, and particularly preferably 10 or more. Cut. [Equation 3] CN=N GB x10 3 xN G -1 (where N GB and N G are the unit area (1 mm 2 ) Ferrite grain boundaries and the number of precipitates with a diameter of 50 nm or less that form within the 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 is produced by reheating a steel slab satisfying the above-mentioned alloy composition, hot rolling it, It can be manufactured by cooling, winding, and cooling again.

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

[0069] If the reheating temperature is less than 1100°C, precipitates containing Ti, Nb, Mo, and V will not be formed sufficiently. It is not redissolved, and the formation of fine precipitates is reduced in the process after hot rolling, leaving coarse TiN. On the other hand, if the temperature exceeds 1350°C, austenite grain growth may occur. This may result in a decrease in strength.

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

[0071] If the rolling finish temperature exceeds 1150°C, the temperature of the steel sheet will become too high, causing the grain size to decrease. On the other hand, if the temperature is less than 850°C, the surface quality of the steel sheet may deteriorate. If there is too much recrystallization delay, elongated grains develop, resulting in severe anisotropy and poor formability. It may get worse.

[0072] Cooling and winding The above hot-rolled steel sheet is cooled at a rate of 10-70°C / s to a temperature range of 550-700°C. After cooling to 100°C, it can be wound up.

[0073] If the cooling end temperature and coiling temperature are less than 550°C, bainite in the steel is unnecessarily formed, The precipitation strengthening effect of steel is greatly reduced, and MA (Martensite & Austenite) On the other hand, if the temperature exceeds 700°C, The ferrite grains become coarse, and coarse precipitates and pearlite are easily formed, resulting in increased strength. It is difficult to ensure a high temperature, and the formability may be poor. After cooling to room temperature, the film can be wound up.

[0074] If the cooling rate is less than 10°C / s, the crystal grains in the matrix structure become coarse and the microstructure becomes uneven. However, when the rate exceeds 70°C / s, bainite and martensite are formed. This can lead to severe deviations in the strength of the steel, which can reduce formability. be.

[0075] cooling The coiled steel sheet is cooled to a temperature of 500°C or less at a cooling rate of 10 to 50°C / h. The cooling may be initiated within 30 minutes after winding.

[0076] Steel sheets coiled at high coiling temperatures in the range of 550 to 700°C maintain a high temperature for a long time. This can cause the precipitates to become coarse, so in the present invention, in order to prevent this, Cooling can be started within 30 minutes of the start of the test. If the cooling rate is less than 10°C / h, the The effect above cannot be obtained if the temperature is maintained for a long time, whereas if the temperature is maintained for a long time, the effect above cannot be obtained. This results in the local formation of bainite or martensite, which leads to severe deviation in the strength of the steel. , the moldability may be reduced.

[0077] The present invention may further include pickling and oiling the cooled steel sheet. The pickled and oiled steel sheet is then heated to a temperature range of 450 to 740°C to melt the steel sheet. A further step of galvanizing may be included.

[0078] The hot dip galvanizing can be carried out using a zinc-based plating bath, and the alloy composition of the plating bath is Although there is no particular limitation on the content, for example, the plating bath contains, by weight %, Mg: 0.01 to 30%, It may contain 0.01 to 50% Al, the remainder Zn, and inevitable impurities.

[0079] The steel sheet of the present invention manufactured as described above has a tensile strength of 590 MPa or more and an elongation The yield ratio is 19% or more, the yield ratio is 0.8 or more, and the yield ratio is 100 to 6% of the yield ratio before the heat treatment. The rate of change in yield ratio after heat treatment at 00°C is 10% or less, which gives it excellent thermal stability, high yield ratio and It can have high strength properties.

