KR20150051839A - High strength hot rolled steel sheet having excellent impact resistance and formability of edge part and method for manufacturing the same - Google Patents

Abstract

Disclosed are a high strength hot-rolled steel sheet with excellent impact resistance and formability on an edge part, and a manufacturing method thereof. According to an aspect of the present invention, the high strength hot-rolled steel sheet with excellent impact resistance and formability on an edge part comprises: 0.03-0.1 wt% of C; 0.01-1.2 wt% of Si; 1.2-1.9 wt% of Mn; 0.01-0.08 wt% of Al; 0.005-0.8 wt% of Cr; 0.01-0.12 wt% of Mo; 0.001-0.05 wt% of P; 0.001-0.005 wt% of S; 0.001-0.01 wt% of N; 0.001-0.15 wt% of at least one selected from groups formed of Ti, Nb, and V; and the remainder consisting of Fe and inevitable impurities. Each or carbonitrides of Ti, Nb, and V with 100 nm or greater of a diameter of 3x108 unit/cm^2 or less. The product of a tensile strength (TS) and a shared-edge bending ratio (SBR) is 40,000 MPa·% or greater.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high strength hot-rolled steel sheet excellent in impact resistance and edge formability, and a method for manufacturing the same. BACKGROUND ART < RTI ID = 0.0 >

The present invention relates to a high-strength hot-rolled steel sheet suitable for use as a steel for chassis structures such as a lower arm, a suspension, and a wheel disc among automobile parts, and a manufacturing method thereof.

BACKGROUND ART [0002] In order to impart a high strength and light weight effect to an automobile, much research has been conducted on materials of various parts constituting an automobile. On the other hand, as a steel for a chassis structure such as a lower arm, a suspension, and a wheel disc of an automobile part, a ferrite-martensite two-phase composite structure steel or a high strength A ferrite-bainite two-phase composite structure steel excellent in flangeability is being applied.

On the other hand, in Patent Documents 1 to 3, on the basis of Si-Mn and Mn-P-Cr components, hot rolled and maintained at the ferrite transformation zone for several seconds and cooled to a martensite transformation start temperature (Ms) Ferrite and martensite composite structure, and in Patent Document 4, after hot-rolling based on Si-Mn-Cr component system or Si-Mn-Cr-Mo component system, it is held for several seconds in the ferrite transformation region, (Ms) or more to form a composite structure of ferrite and martensite to secure both ductility and strength.

However, the ferrite-martensite two-phase composite steel as described above has a problem that its application to automobile chassis parts requiring excellent stretch flangeability is limited because elongation flangeability is weakened.

On the other hand, Patent Document 5 discloses that polygonal ferrite and bainite composite structure are formed by cooling to a temperature of about 700 캜 after hot rolling and then cooling by air for a certain period of time and then cooling and winding to form a composite structure of polygonal ferrite and bainite, At the same time.

However, the ferrite-bainite two-phase complex-structured steel as described above mainly improves elongation flangeability and strength of hot-rolled steel sheet by utilizing alloy components such as Si, Mn, Al and Mo, It causes segregation of the alloy component and unevenness of the microstructure, and also causes severe segregation in the slab after casting, thereby deteriorating the endothelial property and the impact resistance characteristic.

In addition, different microstructures are formed depending on the cooling conditions, and particularly in the edge part where the cooling rate is higher than that in the center of the hot-rolled steel sheet when cooling the hot-rolled steel sheet immediately after hot rolling, the surface of martensite or bainite The fractions thereof are greatly increased, so that ductility, bending workability and elongation flangeability are all deviated. As described above, when the edge formability of the hot-rolled steel sheet is inferior, the edge portion must be removed, resulting in a reduction in productivity due to an unnecessary process and a reduction in the inspection capacity of the hot-rolled steel sheet, which is economically disadvantageous.

Furthermore, when excessive amounts of alloying elements such as Ti, Nb, V, and W used for additional strength improvement are added, coarse precipitates remain or dynamic deformation organic precipitation occurs, so that the deformation resistance during hot rolling is drastically increased, And the precipitation strengthening effect is reduced, so that the desired strength can not be secured.

Japanese Patent Application Laid-Open No. 1995-278731 Japanese Laid-Open Patent Publication No. 1997-241790 Japanese Laid-Open Patent Publication No. 1994-049591 U.S. Patent No. 4502897 Japanese Laid-Open Patent Publication No. 1994-293910

One aspect of the present invention is to provide a high-strength hot-rolled steel sheet excellent in impact resistance and edge formability by controlling the composition and manufacturing method of the steel sheet to suppress formation of coarse carbonitride and controlling the microstructure of the steel sheet, .

