KR101480497B1 - Method for manufacturing twinning-induced plasticity steel sheet by utilizing dynamic recrystallization and twinning-induced plasticity steel manufactured by the same - Google Patents

Abstract

The present invention relates to a method of manufacturing a TWIP steel sheet with improved elongation and yield strength and a steel sheet produced by the method. The steel sheet comprises 15 to 20% of Mn, 0.5 to 1.0% of C, 0.5 to 2.0% of Al, (TWIP) steel consisting of Fe and unavoidable impurities is heated, the heated TWIP steel is hot-rolled, the hot-rolled TWIP steel is cooled, the cooled TWIP steel is warm-rolled and processed, A method of manufacturing a TWIP steel sheet including winding a TWIP steel is disclosed. Also disclosed is a TWIP steel sheet produced thereby.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing a twinned organic plastic steel sheet using dynamic recrystallization and a twin-

The present invention relates to a method of manufacturing a twinned organic plastic steel sheet and a twinned organic plastic steel sheet produced thereby. More particularly, the present invention relates to a twinned organic plastic steel sheet produced by partially dynamic recrystallization, And a twin-crystal organic plastic steel sheet produced by the method.

In general, twinning-induced plasticity steel (TWIP) is a microstructure of austenite with stacking fault energy (SFE) adjusted to 10 to 40 mJ / m ² by adding alloys such as manganese and carbon. . Since the TWIP steel utilizes the potential and twin as the deforming mechanism, the TWIP steel is superior in strength and ductility to conventional carbon steel, DP (dual-phase) and TRIP (Transformation Induced Plasticity) steels, Respectively.

Industrial barriers to TWIP steels are cost increases, delayed fracture and low yield strength due to the addition of high alloy. The cost increase of the base material is an alternative to the cost reduction by omission of the subsequent process (welding, cutting) through good processability. The occurrence of delayed fracture is improved by adding alloying elements such as Al.

However, in order to improve the yield strength, the yield strength did not exceed 600 MPa with the addition of the proposed carbide element (V, Ti) and the austenitic grain refinement method. In addition, the pre-straining method through cold rolling has a disadvantage in that the yield strength reaches 1000 MPa, but the processability, which is an advantage of TWIP steel, sharply decreases.

Prior art patents for TWIP steels reported in the prior art include Korean Patent Laid-Open Publication Nos. 2011-0072791, 2010-0106649, and 2010-0108610. However, the above patent also relates to a manufacturing method of a TWIP plate, and suggests only a method for improving tensile strength and ductility without considering a low yield strength. In Patent No. 2010-0113679, a method of manufacturing a 550 MPa-class TWIP steel is described using a method of grain refinement through heat treatment after cold rolling as a method for improving the yield strength.

TWIP steels, which can be applied to automotive exterior panels, are required to have both excellent yield strength for safety and ductility required for workability.

To solve the above problems, the present invention provides a TWIP steel having improved yield strength, tensile strength and elongation, and a TWIP steel sheet produced thereby.

In one or more embodiments of the present invention, the twinning-induced plasticity (TWIP), which comprises 15 to 20% Mn, 0.5 to 1.0% C, 0.5 to 2.0% Al and the balance Fe and unavoidable impurities, Heating the steel; Hot-rolling the heated TWIP steel; Cooling the hot rolled TWIP steel; Warm-rolling and processing the cooled TWIP steel; And winding the hot-rolled TWIP steel.

The heating is performed at 1100 to 1200 ° C, and the cooling and warm rolling is performed at 500 to 700 ° C.

The winding is performed at 500 ° C or less, and the heating is performed for 1 to 3 hours.

The cooling step may be a step of cooling the cold-rolled steel sheet to room temperature, and reheating the steel sheet to 500 to 700 ° C.

The hot rolling is performed such that the reduction rate is 50% or more.

In one or more embodiments of the present invention, the weight% is 15 to 20% of Mn, 0.5 to 1.0% of C, 0.5 to 2.0% of Al, the balance of Fe and unavoidable impurities, A twinning-induced plasticity (TWIP) steel sheet may be provided that includes fine grains recrystallized by a twinning-induced plasticity (TWIP) steel sheet.

The TWIP steel sheet has a yield strength of 600 MPa or more, an elongation of 25% or more, and a size of the fine grain size of 3 μm or less.

The volume fraction of the fine grains in the steel sheet is 10 to 20%.

According to the embodiment of the present invention, by improving the yield strength of the TWIP steel to 600 MPa or more by using the partial dynamic recrystallization, the stability can be increased when the TWIP steel is applied to the automobile exterior panel.

In addition, high ductility through recrystallized fine grains is ensured, and TWIP steel having high yield strength and excellent workability can be produced.

