KR101008099B1 - High strength steel sheet amd galvenized steel sheet having excellent ducility and free edge crack and method of manufacturing the same - Google Patents

Abstract

The present invention is to provide a steel sheet having a tensile strength of 980MPa or more and an elongation of 28% or more and no cracks at edges.

The present invention is in the weight%, C: 0.1 ~ 0.25%, Si: 1.0 ~ 1.9%, Mn: 1.5 ~ 2.5%, Al: 0.5 ~ 1.6%, Ti: 0.005 ~ 0.03%, B: 5 ~ 30ppm, Sb: It comprises 0.01 ~ 0.03%, the remainder is made of Fe and unavoidable impurities, and provides a high strength steel sheet, hot-dip galvanized steel sheet and a method for producing them characterized by satisfying 1.75≤Si + Al≤3.25.

AUTOMOBILE STEEL SHEET, TRANSFORMATION INDUCED PLASTICITY, REMAINED AUSTENITE, MARTENSITE

Description

High strength steel plate with excellent ductility and no cracks at edges, hot-dip galvanized steel sheet and method for manufacturing the same

The present invention relates to a steel sheet for automobiles excellent in strength and workability mainly used in various structural members of automobiles, and a method of manufacturing the same.

More specifically, a method of manufacturing a high strength cold rolled steel sheet and hot dip galvanized steel sheet having a strength of 780 to 980 MPa at the same time and an elongation of at least 28% and no cracking at the edge of the rolled plate after cold rolling. It is about.

Steel plate used as structural member among automobile components should be able to absorb external shocks well and improve the stability of passengers when the vehicle crashes. Recently, steel sheets that are excellent in both strength and formability have been mainly used for such components as tensile strength of 780 MPa or more and elongation of 25% or more, and have high tensile strength, high elongation and low yield ratio (ratio of yield strength / tensile strength). .

Recently, as the environmental pollution problem caused by automobile exhaust gas has emerged, research to achieve light weight of automobiles using ultra high strength steel with tensile strength of 780 MPa or more in the direction of technology development to further improve fuel economy has been increasing.

Automotive high strength steels include Transformation Induced Plasticity (TRIP) and Dual Phase (DP).

The manufacturing process of super high strength steel with excellent workability is divided into slab manufacturing, hot rolling, cooling and winding of hot rolled sheet, cold rolling, and annealing process. After annealing at a temperature above A ₁ transformation point and below A ₃ , and then transforming the austenite formed during the annealing process into martensite by controlling the cooling rate during cooling, this steel is called an abnormal texture. The abnormal tissue steel has a strength determined by the fraction of martensite and ferrite. As the ratio of martensite in the whole structure increases, strength increases and ductility decreases, and therefore, the ratio of martensite must be appropriate. On the other hand, after forming austenite in the annealing process, such as the method for manufacturing the abnormal tissue steel, by controlling the cooling rate and the end of the cooling temperature in the cooling process by increasing the strength and ductility at the same time by remaining austenite at room temperature There is this. The residual austenite thus formed is transformed into martensite during plastic deformation, thereby increasing the strength and relieving stress concentration by plastic organic transformation. This is called ductile organic plastic steel (TRIP). It is used as the high strength steel which has.

In the metamorphic organic plastic steels, it is very important to increase the strength and ductility by maintaining metabolic stable austenite at a constant fraction or more by suppressing transformation to martensite at room temperature. To this end, by adding Si, Al, etc. to increase the activity of the carbon in the ferrite to suppress the formation of carbides to increase the concentration of carbon in the austenite phase to ensure the stability of the retained austenite.

As a known technique for securing such high strength and elongation, there is Japanese Patent Laid-Open No. 6-1458920. According to this technique, Mo: 0.03 ~ 0.3wt% in the case of steel containing C: 0.6 ~ 0.22wt%, Si: 0.05 ~ 1.0wt%, Mn: 0.5 ~ 2.0wt%, Al: 0.25 ~ 1.5wt% Is added to provide a steel having a ductility of at least 490 MPa and an elongation of at least 35%.