In order to accomplish the above object, one aspect of the present invention provides a method of manufacturing a semiconductor device, comprising: 0.03 to 0.1% of C, 0.01 to 1.2% of Si, 1.2 to 1.9% of Mn, 0.01 to 0.08% of Al, At least one selected from the group consisting of Ti, Nb and V, in an amount of 0.005 to 0.8%, Mo: 0.01 to 0.12%, P: 0.001 to 0.05%, S: 0.001 to 0.005% 0.001 to 0.15% in total, the balance Fe and inevitable impurities,

An impact resistance characteristic in which the number of Ti, Nb and V alone or complex carbonitrides having a diameter of 100 nm or more is 3 x ^{10 8} / cm 2 or less, the product of TS (tensile strength) and SBR (sheared-edge bending ratio) is 40,000 MPa ·% or more, Strength hot-rolled steel sheet excellent in strength.

In another aspect of the present invention, there is provided a ferritic stainless steel comprising, by weight, 0.03 to 0.1% of C, 0.01 to 1.2% of Si, 1.2 to 1.9% of Mn, 0.01 to 0.08% of Al, 0.005 to 0.8% 0.001 to 0.15% in total of at least one selected from the group consisting of Ti, Nb and V, including 0.01 to 0.12% of P, 0.001 to 0.05% of S, 0.001 to 0.005% of S and 0.001 to 0.01% And continuously casting molten steel containing the remaining Fe and unavoidable impurities to obtain a slab; Cooling the slab so as to satisfy the condition of the following expression (1); Reheating the cooled slab to 1200 to 1300 占 폚; Hot-rolling the reheated slab at a temperature equal to or greater than Ar3 to obtain a hot-rolled steel sheet; Cooling the hot-rolled steel sheet to 550 to 750 ° C at a cooling rate of 10 to 100 ° C / s; Air cooling the primary cooled hot rolled steel sheet for 4 seconds or more; And winding the air-cooled hot-rolled steel sheet at 300 to 500 ° C,

The step of obtaining the hot-rolled steel sheet provides a method of manufacturing a high-strength hot-rolled steel sheet excellent in impact resistance characteristics and edge formability including finishing hot rolling so as to satisfy the following conditional expression (2).

[Relation 1]

Cr (° C./sec) = 45.5 - 56.1 [C] + 2.1 [Si] - 19.2 [Mn] - 8.9 [Cr] + 8.0 [

(Wherein CR represents the cooling rate of the slab, and [C], [Si], [Mn], [Cr], [Al], and [Mo]

[Relation 2]

(FET-FDT) (C) = 166 - 456 [C] - 27.9 [Mn] + 4.39 [Si] - 28.5 [Mo] - 28.2 [Ti]

(C), [Mn], [Si], [Mo], [Ti] and [Nb] of the element are the same as those of the element Content (% by weight)

In addition, the solution of the above-mentioned problems does not list all the features of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS The various features and advantages and effects of the present invention will become more fully understood with reference to the following specific embodiments.

According to the present invention, a high-strength hot-rolled steel sheet having an impact energy of 60 J or more at -20 캜, a product of TS (tensile strength) and SBR (Shared-edge Bending Ratio) of 40,000 MPa ·% Can be provided.

1 is a graph showing tensile strength (TS) x edge formability (SBR) values and impact energy values of Comparative Examples 1 to 9 and Inventive Examples 1 to 6.

As a result of deep research to solve the problems of the prior art described above, the inventors of the present invention have found that it is possible to appropriately control an alloy component, to control the cooling rate CR of the slab after casting, the start temperature (FET) It was confirmed that the impact resistance and edge formability of the hot-rolled steel sheet can be secured at the same time, and the present invention has been accomplished.

Hereinafter, the 'edge portion' means a portion from the edge portion of the hot-rolled steel sheet up to 50 mm from the plate center portion, and the 'center portion' means the remaining portion except the edge portion of the hot-rolled steel sheet.

Hereinafter, a high-strength hot-rolled steel sheet excellent in impact resistance and edge formability, which is one aspect of the present invention, will be described in detail.

First, the alloy composition of the hot-rolled steel sheet of the present invention will be described in detail.

Carbon (C): 0.03 to 0.1 wt%

Carbon is the most economical and effective element to strengthen the steel. In the case of ferrite-bainite composite steel, the content of bainite phase increases and the tensile strength of the steel increases. When the content of carbon is less than 0.03% by weight, the ferrite formation rate during cooling after hot rolling is too high to form bainite. On the other hand, when the content of carbon exceeds 0.1% by weight, There is a problem that the bainite phase fraction becomes too high at the edge portion or martensite is formed and the edge formability becomes poor. Therefore, the carbon content is preferably limited to 0.03 to 0.1% by weight.