In addition, by increasing the density of twinning and dislocation of unrecrystallized coarse grains of the partially dynamically recrystallized microstructure, the yield strength can be improved and the elongation can be secured to 0.25 or more.

In addition, by applying hot rolling, it is easy to apply from the viewpoint of productivity and cost can be saved by eliminating the addition of expensive carbide-forming alloy elements (V, Ti) for increasing the yield strength.

1 is an electron backscattering diffraction (EBSD) photograph of a hot rolled material of TWIP steel manufactured according to an embodiment of the present invention and a hot rolled steel material at 500 ° C, 600 ° C, and 700 ° C.
FIG. 2 is a transmission electron microscope (TEM) photograph of unrecrystallized grains of a TWIP steel produced according to an embodiment of the present invention.
FIG. 3 is a graph of nominal tensile curves of hot rolled materials of TWIP steels and hot rolled steels at 500 ° C, 600 ° C and 700 ° C according to an embodiment of the present invention.
4 is a flowchart of a manufacturing process of a TWIP steel according to an embodiment of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS The advantages and features of the present invention and the manner of achieving them will become apparent with reference to the embodiments described in detail below with reference to the accompanying drawings. However, it is to be understood that the present invention is not limited to the disclosed embodiments, but may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. It is intended that the disclosure of the present invention be limited only by the terms of the appended claims.

An embodiment of the present invention discloses a method of manufacturing twinning-induced plasticity steel (TWIP) steels by forming recrystallized fine grains and partially recrystallized elongated coarse grains using partial dynamic recrystallization, A method of making a TWIP steel with enhanced elongation is disclosed.

FIG. 4 is a flow chart of a manufacturing process of a TWIP steel according to an embodiment of the present invention. Referring to FIG. 4, in the embodiment of the present invention, the steel according to the present invention comprises 15 to 20% of Mn, 0.5 to 1.0% The hot TWIP steel is hot-rolled (S120) at a temperature range of 1000 to 1200 占 폚, and the hot-rolled steel is hot-rolled at a temperature of 500 to 1200 占 폚, After cooling to 700 ° C (S130), the TWIP steel is subjected to warm rolling (S140) at 500-700 ° C for partial dynamic recrystallization to obtain a reduction ratio of 50% or more. Thereafter, the steel sheet is wound at 500 DEG C or less (S150) to produce a TWIP steel sheet.

If the reduction rate is less than 50%, dynamic recrystallization does not occur smoothly. Therefore, in the embodiment of the present invention, the rolling reduction during warm rolling is limited to 50% or more.

Further, in step S130, the cold-rolled TWIP steel may be cooled to room temperature and then reheated to 500 to 700 deg. C (S125). At this time, cooling to room temperature is performed by water cooling or air cooling. However, since the TWIP steel according to the present invention is high carbon steel, there is a possibility that a carbide oxide is formed on the grain boundary, so that the water cooling method is more effective.

The reason for limiting the numerical value of the TWIP steel component in the embodiment according to the present invention will be described below.

Manganese (Mn): 15 to 20 wt%

Manganese is an element that stabilizes austenite by increasing stacking fault energy. When stacking fault energy is more than 10 mJ / m ² , twinning is used as a deforming mechanism and excellent mechanical characteristics are exhibited. If the content of manganese is less than 15% by weight, modified epsilon martensite is formed to cause deterioration of mechanical properties. When the content of manganese exceeds 20% by weight, the production cost increases. Therefore, The content is limited to the above range. Further, in order to adjust the optimum stacking fault energy to 20 mJ / m ² , when the manganese content is 20 wt% or less, it is necessary to add an alternative alloying element such as carbon and aluminum.

Carbon (C): 0.5 to 1.0 wt%

Carbon is an austenite stabilizing element that most effectively increases the stacking fault energy, and it is effective to reduce the content of alloying elements such as manganese by adding 0.5 wt% or more. When carbon is added in an amount of less than 0.5% by weight, manganese content should be not less than 25% by weight so that modified epsilon martensite is not formed. On the other hand, when carbon is added in an amount exceeding 1.0% by weight, carbide is formed on the austenite grain boundaries during the cooling process, resulting in embrittlement of hydrogen embrittlement resistance and impact properties, which are main causes of grain boundary fracture. The content is limited to the above range.

Aluminum (Al): 0.5 to 2.0 wt%

Aluminum is not only an austenite stabilizing element that increases the stacking defect energy but also an element that is effective in improving resistance to hydrogen embrittlement by increasing delayed fracture resistance and improving corrosion resistance when added in an amount of 0.5 wt% or more. Aluminum is added to improve the resistance to hydrogen embrittlement, which becomes a problem as the steel is intensified. However, if it is added in an amount exceeding 2.0% by weight, problems may occur in the production process such as clogging of the nozzle during the steelmaking process. In the examples, the content of aluminum is limited to the above range.

