KR20110100868A - High cold-rolled steel with excellent strength and elongation, and method preparing the same - Google Patents

Abstract

The present invention relates to a cold rolled steel sheet having a good tensile strength and yield strength and elongation at the same time and a method for producing the same, more specifically 0.10 ~ 0.15 wt% C; 5.0-8.0 weight% Mn; 0.01-1.2 weight% Al; 0.3-1.5% weight% Si; The present invention relates to a high strength cold rolled steel sheet composed of residual iron (Fe) and other unavoidable impurities, and a method of manufacturing the same. High strength cold rolled steel sheet according to the present invention is composed of 55 ~ 71% ferrite and 29 ~ 45% austenite in the microstructure after continuous annealing, has a tensile strength of 1180MPa or more, yield strength of 600MPa or more and elongation 12% or more As such, it can be usefully used for structural members and inner plate materials for automobiles which require excellent moldability and high strength for passenger safety.

Description

High strength cold rolled steel sheet with excellent tensile strength, yield strength and elongation and manufacturing method thereof {High cold-rolled steel with excellent strength and elongation, and method preparing the same}

The present invention relates to a high strength cold rolled steel sheet having excellent tensile strength, yield strength and elongation and a method of manufacturing the same.

The present invention relates to a high-strength cold-rolled steel sheet having excellent strength and elongation, and a method for manufacturing the same. More particularly, by optimizing the volume fraction and mechanical stability of residual austenite contributing to transformation organic plasticity, It relates to an excellent cold rolled steel sheet and a method of manufacturing the same.

Cold rolled steel applied to structural members and inner plates for automobiles has been strengthened to reduce exhaust gas by improving fuel efficiency and to ensure passenger safety in the event of a vehicle crash. Increasingly, research is being conducted to commercialize 980MPa and 1180MPa. In line with this trend, recent auto industry research shows that the application rate of high-strength steel sheet to the automobile body, which is currently in the 10% range, is expected to increase to more than 50% within 10 years.

Representative high strength cold rolled steels used as structural members and inner plates for automobiles or expected to be used in the future include dual phase steel (DP steel), transformation induced plasticity steel (TRIP steel) and composite steel (Complex Phase Steel) and Twin Induced Plasticity Steel (TWIP).

The ideal tissue steel has a microstructure of ferrite + martensite by heating and annealing a cold rolled steel sheet to an ideal region of ferrite + austenite and then transforming the austenite into martensite upon cooling. The higher the volume fraction of the hard phase martensite, the higher the strength of the ideal tissue steel, but in general, it is difficult to obtain ultra high strength of 1180 MPa or more.

The metamorphic organic plastic steel is produced by stabilizing austenite so as not to transform into martensite during cooling through an ostempering treatment after an abnormal reverse heat treatment of the steel sheet. In the case of metamorphic organic plastic steel, excellent elongation is exhibited by the metamorphic organic plasticity phenomenon in which residual austenite is transformed into martensite during deformation. However, it is known that it is difficult to obtain ultra high strength of 1180 MPa or more in the case of normal metamorphic organic steel containing less than 3% by weight of manganese.

The composite structure steel is a steel in which the final fine structure includes ferrite, bainite, martensite and some austenite through heat treatment of cold rolled steel sheet, and precipitates various carbides to greatly increase the strength of the steel sheet. In the case of composite tissue steel, it is known that a very high tensile strength of 1180 MPa or more can be obtained. However, as the strength is improved, the elongation decreases rapidly. There is a low disadvantage.

Twin strained organic strained steel stabilizes austenite at room temperature by adding 15-25% of manganese in weight ratio and has high strength and elongation due to twins generated during deformation. However, in case of twin-organic strained steel, yield strength is 300 ~ 400MPa, and it is difficult to obtain sufficient strength by the normal deformation amount involved in part molding because a large amount of deformation must be accompanied to obtain super high strength of composite tissue steel level. There are disadvantages.

On the other hand, the yield strength is 400 ~ 600MPa even for steels which are known to have high strength and elongation due to metamorphic organic plasticity, which transforms into a martensite during deformation of the austenitic structure having a manganese content of 10 to 15% by weight and stable at room temperature. It has a low drawback similar to that of twin strained organic steels.

As described above, in the case of high strength cold rolled steel sheets for automobiles currently applied or under development, it is important to manufacture high strength steel having a certain level of elongation required for manufacturing parts while achieving ultra high strength for improving fuel efficiency and improving passenger safety. Remains a challenge

It is an object of the present invention to provide a high strength cold rolled steel sheet having excellent tensile strength, yield strength and elongation and a method of manufacturing the same.

