KR20090046236A - Cold-rolled high mn steel with ultra-high strength and good ductility, and manufacturing method thereof - Google Patents

Abstract

The present invention relates to an ultra-high strength, high ductility cold rolled steel sheet used in automobile steel sheets and the like, which require excellent formability and high strength, and a method of manufacturing the same.

The high manganese type ultra high strength cold rolled steel sheet excellent in ductility according to the present invention is 0.02 to 0.2% carbon (C), 10 to 15% manganese (Mn), 0.01 to 3% aluminum (Al), It comprises a balance of iron (Fe) and other unavoidable impurities, characterized in that the tensile strength of 1.2GPa or more, the product of the tensile strength and the total elongation (TS × T.El) is 25000MPa% or more. And the cold-rolled steel sheet is characterized in that the austenite fraction is 50% or more. According to the present invention as described above there is an advantage that the production of ultra-high strength cold rolled steel sheet excellent in strength-elongation balance value in a simple composition system.

Automotive steel plate, ultra high strength, cold rolled steel, metamorphic organic plasticity, high manganese

Description

High manganese type super high strength cold rolled steel sheet with excellent ductility and its manufacturing method {Cold-rolled high Mn steel with ultra-high strength and good ductility, and manufacturing method

1 is an X-ray diffraction image showing the configuration of a high manganese ultra-high strength cold rolled steel sheet excellent in ductility according to the present invention.

Figure 2 is a microstructure photograph of the back plate plate electron diffraction (EBSD) of a high manganese type ultra high strength cold rolled steel sheet excellent in ductility according to the present invention.

3 is a nominal stress-nominal strain curve of a high manganese type ultra high strength cold rolled steel sheet excellent in ductility according to the present invention.

Figure 4 is a manufacturing process diagram showing a method of manufacturing a high manganese type ultra high strength cold rolled steel sheet excellent in ductility according to the present invention.

Explanation of symbols on the main parts of the drawings

S100. Hot rolling step S200. Cold rolling stage

S300. Grain forming step S400. Room temperature cooling stage

The present invention relates to a cold rolled steel sheet and a method for manufacturing the same, and more particularly, to a high manganese cold rolled steel sheet and a method for manufacturing the same to improve the strength and ductility by overcoming the ductility degradation due to the ultra-high strength by using metamorphic organic plasticity will be.

Cold rolled steel applied to automobile structural members and inner plates is becoming stronger in order to ensure passenger safety in the event of a vehicle crash. The 780MPa and 1180MPa grades have been gradually used to secure steel sheets with a tensile strength of 780MPa. Research to commercialize is ongoing.

In addition, as the regulations on environmental pollution caused by automobile exhaust gas have become stricter, the use of steel plates having lighter and higher strength is increasing to increase fuel efficiency.

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 tissue steel. (Complex Phase Steel) and Twin Induced Plasticity Steel (TWIP).

In general, the manufacturing process of the cold rolled steel sheet may have a desired phase ratio by reheating the cast slab, hot rolling at high temperature, cooling to room temperature after winding, and hot rolling of the hot rolled steel sheet at room temperature and annealing including annealing. It can be divided into steps to make it.

The abnormal structure steel is a steel produced by transforming austenite into martensite by heating and annealing a cold rolled steel sheet in an abnormal region of ferrite + austenite and then rapidly cooling it.

The strength and ductility of the ideal tissue steels change depending on the fraction of the soft phase ferrite and the hard phase martensite. However, in the case of the ideal tissue steel with a total elongation of 20% or more, it is difficult to obtain ultra high strength of 1 GPa or more.

The metamorphic organic-plastic steel refers to a steel sheet having an elongation of steel by improving the elongation of the steel sheet by retaining a portion of the steel sheet microstructure by austenite after an abnormal reverse heat treatment of the cold rolled steel sheet.

In the case of the modified organic plastic steel thus produced, the uniform elongation is excellent due to the transformation of the retained austenite into martensite during deformation, and the tensile organic plastic steel has an elongation of 30% at the tensile strength level of 780 MPa.