In addition, Korean Patent Publication No. 2002-0045212 discloses that the steel containing C: 0.15 ~ 0.30wt%, Si: 1.5 ~ 2.5wt%, Mn: 0.5 ~ 2.0wt%, Al: 0.02 ~ 0.1wt% has a strength of 780MPa or more. Provides steel with elongation greater than 30%.

According to Japanese Laid-Open Patent Publication No. 2005-336526, steels containing C: 0.06 ~ 0.6wt%, [Si + Al]: 0.5 ~ 3.0wt%, Mn: 0.5 ~ 3.0wt% have 800MPa grade strength and 40% elongation. Provide the river. However, from the actual content of the component it can be seen that the amount of [Al + Si] is less than 1.5wt%. For example, when 1 wt% of Al is added, Si is added to 0.5 wt%. Therefore, 1.5wt% is the correct expression, not [Si + Al] which is 0.5 ~ 3.0wt%. In the contents, the manufacturing process is classified into two types: first, manufacturing process of annealing or plating after hot rolling, and second, manufacturing process of performing two stage annealing after hot rolling and cold rolling. In the second stage annealing, after cold rolling, martensite is formed in the base structure by the first stage annealing, and in the second stage annealing, annealing is performed in a conventional metamorphic organic-plastic steel, which is economically lost and has low applicability.

The other technologies are only 490MPa and 780MPa, respectively, and because the manufacturing process is complicated, it is economically advantageous and highly applicable manufacturing technology to secure tensile strength of 780MPa or more and elongation of 28% or more simultaneously. need.

The present invention controls the composition, adjusts the cooling stage during hot rolling to control the martensite fraction 30 to 70% to have a tensile strength of 980 MPa or more and an elongation of 28% or more, and at the same time the edge It is to provide a high strength steel sheet without cracks, hot dip galvanized steel sheet and a method of manufacturing the same.

The present invention is in the weight%, C: 0.1 ~ 0.25%, Si: 1.0 ~ 1.9%, Mn: 1.5 ~ 2.5%, Al: 0.5 ~ 1.6%, Ti: 0.005 ~ 0.03%, B: 5 ~ 30ppm, Sb: It comprises 0.01 ~ 0.03%, the remainder is made of Fe and unavoidable impurities, to provide a high strength steel sheet and hot-dip galvanized steel sheet characterized by satisfying 1.75≤Si + Al≤3.25%.

The present invention is in the weight%, C: 0.1 ~ 0.25%, Si: 1.0 ~ 1.9%, Mn: 1.5 ~ 2.5%, Al: 0.5 ~ 1.6%, Ti: 0.005 ~ 0.03%, B: 5 ~ 30ppm, Sb: 0.01 to 0.03%, the remainder is composed of Fe and unavoidable impurities, hot-rolled steel slab that satisfies 1.75≤Si + Al≤3.25% in the temperature range of A ₃ or more and a cooling rate of 30 ~ 200 ℃ / s Primary cooling to a temperature range of 600 ~ 800 ℃;

Air cooling in a temperature range of 600 to 800 ° C;

Second cooling to a temperature range of room temperature to 300 ° C. at a cooling rate of 50 ° C. to 200 ° C./s; And

Winding step in the temperature range of room temperature ~ 300 ℃

It provides a method for producing a high strength hot rolled steel sheet comprising a.