In order to achieve the above object, the present invention is 0.10 to 0.15% by weight C; 5.0-8.0 weight% Mn; 0.01-1.2 weight% Al; 0.3-1.5% weight% Si; It provides a high strength cold rolled steel sheet consisting of residual iron (Fe) and other unavoidable impurities.

In addition, the present invention is 0.10 ~ 0.15 wt% C; 5.0-8.0 weight% Mn; 0.01-1.2 weight% Al; 0.3-1.5% weight% Si; Heating the slab of steel consisting of residual iron (Fe) and other unavoidable impurities to perform a final hot rolling (step 1); Winding up (step 2); Cold rolling after pickling (step 3); And continuous annealing and cooling (step 4).

High strength cold rolled steel sheet according to the present invention is composed of 55 ~ 71% ferrite and 29 ~ 45% austenite in the microstructure after continuous annealing, 12% despite the high strength of 1180MPa or more tensile strength, 600MPa or more yield strength By having the above good elongation, it can be usefully used for automobile structural members and inner plate materials which require high strength for passenger safety and formability for manufacturing parts.

1 is a result of EBSD (Electron Back Scatter Diffraction) analysis showing the phases constituting the microstructure of the high strength cold rolled steel sheet having excellent tensile strength, yield strength and elongation at the same time (gray portion: ferrite) , Black part: austenite).
FIG. 2 is an XRD graph showing phases constituting a high strength cold rolled steel sheet having excellent tensile strength, yield strength, and elongation simultaneously produced by the present invention (α: peak corresponding to ferrite, γ: austenite Peak).

The present invention is 0.10 ~ 0.15 wt% C; 5.0-8.0 weight% Mn; 0.01-1.2 weight% Al; 0.3-1.5% weight% Si; It provides a high strength cold rolled steel sheet consisting of residual iron (Fe) and other unavoidable impurities.

Hereinafter, the high strength cold rolled steel sheet of the present invention will be described in more detail.

High strength cold rolled steel sheet according to the present invention is 0.10 ~ 0.15 wt% C; 5.0-8.0 weight% Mn; 0.01-1.2 weight% Al; 0.3-1.5% weight% Si; It is composed of residual iron (Fe) and other unavoidable impurities, and the microstructure fraction of the cold rolled steel sheet after continuous annealing is 55-71% ferrite and 29-45% austenite (see FIGS. 1 and 2), The particle size of austenite is 1 micrometer or less here (refer FIG. 1).

On the other hand, the alloy elements used in the present invention, except for iron (Fe) and other unavoidable impurities, although only four kinds of carbon (C), manganese (Mn), aluminum (Al) and silicon (Si) Extremely high tensile and yield strengths and high elongation which are difficult to obtain in the existing ultra high strength steels can be obtained.

Hereinafter, the role and effects of the elements added to the high strength cold rolled steel sheet according to the present invention.

1) Carbon (C)

Since the carbon (C) is the most important component in the steel material and contributes to stabilization of the austenite phase, as the amount of addition increases, the fraction of retained austenite and mechanical stability increases, but the Mn content is 5-8%. If the carbon content is increased by 0.15% or more, the cold workability is lowered due to the increase in the strength of the low temperature transformation phase present in the hot rolled steel sheet, which makes the cold rolling difficult and the productivity decreases. On the other hand, when the amount of carbon added is less than 0.10%, the volume fraction and mechanical stability of the retained austenite are lowered, so that the ductility is lowered and the hardness of the martensite phase is insufficient, resulting in lowering the final strength of the cold rolled steel sheet. Therefore, the content of carbon is preferably limited to 0.10 ~ 0.15%.

2) Manganese (Mn)

The manganese (Mn) contributes to stabilization of austenite as a solid solution strengthening element in steel. When the manganese content is less than 5%, it is difficult to secure an austenite fraction of 25% or more after annealing the cold rolled steel sheet, and it is difficult to increase the tensile strength to 1180 MPa or more. On the other hand, when the manganese content exceeds 8%, the strength of the hot rolled steel sheet is high, which makes it difficult to cold work. Also, the austenitic fraction formed at the annealing temperature of the cold rolled steel sheet is rapidly increased to 50% or more, thereby reducing the carbon concentration in the austenite. Ductility is lowered by inhibiting thermal stability at room temperature. In addition, excessive addition of Mn causes oxidation scale problems and deterioration of plating properties during hot rolling. Therefore, the content of the manganese is preferably limited to 5 to 8%.