However, in the case of metamorphic organic plastic steel, the decrease in elongation is very significant as the tensile strength increases, and it is known that it is difficult to manufacture the metamorphic organic plastic steel having excellent strength-ductile balance at the tensile strength of 1 GPa or more.

The composite steel is a steel in which the final microstructure includes ferrite, bainite and martensite and some austenite through heat treatment of the cold rolled steel sheet, and precipitates various carbides to greatly increase the strength of the steel sheet.

The tensile strength of the composite steel is known to be about 1 GPa or more. However, in the case of composite tissue steel, the total elongation is lower than 15%, so that the tensile strength-elongation balance value is lower than that of the abnormal tissue steel or metamorphic organic plastic steel, and also has a low formability which is absolutely required for automobile parts manufacturing.

On the other hand, recently, twin-organic strain steels have been developed by adding 15-25% by weight of manganese to stabilize austenite at room temperature and increase the strength and toughness of cold-rolled steel sheets using twins generated during deformation.

Strained organic strained steel has a tensile strength of about 1 GPa and a total elongation of about 50%, showing the best mechanical properties among the cold rolled steels that have been studied or commercialized.

However, in the case of twin-organic strained steel, the yield strength is 300 ~ 400MPa, and it is difficult to obtain sufficient strength as the deformation amount according to the part molding because a large amount of deformation must be accompanied in order to show the super high strength of the composite tissue steel level. In addition, as the carbon is added in a weight ratio of 0.2 to 1% and manganese in a large amount of 15 to 25%, weldability is reduced.

As such, the high strength cold rolled steel sheet for automobiles, which is currently commercialized or under development, does not satisfy both the conditions of ultra high strength and strength-ductile balance at the same time, and therefore, development of steel sheet which simultaneously secures strength-ductile balance due to ultra high strength This remains a technical challenge.

An object of the present invention is to solve the above problems, control the content of carbon (C) 0.02 ~ 0.2% by weight, the content of manganese (Mn) to 10-15% by weight and the cold rolling process of hot rolled steel sheet By controlling the fraction and structure of austenite in the subsequent annealing process, it secures fine austenite grains with a phase ratio of 50% or more at room temperature, and through this, a high manganese type having ultra high strength and high ductility due to the deformation organic transformation of austenite. It is to provide a cold rolled steel sheet and a method of manufacturing the same.

The high manganese type ultra high strength cold rolled steel sheet excellent in ductility according to the present invention is 0.02 to 0.2% carbon (C), 10 to 15% manganese (Mn), 0.01 to 3% aluminum (Al), It comprises a balance of iron (Fe) and other unavoidable impurities, characterized in that the tensile strength of 1.2GPa or more, the product of the tensile strength and the total elongation (TS × T.El) is 25000MPa% or more.

The cold rolled steel sheet is characterized in that the austenite fraction is 50% or more.

The method for producing a high manganese type ultra high strength cold rolled steel sheet excellent in ductility according to the present invention is 0.02 to 0.2% carbon (C), 10 to 15% manganese (Mn), and 0.01 to 3% aluminum (Al) in weight%. And a hot rolling step of hot rolling a slab of steel composed of residual iron (Fe) and other unavoidable impurities, a cold rolling step of cold rolling a hot rolled hot rolled steel sheet at room temperature, and a cold rolled cold rolled steel sheet. Annealing in a temperature range of ₃ ± 50 ^° C to form a fine austenite grains, and characterized in that it consists of a room temperature cooling step of cooling the cold rolled steel sheet on which the fine austenite grains are formed.

The cold rolling step, characterized in that the process of lowering the residual austenite fraction in the hot rolled steel sheet to less than 1% by weight.

The room temperature cooling step is characterized in that the process of maintaining the austenitic fraction in the cold rolled steel sheet to 50% by weight or more.

According to the present invention as described above there is an advantage that the production of ultra-high strength cold rolled steel sheet excellent in strength-elongation balance value in a simple composition system.