Silicon (Si): 0.01 to 1.2 wt%

Silicon is an element added for deoxidizing molten steel and improving the strength by solid solution strengthening. Silicon is an element stabilizing ferrite, which is effective in accelerating ferrite transformation during cooling after hot rolling, and is an element effective in increasing the ferrite phase fraction constituting the base of the ferrite-bainite composite structure steel. When the content of silicon is less than 0.01% by weight, the effect of stabilizing ferrite is small and it is difficult to make the base structure into a ferrite structure. On the other hand, when the content of silicon exceeds 1.2% by weight, a red color scale is formed on the steel sheet surface by hot rolling There is a problem that not only the surface quality of the steel sheet is deteriorated but also ductility and weldability deteriorate. In addition, excess silicon greatly delays formation of carbide, so that excessively coagulated austenite remains in the edge portion of the steel sheet having a relatively high cooling rate to form a metastable austenite phase or a martensite phase, There is a problem in that the formability of the shear deformed portion of the edge portion is degraded. Therefore, the content of silicon is preferably limited to 0.01 to 1.2% by weight.

Manganese (Mn): 1.2 to 1.9 wt%

Manganese, like silicon, is an effective element for strengthening the steel and enhances the hardenability of the steel to facilitate the formation of bainite during cooling after hot rolling. In order to exhibit such effects, the content of manganese in the present invention is preferably 1.2 wt% or more. On the other hand, if it exceeds 1.9% by weight, there is a problem that the ferrite transformation is excessively delayed to make it difficult to secure a proper fraction of ferrite constituting the base of the ferrite-bainite composite steel. In the casting process, There is a problem that the stone part is greatly developed and the impact resistance characteristic of the final product is impaired. Further, the curing ability during cooling after hot rolling is excessively increased, and the bainite phase fraction is excessively increased at the edge portion, or martensite is formed and the edge formability is poor. Accordingly, the content of manganese is preferably limited to 1.2 to 1.9 wt%.

Aluminum (Sol.Al): 0.01 to 0.08 wt%

Aluminum is an element added mainly for deoxidation, and in order to exhibit such an effect in the present invention, it is preferable that the aluminum content is 0.01 wt% or more. On the other hand, when the content is more than 0.08% by weight, corner cracks are likely to occur in the slab during continuous casting, defects due to formation of inclusions are likely to occur in the edge portion of the hot-rolled steel sheet, there is a problem. Therefore, the content of aluminum is preferably limited to 0.01 to 0.08% by weight.

Cr (Cr): 0.005 to 0.8 wt%

Chromium is an effective element for strengthening the steel, and it plays a role in retarding the ferrite phase transformation during cooling after hot rolling to help form bainite in the winding phase. In order to exhibit such effects in the present invention, it is preferable that the content of chromium is 0.005 wt% or more. On the other hand, when the content exceeds 0.8% by weight, the ferrite transformation is excessively retarded to excessively increase the bainite phase fraction, and the ductility and elongation flangeability of the hot-rolled steel sheet become poor. Especially, There is a problem in that it becomes more inferior. Also, the impact resistance characteristics of the hot-rolled steel sheet may be adversely affected. Therefore, the content of chromium is preferably limited to 0.005 to 0.8% by weight.

Molybdenum (Mo): 0.01 to 0.12 wt%

Molybdenum is an element that facilitates the formation of bainite structure by increasing the hardenability of steel. In order to exhibit such effects, the content of the molybdenum is preferably 0.01 wt% or more. On the other hand, if the content exceeds 0.12% by weight, the impact resistance and weldability are deteriorated due to an increase in the extrudability, which is economically disadvantageous. In addition, the curing ability during cooling after hot rolling is excessively increased, so that the bainite phase fraction is excessively increased at the edge portion, or martensite is formed, and edge formability is poor. Therefore, the content of the molybdenum is preferably limited to 0.01 to 0.12% by weight.

Phosphorus (P): 0.001 to 0.05 wt%

Phosphorus is a very important element in the ferrite-bainite composite structure steel because it has the effect of strengthening the solution and promoting the ferrite transformation similarly to silicon. However, when the content of phosphorus is less than 0.001 wt%, it is insufficient to obtain the desired strength. When the content of phosphorus exceeds 0.05 wt%, the band structure due to micro segregation causes deterioration of ductility of hot rolled steel sheet, . Therefore, the content of phosphorus is preferably limited to 0.001 to 0.05% by weight.