In addition, the present invention is a weight%, C: 0.1 ~ 0.25%, Si: 1.0 ~ 1.9%, Mn: 1.5 ~ 2.5%, Al: 0.5 ~ 1.6%, Ti: 0.005 ~ 0.03%, B: 5 ~ 30ppm, Sb: 0.01 to 0.03%, the remainder is composed of Fe and unavoidable impurities, hot-rolled steel slab that satisfies 1.75≤Si + Al≤3.25% in the temperature range of A ₃ or more and 30 ~ 200 ℃ / s First cooling to a temperature range of 600 to 800 ° C. at a cooling rate;

Air cooling in a temperature range of 600 to 800 ° C;

Second cooling to a temperature range of room temperature to 300 ° C. at a cooling rate of 50 ° C. to 200 ° C./s;

Winding in a temperature range of room temperature to 300 ° C; And

Cold rolling and annealing at a rolling reduction of 30 to 50%;

It provides a method for producing a high strength cold-rolled steel sheet and a hot-dip galvanizing step further comprising the step of hot-dip galvanizing steel sheet comprising a.

The present invention has a tensile strength of 980 MPa or more and an elongation of 28% or more by controlling the composition of components and manufacturing conditions, so that the present invention can be used for structural parts having high strength and workability, and at the same time, it is economical by preventing cracks at edges. It can increase the effect.

Hereinafter, the composition range of the present invention will be described in detail (hereinafter,% by weight).

The content of carbon (C) is 0.1 to 0.25%. C is the most important component and is closely related to all physical and chemical properties such as strength and ductility. In the present invention, if the amount of carbon is less than 0.1%, the fraction and stability of the retained austenite decreases, and if it exceeds 0.25%, the weldability is lowered, and the workability is lowered due to excessive increase of the second phase fraction. Therefore, the composition range of C was limited to 0.1 to 0.25%.

The content of silicon (Si) is 1.0 to 1.9%. Si is a component that stabilizes ferrite by solid solution in ferrite. In this cold-rolled steel sheet, it is dissolved in ferrite to increase the activity of carbon, thereby increasing the concentration of carbon in the austenite phase and suppressing the formation of the bite phase, thereby maintaining the stability of the retained austenite. It serves to raise the And employment increases the strength. If the amount of Si is less than 1.0%, the strength of the steel is lowered, and the carbide formation inhibitory effect such as carbide phase is reduced, and if it exceeds 1.9%, the hot rolled scale is caused, the plating property is deteriorated, and the weldability is also deteriorated. have. Therefore, the composition range of Si was limited to 1.0 to 1.9%.

The content of manganese (Mn) is 1.5 to 2.5%. Mn is a component that stabilizes austenite as a component that increases hardenability by facilitating the formation of low temperature transformation phases such as acicular ferrite and bainite by increasing the hardenability. Therefore, when less than 1.5% is added, the above effect cannot be expected, and when excessively added in excess of 2.5%, weldability is lowered, and segregation zones are formed at the center of the sheet during hot rolling, and hydrogen embrittlement is caused by inclusions. Let's do it. Therefore, the content of Mn is 1.5 to 2.5%.

The content of aluminum (Al) is 0.5 to 1.6%. Al is inferior in solid solution strengthening effect to Si, but exhibits a solid solution strengthening effect as a ferrite stabilizing element, suppresses the formation of carbides such as carbides, and increases stability by increasing the carbon concentration of residual austenite. If the content of Al is less than 0.5%, the stability of austenite is reduced and it is difficult to suppress the formation of carbides. If the content of Al is more than 1.6%, the fraction of austenite is lowered, so the ductility is relatively lowered, and the surface properties are deteriorated. Therefore, the content of Al is 0.5 ~ 1.6%.

The content of titanium (Ti) is 0.005 to 0.03%. Ti is a component that forms TiN to inhibit AlN nitride formation through Al-bonding with N, and it is difficult to play such a role at less than 0.005% and more than 0.03% Since the effect of addition cannot be expected, the content of Ti was limited to 0.005 to 0.03%.

The content of boron (B) is 5 to 30 ppm. B is a component that improves the hardenability even when a small amount is added to the steel, and when it is added 5 ppm or more, it is segregated at the austenite grain boundary at high temperature to suppress the formation of ferrite, which contributes to the increase of the hardenability, but when excessively added in excess of 30 ppm, the recrystallization temperature is increased. It lowers drawing property and deteriorates weldability. Therefore, the content of B is limited to 5 ~ 30ppm.