Hereinafter, with reference to the accompanying Figures 1 to 3 will be described in detail the present invention.

1 is an X-ray diffraction photograph showing the configuration of a high manganese type ultra high strength cold rolled steel sheet excellent in ductility according to the present invention, Figure 2 is a high manganese type ultra high strength cold rolled steel sheet excellent in ductility according to the present invention A microstructure photograph by back diffusion electron diffraction (EBSD) is shown, and FIG. 3 shows a nominal stress-nominal strain curve of a high manganese type ultra high strength cold rolled steel sheet having excellent ductility according to the present invention.

Tensile organic plastic steel, which is a representative high strength steel sheet using strained organic transformation, usually retains 10 to 15% of austenite, and the austenite transforms into martensite during deformation, contributing to the improvement of strength and uniform elongation. Known.

In the metamorphic organic plastic steel, the mechanical properties of the steel sheet depend largely on the mechanical stability of the retained austenite. If the mechanical stability of the austenite is low, the transformation is concentrated at the initial stage of deformation, so that it is difficult to obtain sufficient stretching, and the mechanical stability of the austenite is excessive. Too high a strength does not produce a sufficient amount of martensite due to deformation.

On the other hand, when the weight percent of the austenite stabilizing element Mn is increased to 10 to 15%, austenite remains as shown in FIG. 1 without additional heat treatment after hot rolling.

However, such austenite has a large grain size, very low mechanical stability, difficult to control the structure by the subsequent heat treatment process, and can not contribute to the increase in elongation of the steel sheet by transforming most of the martensite at the initial stage of deformation.

In order to solve this problem, as shown in FIG. 1, the hot rolled steel sheet is cold rolled at room temperature to control the fraction of retained austenite to less than 1%, followed by annealing at a temperature range of Ac ₃ ± 50 ^o C. By securing more than 50% of the austenitic grain fraction of the micronized austenitic grains, it improves the proper mechanical stability of austenite at room temperature, thereby increasing the uniform elongation due to strained organic transformation as well as increasing the strength by forming a large amount of martensite during deformation. High ductility and ultra high strength of the steel sheet were achieved.

Hereinafter, the present invention will be divided into steel compositions and manufacturing methods.

[Lecture composition]

Carbon (C): 0.02 to 0.2% by weight (hereinafter referred to as only%)

The carbon (C) is the most important component in the steel material, which greatly contributes to the stabilization of the austenite phase, so as the amount added increases the fraction of austenite and mechanical stability, but when the carbon content is increased by more than 0.2%, high manganese hot rolled Cold workability of the steel sheet is lowered, making cold rolling difficult.

In addition, when the addition amount of carbon is less than 0.02%, the mechanical stability of the austenite is lowered, the ductility is lowered, the hardness of the martensite phase is insufficient, there is a problem that the strength of the cold rolled steel sheet is lowered. Therefore, the carbon is preferably limited to the content of 0.02 ~ 0.2%.

Manganese (Mn): 10-15%

The manganese (Mn) contributes to stabilization of austenite as a solid solution strengthening element in steel. If the manganese content is less than 10%, it is difficult to secure an austenite fraction of 50% or more after annealing of the cold rolled steel sheet, and if the manganese content exceeds 15%, the austenite in the steel sheet is excessively stabilized, which makes it difficult to cause deformation organic transformation. The strength increase effect by martensite formation is inhibited. Therefore, the content of the manganese is preferably limited to 10-15%.

Aluminum (Al): 0.01 ~ 3%

The aluminum is known to inhibit the production of ε-martensite as an element that increases the stacking defect energy in high manganese steel. When austenite is transformed into ε-martensite during deformation in high manganese steel, the effect of increasing strength is small, and therefore, it is preferable to suppress it to secure high strength.