Sulfur (S): 0.001 to 0.005 wt%

Sulfur is an impurity present in the steel and is preferably controlled as low as possible. Theoretically, it is advantageous to control the sulfur content to 0%, but it is inevitably contained inevitably in the manufacturing process. In order to make the content of sulfur to 0.0005% by weight or less, it takes too much time for steelmaking and the productivity is lowered. On the other hand, when the content of sulfur exceeds 0.005% by weight, it forms a nonmetallic inclusion by binding with Mn and the like, and thus the stretch flangeability and impact resistance characteristics of the hot-rolled steel sheet are inferior. Particularly, there is a problem that the stretch flangeability and the impact resistance characteristics of the edge portion of the hot-rolled steel sheet are further reduced. Therefore, the sulfur content is preferably limited to 0.001 to 0.005% by weight.

Nitrogen (N): 0.001 to 0.01 wt%

Nitrogen is a typical solid solution strengthening element together with carbon and forms coarse precipitates together with titanium, aluminum and the like. Generally, the solid solution strengthening effect of nitrogen is superior to carbon, but the toughness is significantly decreased as the amount of nitrogen in the steel is increased. If the content of nitrogen is less than 0.001% by weight, it takes a long time to perform the steelmaking process, resulting in a low productivity. On the other hand, when the content exceeds 0.01% by weight, segregated or coarse nitride is formed in grain boundaries to cause brittleness there is a problem. Therefore, the content of nitrogen is preferably limited to 0.001 to 0.01% by weight.

On the other hand, it is preferable to include at least one selected from the group consisting of Ti, Nb and V. In this case, the sum of the contents of Ti, Nb and V is preferably limited to 0.001 to 0.15% in total.

Ti, Nb and V are effective components for refining the crystal grains, and Ti exists as TiN in the steel and has an effect of inhibiting the growth of crystal grains during the heating process for hot rolling. Also, it is a useful component to improve the strength of steel by forming TiC precipitate by reacting with nitrogen and remaining Ti solved in steel and bonding with carbon.

Nb and V form carbides in the steel, which are effective in grain refinement and form fine precipitates to improve the strength and toughness of the steel. In addition, it also stabilizes the elements such as C and N that increase local variations in microstructure and physical properties due to segregation in the steel, thereby improving the impact resistance.

The remainder Fe and unavoidable impurities. Addition of an effective component other than the above-mentioned composition is not excluded.

Hereinafter, the microstructure and precipitates of the hot-rolled steel sheet according to the present invention will be described in detail.

In the hot-rolled steel sheet according to the present invention, the microstructure of the center portion of the hot-rolled steel sheet has a cross-sectional area ratio of 5 to 20% with the bainite having ferrite as a base structure, and the microstructure of the hot- It is preferable that the cross sectional area ratio is 5 to 25% and that the martensite is less than 3% at the cross sectional area ratio. By securing the microstructure as described above, the impact resistance characteristic and the edge formability can be secured at the same time.

In the hot-rolled steel sheet according to the present invention, the Ti, Nb and V alone or composite carbonitrides having a diameter of 100 nm or more are preferably 3 x ^{10 8} / cm 2 or less. By reducing the coarse carbonitride, the local concentration of stress on the external impact is suppressed, and the impact resistance of the hot-rolled steel sheet is improved.

On the other hand, the hot-rolled steel sheet according to the present invention as described above can secure a product of TS (tensile strength) and SBR (Shear-edge Bending Ratio) of 40,000 MPa ·% or more, It is possible to prevent a decrease in the productivity due to the hot-rolled steel sheet and a decrease in the inspection capacity of the hot-rolled steel sheet, which is economically advantageous.

On the other hand, the hot-rolled steel sheet according to the present invention as described above can secure an impact energy of 60 J or more at -20 캜 and can be suitably applied to automobile chassis parts and structural members.

On the other hand, the hot-rolled steel sheet according to the present invention can also be used as a hot-dip galvanized steel sheet by forming a hot-dip galvanized layer on its surface.

Hereinafter, a method of manufacturing a high-strength hot-rolled steel sheet excellent in impact resistance and moldability, which is another aspect of the present invention, will be described in detail.