3) Aluminum (Al)

The aluminum (Al) is thermodynamically known as a ferrite stabilizing element to reduce the equilibrium fraction of austenite during annealing of cold rolled steel sheet, and the effect of solid solution strengthening is very small. Meanwhile, the stacking fault energy of residual austenite is increased to improve mechanical stability, thereby contributing to the improvement of elongation. If the content of Al is less than 0.01%, the effect of improving the mechanical stability of the retained austenite is insignificant. If the content of Al exceeds 1.2%, the strength is significantly reduced due to the low solid solution strengthening effect. Therefore, the content of Al is preferably limited to 0.01 ~ 1.2%.

4) Silicon (Si)

The silicon (Si) contributes to the increase of the strength of ferrite as a solid solution strengthening element in steel and also inhibits the formation of carbides, thereby contributing to the carbon concentration in the retained austenite, thereby increasing the thermal and mechanical stability of austenite. When the amount of Si added is less than 0.3%, the effect of strengthening the solid solution and inhibiting carbide production is significantly reduced. If the amount of the added Si exceeds 1.5%, the surface oxidation occurs excessively during hot rolling, so the surface quality is high. , The content of Si is preferably limited to 0.3 ~ 1.5%.

In addition, the present invention is 0.10 ~ 0.15 wt% C; 5.0-8.0 weight% Mn; 0.01-1.2 weight% Al; 0.3-1.5% weight% Si; Heating the slab of steel consisting of residual iron (Fe) and other unavoidable impurities to perform a final hot rolling (step 1); Winding up (step 2); Cold rolling after pickling (step 3); And continuous annealing and cooling (step 4).

The present invention is 0.10 ~ 0.15 wt% C; 5.0-8.0 weight% Mn; 0.01-1.2 weight% Al; 0.3-1.5% weight% Si; Slabs of steel composed of residual iron (Fe) and other unavoidable impurities,

Heating at 1200 to 1250 ° C., performing finish hot rolling at a temperature range of 850 to 950 ° C. (step 1);

Winding at 550-650 ° C. (step 2);

Cold rolling after pickling (step 3); And

Continuously annealing and cooling for 60 to 300 seconds in a temperature range of 680 to 720 ° C. (step 4);

It provides a method of manufacturing a high strength cold rolled steel sheet comprising a.

Hereinafter, the high strength cold rolled steel sheet manufacturing method of the present invention will be described in more detail.

Step 1:

First, step 1 is 0.10 to 0.15 weight% C; 5.0-8.0 weight% Mn; 0.01-1.2 weight% Al; 0.3-1.5% weight% Si; A slab of steel composed of residual iron (Fe) and other unavoidable impurities is heated at 1200 to 1250 ° C and hot finish rolling at 850 to 950 ° C.

Step 2:

Next, step 2 is a step of winding at 550 ~ 650 ℃. Cold rolling is difficult due to the deterioration of plate shape and increase in strength at the winding temperature below 550 ℃. The winding temperature is 550 ~ 650 ℃ because the oxidation of the surface of hot rolled steel becomes more difficult at the winding temperature above 650 ℃. Limited.

Step 3:

Next, step 3 is a step of cold rolling after pickling. That is, it is cold rolled to 50 to 80% of range after pickling with hydrochloric acid. When the rolling reduction is less than 50%, the effect of reducing the thickness is low. When the rolling reduction is more than 80%, the strength is increased due to the work hardening of the cold rolled material, which makes rolling difficult.

Step 4:

Next, step 4 is a step of continuous annealing and cooling for 60 to 300 seconds in the temperature range of 680 ~ 720 ℃. In other words, the cold-rolled steel sheet above the ferrite and austenite ideal temperature Annealed in the section is maintained at a temperature range of 680 ~ 720 ℃ to have a reverse amorphous fine austenite having a fraction of 29 ~ 45% level. The annealing time is preferably 60 to 300 seconds in order to prevent the austenite crystal grains from becoming too coarse while carbon can diffuse and concentrate in the austenite grains. The cooling rate after the continuous annealing is not particularly limited, and conventional cooling conditions including air cooling or water cooling can be used.

The cold rolled steel sheet manufactured by the manufacturing method as described above is made of ferrite and austenite as shown in FIGS. 1 and 2, and as shown in FIG. 1, the austenite has a grain size of 1 μm or less.

The high strength cold rolled steel sheet according to the present invention exhibits excellent elongation compared to ultra high strength and strength by simultaneously having a tensile strength of 1180 MPa or more, a yield strength of 600 MPa or more and an elongation of 12% or more.

Therefore, the high strength cold rolled steel sheet according to the present invention can be usefully used for structural members and inner plates for automobiles that require high formability for excellent formability and passenger safety.