The content of antimony (Sb) is 0.01 to 0.03%. When Sb is added in an appropriate amount of 0.01 to 0.03%, the surface properties are improved. However, when Sb is added in excess of 0.03%, thickening occurs on the surface, resulting in poor surface properties. Sb is a kind of impurity that can be inevitably included in steel materials. If the content is less than 0.01%, it can be obtained even when steel is manufactured without a specific purpose. If the content of Sb is less than 0.01%, the surface is thickened. It is set to 0.01% or more because it is too small to generate a change in surface properties. Therefore, the content of Sb is limited to 0.01 ~ 0.03%.

In the present invention, 1.75 ≦ Si + Al ≦ 3.25 is satisfied. Both Si and Al play a role of inhibiting carbide formation in steel to increase the content of solid solution carbon in residual austenite and to improve the residual austenite stability, thereby controlling the contents of both components together when considering the strength and elongation of TRIP steel. It is necessary to do If the content of the two components exceeds 3.25%, the surface quality such as plating property due to the surface oxide over-formation decreases, and the strength and ductility may decrease due to the decrease of the austenite fraction during the two-phase annealing annealing. In addition, if it is less than 1.75%, the basic solid solution strengthening effect required for manufacturing TRIP steel with a target tensile strength of 780 MPa or more is reduced, and it is difficult to secure residual austenite stability. Therefore, the content of Si + Al is 1.75 ~ 3.25%.

The present invention comprises the above composition and the remainder consists of Fe and unavoidable impurities.

Hereinafter, the manufacturing method of the present invention will be described in detail.

The present invention primarily cools the steel slab that satisfies the above composition at a cooling rate of 30 ~ 200 ℃ / s after hot rolling in the austenite region of A ₃ or more. At this time, when the cooling rate is less than 30 ℃ / s, it is difficult to secure the target material because the pearlite structure can be formed, and if it exceeds 200 ℃ / s, the distortion of the material may occur due to residual stress caused by the temperature deviation of the steel sheet It limits to 30-200 degreeC / s.

The primary cooling is maintained for 5 to 20 seconds in a temperature section of 600 ~ 800 ℃. This means that it is cooled by natural convection in the atmosphere at room temperature without forced cooling. Below 600 ° C it is difficult to secure the ferrite phase formation fraction and above 800 ° C excess ferrite may be formed or pearlite tissue may be formed.

The air-cooled After the second cooling at a cooling rate of 50 ~ 200 ℃ / s and wound up at room temperature ~ 300 ℃. If the cooling rate is less than 50 ℃ / s, it is difficult to obtain the target structure by the bainite phase formation, and if it exceeds 200 ℃ / s, excessive martensite phase may be formed and the shape of the rolled plate may occur, the secondary cooling rate is 50 ~ It limits to 200 degree-C / s.

In addition, if the coiling temperature exceeds 300 ℃, it is difficult to secure the target tissue by the formation of bainite phase, and when wound at room temperature ~ 300 ℃, the matrix structure has a lattice-like fine martensite structure After hot rolling, it has high dislocation density and uniform distribution of solid carbon. Therefore, the coiling temperature is limited to room temperature ~ 300 ℃.

After winding, cold rolling is performed at a rolling reduction of 30 to 50%. In general, in the present invention, cold rolling is performed at a reduction ratio of 30 to 50%. This can sufficiently increase the dislocation density in the martensite structure and the ferrite structure after winding through the cold rolling, and the reusability of carbon and an austenite phase formation uniformly occur during annealing. If the cold reduction rate is less than 30%, the dislocation density in the ferrite structure is not sufficient, so that it is difficult to obtain the target structure. If the cold reduction rate exceeds 50%, microcracking occurs easily at the boundary between the martensite phase and the ferrite phase. Crack defects are generated in the. Therefore, the cold reduction rate is limited to 30-50%.