The step of obtaining the slab and the step of cooling

Continuous casting of molten steel satisfying the above composition is performed to obtain a slab, and the slab is cooled to satisfy the following relational expression (1). This is to prevent formation of coarse carbides and nitrides by avoiding ferrite transformation occurring during cooling and suppressing the segregation or diffusion of the steel components generated during the phase transformation. Furthermore, the slab during re-heating in the heating as will allow the re-employment of the alloying elements to obtain a relatively low reheating rapidly occurs at a temperature to uniform and fine austenite phase, and after hot rolling, coarse precipitates diameter than 100nm in the hot-rolled steel sheet 3x10 ⁸ Cm < 2 > or less, and the impact resistance characteristics are improved.

[Relation 2]

Cr (° C./sec) = 45.5 - 56.1 [C] + 2.1 [Si] - 19.2 [Mn] - 8.9 [Cr] + 8.0 [

(Wherein CR represents the cooling rate of the slab, and [C], [Si], [Mn], [Cr], [Al], and [Mo]

Steps to reheat

The cooled slab is reheated at a temperature of 1200 to 1300 ° C. If the reheating temperature is less than 1200 ° C., the precipitates are not sufficiently reused, so that precipitates such as NbC and TiC are reduced in the process after the hot rolling. If the reheating temperature is higher than 1300 ° C., the austenite grains undergo abnormal grain growth, The reheating temperature is preferably limited to 1200 to 1300 ° C.

Step of obtaining hot-rolled steel sheet

The reheated slab is hot-rolled at a temperature equal to or higher than Ar3, and subjected to finish hot rolling to obtain the hot-rolled steel sheet satisfying the following relational expression (2). This is because it is possible to minimize the precipitation strengthening effect after hot rolling by minimizing precipitation of precipitates due to dynamic deformation of the steel sheet during hot rolling and to maximize the strain energy in the steel sheet during hot rolling to obtain fine and uniform ferrite So that the crystal grains are formed.

[Relation 2]

(FET-FDT) (C) = 166 - 456 [C] - 27.9 [Mn] + 4.39 [Si] - 28.5 [Mo] - 28.2 [Ti]

(C), [Mn], [Si], [Mo], [Ti] and [Nb] of the element are the same as those of the element Content (% by weight)

Cooling step

The hot-rolled steel sheet is first cooled to 550 to 750 ° C at a cooling rate of 10 to 100 ° C / sec. When the temperature is lower than 550 占 폚 in the primary cooling step, bainite phase, metastable austenite phase, and martensite phase are excessively formed at the edge portion of the hot-rolled steel sheet, and the ductility of the edge portion is greatly deteriorated. On the other hand, when it exceeds 750 ° C, coarse ferrite and pearlite structure are formed, and desired strength can not be obtained. When the cooling rate is less than 10 ° C / sec, coarsening of the ferrite grains occurs and the precipitates are coarsened, making it difficult to obtain the desired strength. On the other hand, when the cooling rate exceeds 100 ° C / sec, the low temperature ferrite fraction increases, There is a problem.

The primary cooled hot-rolled steel sheet is air-cooled for 4 seconds or more. If air is not cooled for more than 4 seconds after the primary cooling, the ferrite structure is not sufficiently formed and the ductility of the hot-rolled steel sheet is greatly deteriorated.

Winding  step

Thereafter, the air-cooled hot rolled steel sheet is wound at 300 to 500 ° C. When the coiling temperature is less than 300 ° C, martensite is excessively formed to increase the strength of the steel sheet excessively, and particularly, the moldability of the hot-rolled steel sheet edge portion having a high cooling rate is remarkably lowered. On the other hand, when the temperature exceeds 500 ° C, the bainite phase fraction becomes too low, and coarse carbides are formed, and the desired strength can not be secured. Therefore, the coiling temperature is preferably limited to 300 to 500 占 폚.

On the other hand, the hot rolled steel sheet wound as described above can be manufactured as a pickling steel sheet through a step of removing the surface layer scale by pickling treatment after air-cooling at room temperature (25 캜) to 200 캜 and casting. If the temperature exceeds 200 占 폚 at the pickling step, there is a problem that the surface layer portion of the hot-rolled steel sheet is over-saponified to degrade the surface layer roughness. Therefore, the pickling temperature is preferably limited to 200 占 폚 or lower.

After the winding or pickling, the hot-rolled steel sheet may be heated at 450 to 480 ° C and passed through a hot-dip galvanizing bath to produce a hot-dip galvanized steel sheet. If the heating temperature is lower than 450 ° C, unplating tends to occur. On the other hand, if the heating temperature is higher than 480 ° C, plating defects may occur or it may be difficult to uniformly manufacture the thickness of the plating layer. Therefore, the heating temperature is preferably limited to 450 to 480 ° C.

Hereinafter, the present invention will be described in more detail with reference to examples. However, the description of these embodiments is intended only to illustrate the practice of the present invention, but the present invention is not limited thereto.