[0080] The present invention will be described in more detail below with reference to examples. However, the following examples are not intended to be limiting of the present invention. The present invention is not intended to limit the scope of the present invention. It is important to note that this is not the case. [Example]

[0081] Table 1 below shows the alloying elements for each steel type, the |K| value in relational formula 1, and the RA value in relational formula 2. The |K| value in the relational expression 1 in the present invention is measured by applying a high-temperature compression test, and 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. At the same time, a 30% deformation was applied 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 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 weights of the corresponding alloying elements. %.) [Equation 2] RA=([Ti] / 48+[Mo] / 96+[V] / 51) / ([Nb] / 93) (where [Ti], [Mo], [V] and [Nb] are the weight percentages of the alloying elements.) .)

[0083] The steel types in Table 1 above were used to manufacture steel sheets at the rolling end 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, and the The cooling rate is 10 to 70°C / s, and the cooling rate of the steel sheet after coiling is 10 to 50°C. / h, and cooling was initiated within 30 minutes after winding.

[0084] Table 2 below shows the phase fractions of the microstructure of the steel sheet after cooling and before heat treatment, and the CN values ​​of Relation 3. Ferrite (F), bainite (B), martensite (M) and pearlite The (P) fraction was measured at 1 / 4 of the thickness of each steel type using SEM at x3000 and x500. The results were analyzed at 0 magnification. Here, bainite includes low-temperature ferrite and Ferrite contains carbides with a diameter of 0.1 μm or more. The distribution of precipitates with a diameter of 50 nm or less formed at grain boundaries is calculated based on the unit area (1 mm 2 ) 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 Gare the unit area (1 mm 2 ) Ferrite grain boundaries and the number of precipitates with a diameter of 50 nm or less that form within the grains.)

[0086] Table 3 below shows the mechanical property values ​​before and after the heat treatment. The tensile strength (TS), elongation at break (El), and yield ratio (yield strength / tensile strength) are shown. The change in yield ratio before and after heat treatment was calculated and shown. The test pieces were taken perpendicular to the rolling direction and tested. The physical properties shown in Table 3 were The results were evaluated at room temperature, and the heat treatment was performed by maintaining the temperature at 500°C for 60 minutes and then air-cooling to room temperature. Applied.

[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] The invention steels 1 to 7 satisfying the alloy composition and manufacturing method proposed in the present invention are shown in Tables 2 and 3. As described above, all of the mechanical properties targeted by the present invention were ensured.

[0089] On the other hand, comparative steels 1 to 3 do not satisfy relational formula 1 of the present invention, and comparative steels 1 and 2 are excessively If the C or Mn content is too high, it will fall outside the range of the composition of the present invention, and coarse carbides will be formed, resulting in the formation of particles. After heat treatment, the carbides became coarser and grain growth occurred, resulting in an increase in tensile strength. The temperature drop is large, the yield point phenomenon occurs, and the yield strength also increases slightly. The ratio exceeded 10%, resulting in a decrease in thermal stability. Comparative Steel 3 did not satisfy the alloy composition range of the present invention. However, the relational expression 1 was not satisfied, and the carbide and pearlite structure were excessive. The ferrite fraction did not satisfy the range of the present invention, and the fine precipitates at the grain boundaries and within the grains also decreased, so that the relation 3 was not satisfied, resulting in reduced thermal stability.

[0090] Comparative steels 4 and 5 did not satisfy the range of relational expression 2 of the present invention. Although the range of relation 2 was not satisfied and the precipitate fraction increased, the shape of precipitates larger than 50 nm at the grain boundaries The amount of crystal grains is so large that the relational expression 3 is not satisfied, which causes uneven grain growth during heat treatment. Comparative steel 5 is outside the range of relational formula 2, and the thermal stability characteristics are deteriorated. In comparison, there were insufficient fine precipitates at the grain boundaries, and relational expression 3 was not satisfied. The ferrite fraction was insufficient due to the formation of bainite. The ratio of 10% was higher than 10%, which was due to the lack of stability of the grain boundaries during heat treatment, which led to the occurrence of grain growth. This is because the bainite formed by the annealing process deteriorates.