<Examples>

Ingots of the steel grades A to F having alloy elements and contents as shown in Table 1 were heated at 1200 ° C. for 2 hours, and then hot rolled at 900 ° C. for finishing. The hot rolled steel sheet was cold rolled at a reduction ratio of 75% at room temperature, and then heated to a temperature increase rate of 10 ° C./s to a temperature of 660 to 800 ° C., annealed for 120 seconds, and cooled to room temperature at a cooling rate of 100 ° C./s. Was prepared. For reference, steel grades A to C fall within the composition range of the present invention, and steel grades D to F fall outside the composition range of the present invention.

Steel grade division Annealing Temperature (℃) C (wt%) Mn (wt%) Al (wt%) Si (wt%) A A-1 700 0.11 5.15 0.02 1.39 A-2 720 A-3 740 B B-1 700 0.12 5.10 1.15 0.49 B-2 720 B-3 780 C C-1 680 0.12 7.20 1.07 0.49 C-2 700 C-3 660 D D-1 740 0.12 3.15 1.06 0.49 D-2 760 E E-1 780 0.12 6.01 2.84 0.48 E-2 800 F F-1 720 0.09 4.92 1.15 0.55 F-2 740

The room temperature yield strength, tensile strength and elongation of the specimen prepared as described above were measured by ASTM E8M standard, and the test results are shown in Table 2 below.

Steel grade division Yield strength
(MPa) The tensile strength
(MPa) Elongation
(%) Residual austenite
Volume fraction (%) Remarks A A-1 785 1269 17.3 30.3 Invention 1 A-2 602 1518 12.3 37.3 Invention Material 2 A-3 433 1612 9.9 7.7 Comparative material 1 B B-1 873 1073 19.5 20.2 Invention 3 B-2 691 1190 12.1 29.6 Invention 4 B-3 334 1460 8.6 5.6 Comparative material 2 C C-1 1000 1229 26.3 32.1 Invention 5 C-2 783 1352 13.2 43.6 Invention Material 6 C-3 1083 1124 26.7 20.7 Comparative material 3 D D-1 336 844 20.6 16.8 Comparative material 4 D-2 424 949 13.3 9.4 Comparative material 5 E E-1 687 1082 22.3 31.3 Comparative Material 6 E-2 589 1165 15.4 21.9 Comparative material 7 F F-1 625 1151 15.4 28.7 Comparative Material 8 F-2 434 1314 6.7 4.7 Comparative material 9

As can be seen in Table 2, the inventive materials 1 to 6 of the present invention have a tensile strength of 1180 MPa or more, a yield strength of 600 MPa or more, and an elongation of 12% or more, and thus have very high strength and excellent elongation as an ultra high strength steel.

On the other hand, Comparative Materials 1 to 9, it can be seen that there is no steel that satisfies the conditions of tensile strength 1180MPa or more, yield strength 600MPa or more and elongation 12% or more at the same time.

Claims (8)

0.10 to 0.15 weight% C; 5.0-8.0 weight% Mn; 0.01-1.2 weight% Al; 0.3-1.5% weight% Si; High strength cold rolled steel sheet consisting of balance iron (Fe) and other unavoidable impurities.
The high strength cold rolled steel sheet according to claim 1, wherein the microstructure of the cold rolled steel sheet after continuous annealing is made of 55 to 71% of ferrite and 29 to 45% of austenite.
The high strength cold rolled steel sheet according to claim 2, wherein the austenite is 1 µm or less.
The high strength cold rolled steel sheet according to any one of claims 1 to 3, wherein the cold rolled steel sheet has a tensile strength of 1180 MPa or more, a yield strength of 600 MPa or more, and an elongation of 12% or more.
0.10 to 0.15 weight% C; 5.0-8.0 weight% Mn; 0.01-1.2 weight% Al; 0.3-1.5% weight% Si; Slabs of steel composed of residual iron (Fe) and other unavoidable impurities,
Heating at 1200 to 1250 ° C., performing finish hot rolling at a temperature range of 850 to 950 ° C. (step 1);
Winding at 550-650 ° C. (step 2);
Cold rolling after pickling (step 3); And
Continuously annealing and cooling for 60 to 300 seconds in a temperature range of 680 to 720 ° C. (step 4);
Method for producing a high strength cold rolled steel sheet comprising a.
5. The method of claim 4, wherein the microstructure of the cold rolled steel sheet after continuous annealing in step 4 is made of 55-71% ferrite and 29-45% austenite.
7. The method of claim 6, wherein the austenite is 1 µm or less.
The method according to any one of claims 5 to 7, wherein the cold rolled steel sheet has a tensile strength of 1180 MPa or more, a yield strength of 600 MPa or more, and an elongation of 12% or more.

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