( Example )

Table 1 below shows the steel slab composition having the composition of the inventive and comparative examples based on the present invention. In Table 2, slab cooling conditions, hot rolling conditions, and coiling temperatures are shown for the steel types shown in Table 1. In Table 2, CR, FET, FDT and CT are the slab cooling rate, the finish hot rolling start temperature, the finish hot rolling finish temperature and the coiling temperature, respectively, and the average cooling rate until the first cooling immediately after hot rolling is 25 ° C / sec, the temperature of the steel sheet during air cooling after the first cooling was 680 to 700 ° C, and the air cooling time was kept constant for 4 to 5 seconds.

Table 3 shows the mechanical property evaluation results and microstructure observation results of the inventive and comparative examples. In Table 3, YS, TS, T-El, and SBR mean yield strength, tensile strength, fracture elongation, and Shared-edge Bending Ratio.

The tensile test was performed on specimens taken in accordance with the JIS No. 5 standard with respect to the rolling direction of the rolled plate in the direction of 90 °. The results in Table 3 below are the average values after three runs. In particular, YS means a 0.2% off-set yield strength or lower yield point.

SBR evaluation was carried out using a specimen having a notch at the center of the specimen and having a shear-deformed circular notch with a radius of curvature of 5 mm. At this time, the test specimens were taken from the edge of the hot rolled plate and prepared parallel to the rolling direction. The test was stopped at the time of cracking at the notch portion and the change was measured. That is, in the SBR, the interval (Lo) when the notch portion interval (Lo = 10 mm) was gradually widened by the bending test and cracks occurred at the notch portion was measured and found by the following Equation 3, It is the average value after execution.

[Relation 3]

SBR = (Lf - Lo) x 100 / Lo

(Where Lo is an initial circular notched portion interval (10 mm), and Lf is a circular notched portion spacing that is deformed when a crack is generated)

The bainite phase fraction of the microstructure was obtained by taking the edge and center specimens from the rolled sheet, etching them with Lepera or Nital etchant, and observing them at 500 magnification using an optical microscope and analyzing them with an image analyzer. The coarse Ti, Nb and V single or complex carbonitrides were prepared by Replica method and observed by TEM (Transmission Electron Microscopy).

In addition, the impact resistance of the hot-rolled steel sheet shown in Table 3 is the result obtained by testing with ASTM Standard E8m-04 standard. The impact test specimens were taken in the vertical direction in the rolling direction, and the impact energy was the smallest among the results of three tests at -20 ° C. When the impact absorption energy value is smaller than 60J, it is judged that the impact resistance characteristic is heat.

1 is a graph showing tensile strength (TS) x edge formability (SBR) values and impact energy values of Comparative Examples 1 to 8 and Examples 1 to 6; The hatched area in Fig. 1 corresponds to the range of the present invention.

Psalter C Si Mn Cr Al P S N Ti Mo Nb V Comparative Example 1 0.05 0.3 1.5 0.008 0.03 0.01 0.006 0.004 0.005 0.01 0.05 0.006 Comparative Example 2 0.06 0.6 1.7 0.01 0.03 0.01 0.005 0.004 0.05 0.01 0.005 0.005 Comparative Example 3 0.06 1.3 1.4 0.007 0.03 0.01 0.004 0.004 0.02 0.01 0.06 0.007 Comparative Example 4 0.07 0.9 1.6 0.008 0.03 0.01 0.003 0.004 0.08 0.01 0.007 0.03 Comparative Example 5 0.07 0.8 1.8 0.012 0.03 0.01 0.003 0.004 0.02 0.01 0.02 0.03 Comparative Example 6 0.08 1.2 1.6 0.02 0.1 0.01 0.003 0.004 0.04 0.01 0.03 0.008 Comparative Example 7 0.09 1.5 1.5 0.3 0.03 0.01 0.003 0.004 0.08 0.02 0.02 0.007 Comparative Example 8 0.09 1.0 1.8 0.5 0.03 0.01 0.003 0.004 0.05 0.01 0.06 0.02 Comparative Example 9 0.07 0.9 1.6 0.008 0.03 0.01 0.003 0.004 0.08 0.01 0.007 0.03 Inventory 1 0.06 0.05 1.5 0.005 0.03 0.0025 0.003 0.005 0.005 0.01 0.04 0.008 Inventory 2 0.04 0.7 1.4 0.3 0.03 0.01 0.003 0.005 0.04 0.05 0.02 0.007 Inventory 3 0.05 1.0 1.7 0.7 0.03 0.01 0.003 0.005 0.06 0.01 0.03 0.007 Honorable 4 0.07 0.1 1.8 0.4 0.03 0.01 0.003 0.005 0.07 0.1 0.02 0.02 Inventory 5 0.08 0.6 1.7 0.4 0.03 0.01 0.003 0.004 0.05 0.01 0.05 0.03 Inventory 6 0.09 0.2 1.6 0.6 0.03 0.01 0.003 0.004 0.07 0.01 0.04 0.02