[0091] Comparative steels 6 and 7 met the alloy composition proposed in 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 and fell outside the range proposed by the present invention. In the initial microstructure, pearlite was formed and the precipitates were coarse. The precipitates after heat treatment tend to become coarser, and the tensile strength decreased due to heat treatment. In the case of Comparative Steel 7, the cooling end temperature was too low and fell outside the range proposed by the present invention. Bainite and martensite are formed, and as a result, the yield ratio before heat treatment falls within the range of the present invention. The change in YR ratio before and after the heat treatment also exceeded the range of the present invention.

[0092] Although the present invention has been described in detail above using the embodiments, other embodiments are also possible. Therefore, the spirit and scope of the following claims are not limited to the examples. Not determined.

Claims

1. In weight percent, C: 0.02 to 0.08%, Si: 0.01 to 0.5%, Mn: 0.8 to 1 .. 8%, Al: 0.01-0.1%, P: 0.001-0.02%, S: 0.001-0 .. 01%, N: 0.001 to 0.01%, Ti: 0.0005 to 0.13%, Nb:

0. 0.005 to 0.06%, Mo: 0.001 to 0.2%, the balance 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 high-strength steel plate having a CN value defined by the following relational expression 3 of 1.5 or more. [Relationship 1] |K|=-0.555-1.27[C]+0.043[Si]-0.113[Mn]+ 0.08 [T] + 0.086 [N] ２ (Here, [C], [Si], [Mn], [Ti] and [Nb] are the weight amounts 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 ) Ferrite grain boundaries and the number of precipitates with a diameter of 50 nm or less formed within the crystal grains.)

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

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

4. The steel sheet has a yield ratio change rate after heat treatment at 100 to 600°C relative to the yield ratio before heat treatment. The high strength steel plate according to claim 1, wherein the content of Cr is 10% or less.

5. In weight percent, C: 0.02 to 0.08%, Si: 0.01 to 0.5%, Mn: 0.8 to 1 .. 8%, Al: 0.01-0.1%, P: 0.001-0.02%, S: 0.001-0 .. 01%, N: 0.001 to 0.01%, Ti: 0.0005 to 0.13%, Nb:

0. 0.005 to 0.06%, Mo: 0.001 to 0.2%, the balance and unavoidable impurities, The |K| value defined by the relational expression 1 is 0.8 or less, and the RA value defined by the following relational expression 2 is 5-9.2 Reheating the steel slab to a temperature range of 1100-1350°C; hot rolling the reheated steel slab at a rolling finish temperature of 850 to 1150°C; The hot-rolled steel sheet is cooled to a temperature range of 550 to 700°C at a cooling rate of 10 to 70°C / s. cooling to 100°C and then winding; and The coiled steel sheet is cooled to a temperature of 500°C or less at a cooling rate of 10 to 50°C / h. A method for manufacturing a high-strength steel plate, comprising the steps of: [Relationship 1] |K|=-0.555-1.27[C]+0.043[Si]-0.113[Mn]+ 0.08 [T] + 0.086 [N] ２ (Here, [C], [Si], [Mn], [Ti] and [Nb] are the weight amounts 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 steel plate further contains one or more of Cr, V, Ni and B in a total content of 0.5% or less. The method for producing a high strength steel plate according to claim 5,

7. 6. The method according to claim 5, wherein the cooling step is initiated within 30 minutes after winding. A method for manufacturing the above high-strength steel plate.

8. The high strength steel according to claim 5, further comprising the steps of pickling and oiling the cooled steel plate. How the board is manufactured.

9. The pickled and oiled steel sheet is heated to a temperature range of 450 to 740°C, and then molten zinc is added. The method for producing a high strength steel plate according to claim 8, further comprising the step of lead plating.

10. The hot dip galvanizing contains, by weight, Mg: 0.01 to 30%, Al: 0.01 to 50%, and The high strength plating bath according to claim 9, wherein the plating bath contains Zn and the balance Zn and unavoidable impurities. Steel plate manufacturing method.

Images