After cold rolling, a conventional annealing process is performed.

The present invention is hot rolled and hot dip galvanized or alloyed hot dip galvanized. After the annealing process, the plated layer is attached to the surface layer by passing the steel plate through a plating bath containing molten zinc. At this time, the temperature of the plating bath is preferably 450 ~ 500 ℃ and to produce a hot-dip galvanized steel sheet by a slow cooling process at a rate of 30 ℃ / s or less.

In addition, the plated steel sheet passed through the plating bath is immediately heated to a temperature range of 500 ~ 600 ℃ to alloy the heat treatment of the hot-dip galvanized layer to produce an alloyed hot-dip galvanized steel sheet fixed by slow cooling at a rate of less than 30 ℃ / s.

Hereinafter, the structure of the steel plate manufactured by this method is demonstrated in detail.

Hot rolled steel sheet after hot rolling in the present invention is characterized in that the martensite fraction is 30 ~ 70%. Lath-like fine martensite structure produced by primary and secondary cooling after hot rolling induces uniform austenite transformation during annealing after cold rolling and increases the fraction of stabilized residual austenite . Typically, the hot rolled steel sheet is manufactured by only one cooling process, and the hot rolled steel sheet contains pearlite structure in which coarse carbides are present. Coarse carbides are re-used during annealing after cold rolling. The carbides are coarse in size, so they are not reusable even at high temperatures above 700 ° C. During the annealing, austenite begins to form mainly near carbides with high carbon concentration. As a result, austenite formation is poor in the carbide-free region. As a result, the austenite fraction is lowered during the two-phase annealing, and local deviations occur, thereby reducing the residual austenite fraction generated during the cooling after annealing. The martensite phase in the hot rolled steel sheet can alleviate this drawback.

In the present invention, when the martensite phase is formed through the first and second cooling processes, a hot rolled steel sheet may be manufactured with almost no carbides formed. The hot rolled steel sheet including the martensite phase is formed of fine carbide in the martensite structure having a high dislocation density during annealing heat treatment, and is re-applied immediately so that the variation in carbon concentration is considerably reduced compared with the case of the pearlite structure. Therefore, the austenite phase formed during annealing is formed evenly distributed in the vicinity of the grain boundary and the near-martensite structure, so that the residual austenite fraction and stability can be improved.

In addition, by utilizing the dislocation density increased by the martensite phase, by applying a relatively low cold reduction rate, it is possible to suppress the formation of microcracks that easily occur in the edge area after cold rolling. At this time, if the martensite fraction of the hot rolled steel sheet is less than 30%, the effect of increasing the retained austenite fraction is insignificant. If the martensite fraction of the hot rolled steel sheet is more than 70%, fine cracks are generated at the edges during cold rolling. Limited to 70%.

After the cold rolling and annealing process of the present invention, the cold rolled steel sheet generates residual austenite in the ferrite and bainite structure. At this time, the fraction of retained austenite in the present invention has 5 to 15%. The residual austenite phase appears in the form of a wrestle and is more stable than other shapes of retained austenite due to the influence of the wrestling martensite structure on the hot rolled steel sheet. In addition, the fraction of bainite tissue has 20 to 40% and the remainder consists of ferrite.

According to this method, a high strength cold rolled steel sheet having a tensile strength of 780 to 980 MPa and an elongation of 28% or more having excellent workability can be produced.

Hereinafter, the present invention will be described in detail through examples of the present invention.