The dynamic recrystallization in the embodiment according to the present invention means that internal defects generated during deformation act as recrystallization nuclei generation sites and recrystallization of crystal grains occurs during deformation. The dynamic recrystallization shows the highest dynamic recrystallization rate when the temperature is changed in the warm state as compared with the high temperature at which dynamic recovery occurs. Dynamic recrystallization can be classified into full dynamic recrystallization and partial dynamic recrystallization. In a fully dynamic recrystallization, all grains are recrystallized during deformation. Partial dynamic recrystallization has a high Schmid factor for deformation It means that recrystallization occurs only in the crystal grains.

In the embodiment of the present invention, the partial dynamic recrystallization consists of recrystallized fine grains and unrecrystallized coarse grains. The fine grains are free from defects due to dynamic recovery, and coarse grains are cold worked Microstructure.

The recrystallized grains of the partially dynamic recrystallized microstructure of the TWIP steel according to the present invention have high ductility and twinning as a transformation mechanism in the subsequent processing through dynamic recovery, and the non-recrystallized grains have a density of dislocation and twin So that it has a high yield strength at the time of deformation. In the examples according to the present invention, the yield strength of the TWIP steel is 600 MPa or more and the tensile strength is 1000 MPa or more.

In the embodiment according to the present invention, the heating of the TWIP steel is performed at 1100 to 1200 ° C for 1 to 3 hours. If the heating temperature is lower than 1100 ° C, the effect of heat treatment of manganese is reduced, so that a long heat treatment time is required to prevent segregation of manganese. On the other hand, when the heating temperature exceeds 1200 ° C, decarburization occurs rapidly, A gradient of < / RTI >

If the heating time at the heating temperature is less than 1 hour, manganese segregation may occur. If the heating time exceeds 3 hours, there is no difference in heating effect, which is economically disadvantageous. Is limited to the above range.

In the cooling step (S130), the temperature is cooled to 500 to 700 deg. C by air cooling or water cooling. When the temperature is higher than 700 deg. C or lower than 500 deg. C, the partial dynamic recrystallization effect through warm rolling is lowered. This is the same in the case of reheating to 500 to 700 ° C after cooling to room temperature in step S125.

In the warm rolling step, the reduction rate is 50% or more at a processing temperature of 500 to 700 ° C. When the processing temperature is less than 500 ° C, the elongation of the TWIP steel may be less than 25% The work hardening effect can not be expected in the non-recrystallized grains due to the dynamic recovery effect during the warm rolling, so that the yield strength of 600 MPa or more can not be obtained. Therefore, the temperature of the warm rolling in the embodiment of the present invention Is limited to the above range.

Finally, the winding step is carried out at a temperature of 500 ° C or lower. If the temperature exceeds 500 ° C, static recovery of the dislocations occurs in the non-recrystallized grains, which may lead to a decrease in the yield strength. Therefore, winding of the TWIP steel is performed at 500 ° C or higher .

Hereinafter, embodiments according to the present invention will be described in more detail.

[Example]

In an embodiment of the present invention, TWIP steel of alloy composition Fe-18Mn-0.6C-1.5Al was used, and a 50 Kg ingot was dissolved in a vacuum heating furnace.

The ingot was first heat treated in a heating furnace at 1200 ° C for 2 hours for solution heat treatment.

These ingots were taken out of the furnace, hot rolled at 1100 ° C, hot rolled to a length of 750 mm, a width of 250 mm, and a thickness of 35 mm, followed by air cooling to room temperature.

Then, the hot rolled material was cut into a length of 250 mm, a width of 35 mm, and a thickness of 34 mm using a water jet.

The hot rolled material was reheated and maintained in a heating furnace heated to 500 ° C, 600 ° C, and 700 ° C for 30 minutes, followed by warm rolling. At this time, a rolled steel sheet having a thickness of 35 mm was hot-rolled and processed to a thickness of 17 mm and a hot-rolled reduction ratio of 50% to produce a hot rolled TWIP steel sheet.

FIG. 1 is a microstructure showing the microstructure of TWIP hot rolled material produced according to the above process and hot-rolled TWIP steel at 500.degree. C., 600.degree. C., and 700.degree. C. by electron backscattering diffraction (EBSD). 1, it can be seen that the hot-rolled material is an equiaxed crystal grain, the partial recrystallization ratio of the hot intermediate-tensioned material at 500 ° C is high, and the hot-rolled material at 700 ° C has a low partial recrystallization ratio and the crystal grain is elongated.