 If the content of Al is less than 0.01%, the inhibitory effect on the transformation of ε-martensite is small, and if the amount is more than 3%, nozzle clogging occurs during steelmaking and continuous casting, and excessive surface oxidation occurs during hot rolling. Decreased surface quality. Therefore, the content of Al is preferably limited to 0.01 ~ 3%.

[Manufacturing method]

Hereinafter, a high manganese type ultra high strength cold rolled steel sheet manufacturing method excellent in ductility according to the present invention configured as described above will be described with reference to FIG. 4.

4 is a manufacturing process chart showing a method of manufacturing a high manganese type ultra high strength cold rolled steel sheet excellent in ductility according to the present invention.

As shown in the drawing, a method for producing a high manganese type ultra high strength cold rolled steel sheet excellent in ductility according to the present invention is 0.02 to 0.2% carbon (C), 10 to 15% manganese (Mn), and 0.01 to 3% by weight. Hot rolling step (S100) of hot rolling a slab of steel including aluminum (Al), balance iron (Fe) and other unavoidable impurities, and cold rolling step of cold rolling the hot rolled hot rolled steel sheet at room temperature (S200). And annealing the cold rolled cold rolled steel sheet at a temperature range of Ac ₃ ± 50 ^o C to form fine austenite grains (S300), and to cool the cold rolled steel sheet having fine austenite grains formed at room temperature. It consists of a cooling step (S400).

Each step will be explained in more detail.

The hot rolling step (S100) is a process of hot rolling and winding a slab of steel having the composition ratio as described above.

In the cold rolling step (S200), the austenitic phase remaining in the hot rolled steel sheet is cold rolled at a sufficient cold rolling rate so that the phase fraction may be reduced to less than 1% by deformation organic martensite transformation.

This is because when austenite phase is present at 1% or more after cold rolling, growth of austenite remaining in the subsequent annealing process occurs to form coarse grains, and grain coarsening is performed in a grain forming step (S300) and a room temperature cooling step. In (S400) to reduce the mechanical stability of the austenite to inhibit the uniform elongation of the steel sheet.

The grain forming step (S300) is a process of annealing the cold rolled steel sheet at a temperature range of Ac ₃ ± 50 ^o C to heat-treat the fine austenite grains to have a phase fraction of 50% or more.

The holding time at the annealing temperature of the cold rolled steel sheet is carried out within a range in which coarsening of austenite grains does not proceed, and more particularly, the annealing time of the cold rolled steel sheet is preferably within 30 minutes.

And the temperature increase rate up to annealing temperature can be performed on normal conditions.

Since the room temperature cooling step (S400) it is possible to use the usual cooling conditions, including air cooling and water cooling, the process of maintaining the austenite volume fraction in the cold rolled steel sheet to 50% or more.

The microstructure of the cold rolled steel sheet manufactured according to the above process will have a grain size of less than 5 ㎛ as shown in FIG.

Hereinafter, the present invention will be described in more detail with reference to Examples.

EXAMPLE

The steel ingot formed as shown in Table 1 was heated at 1200 ^° C. for 2 hours, followed by finishing hot rolling and air cooling at 900 ^° C. (S100: hot rolling step). The hot rolled steel sheet was cold rolled at a reduction ratio of 60% at room temperature to reduce the fraction of austenite phase remaining in the steel sheet to less than 1% (S200: cold rolling step).

Then, the cold rolled steel sheet was annealed at a temperature range of Ac ₃ ± 50 ^o C for 10 minutes so that the phase fraction of the fine austenite grains was 50% or more (S300: grain forming step) and then cooled to room temperature (S400: normal temperature cooling step). The specimen was prepared.

Using the specimen prepared as described above was measured the strength and elongation at room temperature and the results are shown in Table 2 below. Figure 3 shows a typical nominal stress-nominal strain curve of the ultra-high strength steel excellent in ductility produced by the present invention.