(Example)

Table 1 shows the composition ranges of the steels used in the examples and in the present invention proper adjustment of the components is important to obtain the desired tensile strength and elongation.

division C Mn Si P S Al Ti B Sb N Si + Al Remarks One 0.11 1.95 1.51 ≤0.02 ≤0.005 1.05 0.018 0.001 0.022 ≤0.003 2.56 Invention steel 2 0.15 1.98 1.45 ≤0.02 ≤0.005 1.23 0.016 0.001 0.017 ≤0.003 2.68 Invention steel 3 0.19 1.52 1.47 ≤0.02 ≤0.005 0.55 0.014 0.001 0.02 ≤0.003 2.02 Invention steel 4 0.21 1.55 1.49 ≤0.02 ≤0.005 1.03 0.011 0.001 0.015 ≤0.003 2.52 Invention steel 5 0.19 1.51 1.5 ≤0.02 ≤0.005 1.45 0.008 0.001 0.027 ≤0.003 2.95 Invention steel 6 0.25 1.89 1.44 ≤0.02 ≤0.005 0.82 0.019 0.001 0.02 ≤0.003 2.26 Invention steel 7 0.24 2.44 1.56 ≤0.02 ≤0.005 1.25 0.015 0.001 0.022 ≤0.003 2.81 Invention steel 8 0.23 1.62 1.65 ≤0.02 ≤0.005 1.39 0.014 0.001 0.015 ≤0.003 3.04 Invention steel 9 0.08 1.88 0.05 ≤0.02 ≤0.005 0.03 0.017 0.001 0.02 ≤0.003 0.08 Comparative steel 10 0.21 1.56 1.83 ≤0.02 ≤0.005 0.03 0.012 0.001 0.022 ≤0.003 1.86 Comparative steel 11 0.18 1.78 0.06 ≤0.02 ≤0.005 0.56 0.015 0.001 0.019 ≤0.003 0.62 Comparative steel 12 0.21 1.82 0.52 ≤0.02 ≤0.005 0.62 0.014 0.001 0.02 ≤0.003 1.14 Comparative steel 13 0.19 1.65 1.07 ≤0.02 ≤0.005 0.58 0.01 0.001 0.021 ≤0.003 1.65 Comparative steel 14 0.18 1.48 1.52 ≤0.02 ≤0.005 0.07 0.015 0.001 0.022 ≤0.003 1.59 Comparative steel 15 0.23 2.35 1.63 ≤0.02 ≤0.005 1.78 0.015 0.001 0.022 ≤0.003 3.41 Comparative steel

The steel slab having the composition shown in Table 1 was cold rolled and annealed after cooling and winding through a hot rolling process, and the cooling conditions, cold rolling rate, and tensile test results are shown in Table 2 below.

Tensile properties, among the mechanical properties of the material, depend on the composition and the fabrication conditions of the microstructure. The steel sheet of the present invention is subjected to deformation by inducing a uniform austenite transformation in the annealing process by the lath-like fine martensite structure produced immediately after hot rolling and increasing the fraction of retained austenite stabilized therefrom. When you want to get excellent mechanical properties through transformation.

In addition, by applying a relatively low cold reduction rate by utilizing the dislocation density increased by the martensite phase, it was intended to suppress the formation of microcracks easily occurring at the edge part after cold rolling. In order to obtain such characteristics, basically, the components proposed in the present invention should be satisfied, and the cooling process, the martensite fraction, and the cold reduction rate should be optimally adjusted immediately after hot rolling.

division 1st cooling speed
℃ / s middle
Temperature
℃ Air cooling
time
sec 2nd cooling speed
℃ / s Winding
Temperature
℃ Cold
Rolling reduction
% Martensite fraction% TS