Psalter CR (° C / sec) ① FET (° C) FDT (占 폚) FET-FDT (占 폚) ② CT (° C) Comparative Example 1 6.9 14.4 1003 895 108 100 450 Comparative Example 2 5.5 10.6 988 899 89 92 455 Comparative Example 3 18.5 17.9 1000 908 92 101 443 Comparative Example 4 10.5 12.6 1005 910 95 90 453 Comparative Example 5 5.2 8.6 1002 908 94 86 450 Comparative Example 6 15.2 13.2 982 902 80 87 520 Comparative Example 7 6.2 11.8 992 911 81 86 441 Comparative Example 8 5.0 3.5 980 887 93 74 438 Comparative Example 9 10.5 12.6 998 905 93 90 285 Inventory 1 15.5 13.4 982 892 90 95 443 Inventory 2 17.4 14.1 987 887 100 108 453 Inventory 3 10.5 5.9 975 886 89 97 460 Honorable 4 5.0 1.2 973 898 75 78 442 Inventory 5 9.2 6.0 980 900 80 80 438 Inventory 6 11.6 4.8 978 904 74 77 445 [Mo] - 8.9 [Cr] + 8.0 [Al] - 26.9 [Mo]
- = 28.9 [Si] - 28.5 [Mo] - 28.2 [Ti] - 51.1 [Nb]

Psalter YS
(MPa) TS
(MPa) T-El
(%) SBR
(%) TSxSBR
(MPa 占%) Central Bainite phase fraction (%) Edge Bainite phase fraction (%) Martensite phase fraction
(%) Number of coarse precipitates
(EA / cm ² ) Shock
Energy (J) Impact resistance Comparative Example 1 498 575 30 64 36800 9 12 0 4.2 x ^{10 8} 57 X Comparative Example 2 522 634 25 56 35504 10 16 3 3.8 x ^{10 8} 52 X Comparative Example 3 572 742 23 48 35616 18 24 5 1.6x10 ⁸ 62 O Comparative Example 4 543 656 23 50 32800 21 21 0 4.3 x ^{10 8} 46 X Comparative Example 5 605 722 21 39 28158 19 23 4 4.5 x ^{10 8} 48 X Comparative Example 6 697 785 18 35 27475 9 12 0 3.4 x ^{10 8} 53 X Comparative Example 7 723 846 18 33 27918 20 27 0 5.8 x ^{10 8} 42 X Comparative Example 8 893 998 15 30 29940 26 31 5 6.2x10 ⁸ 46 X Comparative Example 9 498 704 21 16 11264 8 5 35 5.1x10 ⁸ 41 X Inventory 1 522 605 25 73 44165 11 14 0 1.1 x ^{10 8} 78 O Inventory 2 501 592 26 71 42032 10 15 0 0.9x10 ⁸ 82 O Inventory 3 736 862 21 53 45686 15 21 0 1.7 x ^{10 8} 75 O Honorable 4 742 885 18 56 49560 14 19 0 2.0x10 ⁸ 72 O Inventory 5 794 944 16 48 45312 18 23 One 1.6x10 ⁸ 64 O Inventory 6 804 969 13 46 44574 17 21 One 2.3x10 ⁸ 66 O

In Comparative Examples 1, 2, 4, 5, 7 and 8, all of the steel components corresponded to the composition range proposed by the present invention, but one or both of relational expressions 1 and 2 were not satisfied, so that coarse carbides were generated at grain boundaries, The moldability and the impact resistance were both inferior. In Comparative Example 3, the steel production method satisfied all of the ranges proposed by the present invention, but the Si content in the steel component was excessive, and martensite was excessively formed in the edge portion of the steel sheet, resulting in poor edge formability. Comparative Example 6 satisfied both the steel component and the relational expressions 1 and 2, but the coiling temperature was high and a large amount of coarse precipitates were formed, and a large amount of pearlite structure was formed by the microstructure of the steel sheet, Whereas in Comparative Example 9, the coiling temperature was too low, and martensite was excessively formed in the edge portion of the steel sheet, resulting in poor edge formability and impact resistance characteristics.