TABLE 1 Chemical Composition, Ac ₃ Temperature, Annealing Temperature and Austenitic Volume Fractions of Examples and Comparative Examples

division Composition (wt%) Ac ₃ temperature ( ^oC ) Annealing Temperature ( ^oC ) Austenitic fraction (%) C Mn Al Example 1 0.04 10.5 0.03 632 640 55 Example 2 0.05 13.2 0.05 605 620 75 Example 3 0.08 11.2 0.03 628 620 81 Example 4 0.08 12.9 0.03 601 640 99 Example 5 0.12 11.3 1.53 669 650 95 Example 6 0.12 14.5 1.52 622 650 99 Comparative Example 1 0.008 13.2 0.03 603 640 42 Comparative Example 2 0.07 18.2 1.51 547 600 82 Comparative Example 3 0.12 8.2 1.53 756 760 72

TABLE 2 Room Temperature Tensile Properties of Examples and Comparative Examples

division Yield strength (MPa) Tensile Strength (MPa) Total Elongation (%) Tensile Strength x Total Elongation (MPa%) Example 1 402 1321 22.7 30055 Example 2 526 1265 26.9 34029 Example 3 427 1537 17.4 26744 Example 4 564 1360 25.8 35088 Example 5 465 1699 16.5 28034 Example 6 587 1453 20.8 30222 Comparative Example 1 375 983 23.4 23002 Comparative Example 2 582 1695 12.6 21357 Comparative Example 3 426 956 16.9 16156

As can be seen from the above [Table 2], Examples 1 to 6 have a tensile strength of 1.2 GPa or more and a strength-elongation balance value of 25000 MPa% or more.

However, in the case of Comparative Examples 1 to 3 in which the content of manganese (Mn) or carbon (C) is out of the range of the present invention, the tensile strength was less than 1.2 GPa or the strength-elongation balance value was less than 25000 MPa%.

The scope of the present invention is not limited to the above-exemplified embodiments, and many other modifications based on the present invention may be made by those skilled in the art within the above technical scope.

As described in detail above, the present invention, by controlling the content of carbon (C) and manganese (Mn) to secure more than 50% of the fine austenite grains by annealing after cold rolling, by actively using metamorphic organic plasticity high strength And elongation.

Therefore, there is an advantage in that defects such as bursting at the time of shaping of parts are reduced.

In addition, there is an advantage that the durability and safety of the vehicle is improved when applied as a vehicle component due to the improvement in formability.

Claims (5)

Weight percent, comprising 0.02 to 0.2% carbon (C), 10 to 15% manganese (Mn), 0.01 to 3% aluminum (Al), balance iron (Fe) and other unavoidable impurities, A high manganese type super high strength cold rolled steel sheet having excellent ductility, characterized by a tensile strength of 1.2 GPa or more and a product of tensile strength and total elongation (TS × T.El) of 25000 MPa% or more. According to claim 1, The cold rolled steel sheet is a high manganese ultra-high strength cold rolled steel sheet having excellent ductility, characterized in that the austenite fraction is 50% or more. Slab of steel consisting of 0.02 to 0.2% carbon (C), 10 to 15% manganese (Mn), 0.01 to 3% aluminum (Al), balance iron (Fe) and other unavoidable impurities in weight% Hot rolling step of hot rolling, A cold rolling step of cold rolling the hot rolled hot rolled steel sheet at room temperature, A grain forming step of annealing the cold rolled cold rolled steel sheet at a temperature range of Ac ₃ ± 50 ^o C to form fine austenite grains; A method of manufacturing a high manganese type super high strength cold rolled steel sheet having excellent ductility, comprising a step of cooling the cold rolled steel sheet having fine austenite grains formed at room temperature. The method of claim 3, wherein the cold rolling step, The high manganese type super high strength cold rolled steel sheet manufacturing method characterized in that the process of lowering the residual austenite fraction in the hot rolled steel sheet to less than 1% by weight. The method of claim 4, wherein the room temperature cooling step, A method of manufacturing a high manganese type super high strength cold rolled steel sheet having excellent ductility, characterized in that the process of maintaining the austenite fraction in the cold rolled steel sheet 50% by weight or more.

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