MPa T-El

% Edge crack length
mm One Invention 100 680 5 150 200 50 70 740 30 0.02 2-1 Invention 100 700 5 150 200 45 70 783 32 0.05 2-2 Comparative material 100 600 One 200 200 55 70 785 31 1.2 2-3 Comparative material 120 650 5 250 200 60 70 778 32 3.2 3-1 Invention 60 680 8 120 220 40 55 787 35 0 3-2 Comparative material 80 680 8 120 220 60 55 790 33 2.5 3-3 Comparative material 100 700 2 250 200 40 90 792 31 3.4 4-1 Invention 80 650 8 100 200 30 60 925 32 0 4-2 Comparative material 80 650 One 200 200 30 75 930 30 2.2 4-3 Comparative material 80 650 2 250 200 30 90 840 30 4 5-1 Invention 100 680 5 150 200 40 65 840 31 0.02 5-2 Comparative material 30 700 5 80 200 40 10 773 27 0 6 Invention 80 650 8 120 200 40 50 945 30 0.05 7-1 Invention 80 650 8 120 200 50 45 990 28 0.07 7-2 Comparative material 50 650 9 120 200 60 45 988 28 3 7-3 Comparative material 30 - - - 650 50 0 985 26 0 8-1 Invention 80 680 8 100 200 40 60 935 30 0.08 8-2 Comparative material 30 - - - 650 40 0 920 27 0 9 Comparative material 80 700 8 100 100 40 60 680 20 0.02 10-1 Comparative material 30 650 6 100 200 40 40 930 27 0 10-2 Comparative material 30 - - - 650 40 0 825 28 0 11 Comparative material 80 980 8 100 200 40 50 790 18 0 12-1 Comparative material 100 700 8 100 200 40 50 800 22 0.05 12-2 Comparative material 120 700 10 100 200 60 50 815 22 0.2 12-3 Comparative material 50 - - - 630 40 0 790 20 0 13 Comparative material 80 720 15 100 250 40 40 785 27 0 14 Comparative material 80 710 8 100 200 40 60 740 35 0 15 Comparative material 50 680 8 100 200 40 40 960 23 0

Table 2 shows the results of the cooling process, hot martensite fraction, cold rolling ratio, mechanical properties and edge cracking after hot rolling.

Comparative materials 9 to 15 do not satisfy the basic component requirements, it can be seen that even if the cooling conditions, martensite fraction and cold rolling rate conditions during the manufacturing process is properly adjusted, the strength and ductility targeted by the present invention are not obtained.

In addition, Comparative Materials 2-2, 2-3, 3-2, 3-3, 4-2, 4-3, 5-2, 7-2, 7-3, and 8-2 met the component requirements, but cooling conditions It can be seen that the target mechanical properties cannot be obtained due to failure to meet the requirements such as martensite fraction, cold reduction rate, or microcracks at the edges.

In Table 2, the crack length of the edge was measured by using a 200-magnification optical microscope to measure the martensite fraction, and the result was measured by an image analyzer after etching the microstructure at the thickness t / 4 position with 2% Nital etchant. The fine cracks generated at were randomly selected at the edges of the cold rolled rolled plate and were averaged after selecting at least 30 of the longest cracks occurring within the length of 100 mm.

Figure 1 shows the test results of the tensile strength according to the [Si] + [Al] content vs. martensite volume fraction pre-tested for the present invention.

Figure 2 shows the test results of the elongation according to the martensite volume fraction after [Si] + [Al] content vs. hot rolling pre-tested for the present invention.

Figure 3 shows the crack length of the edge after cold rolling according to the [Si] + [Al] content vs. hot rolled Martensite volume fraction pre-tested for the present invention.

4 shows tensile strength, elongation, and edge crack length after cold rolling according to [Si] + [Al] content vs. martensite volume fraction of inventive steel and comparative steel prepared according to the present invention.

Claims (8)