On the other hand, all of the inventors satisfied the composition range and the manufacturing method proposed in the present invention, and showed excellent materials and impact resistance characteristics.

The tensile strength (TS) x edge formability (SBR) values and impact energy values of Comparative Examples 1 to 9 and Examples 1 to 6 are shown in Fig.

Claims (8)

The steel sheet according to any one of claims 1 to 3, wherein the steel sheet contains 0.03 to 0.1% of C, 0.01 to 1.2% of Si, 1.2 to 1.9% of Mn, 0.01 to 0.08% of Al, 0.005 to 0.8% of Cr, 0.01 to 0.12% 0.001 to 0.15% in total of at least one selected from the group consisting of Ti, Nb and V, and the balance Fe and unavoidable impurities, in an amount of 0.001 to 0.05%, S to 0.001 to 0.005% and N: 0.001 to 0.01% ,
An impact resistance characteristic in which the number of Ti, Nb and V alone or complex carbonitrides having a diameter of 100 nm or more is 3 x ^{10 8} / cm 2 or less, the product of TS (tensile strength) and SBR (sheared-edge bending ratio) is 40,000 MPa ·% or more, High-strength hot-rolled steel with excellent strength.
The method according to claim 1,
Wherein the microstructure of the center of the hot-rolled steel sheet has a cross-sectional area ratio of 5 to 20%, and the microstructure of the hot-rolled steel sheet has a ferrite base structure, wherein the bainite has a cross- By mass, and martensite is 3% or less at a cross-sectional area ratio and excellent in impact resistance and edge formability.
The method according to claim 1,
Wherein the hot-rolled steel sheet has an impact resistance of 60 J or more at -20 캜 and excellent in edge formability.
The method according to claim 1,
Wherein the hot-rolled steel sheet further comprises a hot-dip galvanized layer and has excellent impact resistance and edge formability.
The steel sheet according to any one of claims 1 to 3, wherein the steel sheet contains 0.03 to 0.1% of C, 0.01 to 1.2% of Si, 1.2 to 1.9% of Mn, 0.01 to 0.08% of Al, 0.005 to 0.8% of Cr, 0.01 to 0.12% 0.001 to 0.15% in total of at least one selected from the group consisting of Ti, Nb and V, and the balance Fe and unavoidable impurities, in an amount of 0.05 to 0.05%, S: 0.001 to 0.005% and 0.001 to 0.01% Continuously cast molten steel to obtain a slab;
Cooling the slab so as to satisfy the condition of the following expression (1);
Reheating the cooled slab to 1200 to 1300 占 폚;
Hot-rolling the reheated slab at a temperature equal to or higher than Ar3 and subjecting the hot-rolled steel sheet to a hot-rolled steel sheet to obtain a hot-rolled steel sheet satisfying the following relational expression (2);
Cooling the hot-rolled steel sheet to 550 to 750 ° C at a cooling rate of 10 to 100 ° C / s;
Air cooling the primary cooled hot rolled steel sheet for 4 seconds or more; And
And a step of winding the air-cooled hot-rolled steel sheet at 300 to 500 ° C, and a method of manufacturing a high-strength hot-rolled steel sheet having excellent impact resistance and edge formability.
[Relation 1]
Cr (° C./sec) = 45.5 - 56.1 [C] + 2.1 [Si] - 19.2 [Mn] - 8.9 [Cr] + 8.0 [
(Wherein CR represents the cooling rate of the slab, and [C], [Si], [Mn], [Cr], [Al], and [Mo]
[Relation 2]
(FET-FDT) (C) = 166 - 456 [C] - 27.9 [Mn] + 4.39 [Si] - 28.5 [Mo] - 28.2 [Ti]
(C), [Mn], [Si], [Mo], [Ti] and [Nb] of the element are the same as those of the element Content (% by weight)
6. The method of claim 5,
Picking up the hot-rolled steel sheet after the winding step; And
Further comprising a step of raising the hot-rolled steel sheet subjected to the pickling treatment, and further comprising a step of raising the pickled hot-rolled steel sheet.
6. The method of claim 5,
Picking up the hot-rolled steel sheet after the winding step; And
Further comprising a step of immersing the hot-rolled steel sheet subjected to the pickling treatment in a hot-dip galvanizing bath at 450 to 480 캜 for hot-dip galvanizing, and a method for producing a high-strength hot-rolled steel sheet excellent in impact resistance and edge formability.
The method according to claim 5 or 6,
Wherein the pickling treatment is carried out at a temperature of 200 DEG C or lower, and a method of manufacturing a high strength hot-rolled steel sheet having excellent impact resistance and edge formability.

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