By weight%, C: 0.1-0.25%, Si: 1.0-1.9%, Mn: 1.5-2.5%, Al: 0.5-1.6%, Ti: 0.005-0.03%, B: 5-30 ppm, Sb: 0.01-0.03 A high strength steel sheet comprising%, the remainder being made of Fe and inevitable impurities, and satisfying 1.75 ≦ Si + Al ≦ 3.25%. By weight%, C: 0.1-0.25%, Si: 1.0-1.9%, Mn: 1.5-2.5%, Al: 0.5-1.6%, Ti: 0.005-0.03%, B: 5-30 ppm, Sb: 0.01-0.03 %, The remainder is composed of Fe and inevitable impurities, satisfies 1.75≤Si + Al≤3.25%, the fraction of the microstructure after hot rolling is characterized in that the martensite is 30 ~ 70% and the rest is ferrite High strength hot rolled steel sheet. By weight%, C: 0.1-0.25%, Si: 1.0-1.9%, Mn: 1.5-2.5%, Al: 0.5-1.6%, Ti: 0.005-0.03%, B: 5-30 ppm, Sb: 0.01-0.03 %, The remainder is composed of Fe and inevitable impurities, satisfies 1.75≤Si + Al≤3.25%, the fraction of microstructure after cold rolling is 5-15% residual austenite, 20-40% bainite and High strength cold rolled steel sheet, characterized in that the rest is ferrite. The high strength cold rolled steel sheet according to claim 3, wherein the cold rolled steel sheet has a tensile strength of 780 MPa to 980 MPa and an elongation of 28% or more. By weight%, C: 0.1-0.25%, Si: 1.0-1.9%, Mn: 1.5-2.5%, Al: 0.5-1.6%, Ti: 0.005-0.03%, B: 5-30 ppm, Sb: 0.01-0.03 A high strength hot dip galvanized steel sheet comprising%, the remainder being made of Fe and inevitable impurities, satisfying 1.75 ≦ Si + Al ≦ 3.25%, and having a hot dip galvanized layer. By weight%, C: 0.1-0.25%, Si: 1.0-1.9%, Mn: 1.5-2.5%, Al: 0.5-1.6%, Ti: 0.005-0.03%, B: 5-30 ppm, Sb: 0.01-0.03 include% and the remainder Fe and unavoidable impurities, consists of, 1.75≤Si + a steel slab satisfying the Al≤3.25% and the hot rolling in the temperature range a ₃ or more at a cooling rate of 30 ~ 200 ℃ / s 600 ~ Primary cooling to a temperature range of 800 ° C .; Air cooling in a temperature range of 600 to 800 ° C; Second cooling to a temperature range of room temperature to 300 ° C. at a cooling rate of 50 ° C. to 200 ° C./s; And Winding in a temperature range of room temperature to 300 ° C; Method for producing a high strength hot rolled steel sheet comprising a By weight%, C: 0.1-0.25%, Si: 1.0-1.9%, Mn: 1.5-2.5%, Al: 0.5-1.6%, Ti: 0.005-0.03%, B: 5-30 ppm, Sb: 0.01-0.03 include% and the remainder Fe and unavoidable impurities, consists of, 1.75≤Si + a steel slab satisfying the Al≤3.25% and the hot rolling in the temperature range a ₃ or more at a cooling rate of 30 ~ 200 ℃ / s 600 ~ Primary cooling to a temperature range of 800 ° C .; Air cooling in a temperature range of 600 to 800 ° C; Second cooling to a temperature range of room temperature to 300 ° C. at a cooling rate of 50 ° C. to 200 ° C./s; Winding in a temperature range of room temperature to 300 ° C; And Cold rolling and annealing at a rolling reduction of 30 to 50%; Method for producing a high strength cold rolled steel sheet comprising a. By weight%, C: 0.1-0.25%, Si: 1.0-1.9%, Mn: 1.5-2.5%, Al: 0.5-1.6%, Ti: 0.005-0.03%, B: 5-30 ppm, Sb: 0.01-0.03 include% and the remainder Fe and unavoidable impurities, consists of, 1.75≤Si + a steel slab satisfying the Al≤3.25% and the hot rolling in the temperature range a ₃ or more at a cooling rate of 30 ~ 200 ℃ / s 600 ~ Primary cooling to a temperature range of 800 ° C .; Air cooling in a temperature range of 600 to 800 ° C; Second cooling to a temperature range of room temperature to 300 ° C. at a cooling rate of 50 ° C. to 200 ° C./s; Winding in a temperature range of room temperature to 300 ° C; Cold rolling and annealing at a rolling reduction of 30 to 50%; And Hot dip galvanizing; Method for producing a high strength hot dip galvanized steel sheet comprising a.

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