More specifically, the hot rolled material of Fig. 1 was an equiaxed grain with an average grain size of 20 mu m. When the hot rolled material is warm-rolled, the microstructure is partially dynamic recrystallized to form recrystallized fine grains and unrecrystallized elongated coarse grains. At this time, it can be seen that the recrystallized fine grains are 3 탆 or less, and dynamic recrystallization and dynamic recovery occur at the same time. In particular, the dynamic recrystallization rate was high at 500 ℃ with low rolling temperature, and dynamic recrystallization ratio was low due to dynamic recovery due to relatively high temperature at 700 ℃.

The size of the recrystallized microcrystalline grains in the embodiment of the present invention is limited to 3 탆 as described above. Since the recrystallization is for securing ductility, the size of the recrystallized microcrystalline grains does not greatly affect the size of the microcrystalline grains. However, If the size exceeds 3 mu m, a dislocation mechanism or twinning deforming mechanism may occur and the yield strength may be adversely affected. Therefore, in the embodiment of the present invention, the size of the fine grain is limited to 3 mu m or less.

In the embodiment of the present invention, the volume fraction of the fine crystal grains by recrystallization varies depending on the temperature of warm rolling, but it is advantageous from 10 to 20%. If it is less than 10%, it is difficult to secure ductility. If it exceeds 20%, it becomes difficult to secure the yield strength. Therefore, the volume fraction of the fine crystal grains in the embodiment of the present invention is limited to the above range.

FIG. 2 is a TEM photograph of unrecrystallized grains of a warm-rolled structure of a TWIP steel produced according to an embodiment of the present invention. FIG. Referring to FIG. 2, a number of twinning and dislocations in two directions were observed in the warm rolled material at 500 ° C. In the hot rolled material at 600 ° C, a small number of twinning and potential observations were observed. In the hot rolled material at 700 ° C, Not observed.

In other words, no dynamic recrystallization occurred in the non-recrystallized grains, and dislocations and twinning occurred in the interior of the unreinforced crystal grains. In the case of the warm-edged elongation at 500 ° C, Only twinning was observed. On the other hand, twinning was not observed in the 700 ° C hot-air pressure series and dislocation density was lower than that of the 500 ° C and 600 ° C hot-air pressure series. As the temperature increases, the stacking fault energy of the Fe-18Mn-0.6C-1.5Al steel rises and the value of the critical resolved shear stress for the deformation twin increases due to the increase of the stacking fault energy. The twin is not observed. This is also why a low dynamic recrystallization ratio appears at 700 ° C. That is, because twinning occurs less with higher temperature increase, the recrystallization nucleation sites are reduced and dynamic recrystallization does not occur well.

Therefore, the micro-structure of the non-recrystallized coarse grained grain in the embodiment according to the present invention has a high density of twins and dislocations.

FIG. 3 is a graph showing tensile test results of a hot rolled material of a TWIP steel manufactured according to an embodiment of the present invention and a hot-rolled TWIP steel at 500 ° C, 600 ° C, and 700 ° C. Referring to FIG. 3, it can be seen that the yield strength of the hot pressured steel is increased by 200 to 300% through the dislocations and twinning of the dynamically recrystallized microcrystalline grains and the non-recrystallized grains.

The tensile test was carried out with a specimen of ASTM E8M-subsize standard using INSTRON 8801 equipment at a strain rate of 5x10 ^-3 / sec. It can be seen that the yield strength of the hot pressured steel of 500 ° C is higher than that of the hot pressured steel of 700 ° C, which can be judged by the effect of the non-recrystallized grains having relatively high dislocation and twin density.

In addition, it is possible to manufacture a high strength TWIP steel sheet satisfying an elongation ratio of 30% required for not only an increase in strength but also an automobile exterior plate material, and can contribute to the improvement of the stability of the automobile exterior plate material by using TWIP steel added with only hot rolling.

The TWIP steel according to the embodiment of the present invention is improved by 10 to 35% higher than the yield strength of 550 MPa of the fine grain TWIP steel annealed after the conventional cold rolling.

While the present invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, You will understand.

It is therefore to be understood that the above-described embodiments are illustrative in all aspects and not restrictive. The scope of the present invention is defined by the appended claims rather than the detailed description, and all changes or modifications derived from the meaning and scope of the claims and their equivalents should be interpreted as being included in the scope of the present invention .

Claims (12)

Heating a TWIP (twinning-induced plasticity) steel comprising 15 to 20% of Mn, 0.5 to 1.0% of C, 0.5 to 2.0% of Al, and the balance of Fe and unavoidable impurities;
Hot-rolling the heated TWIP steel;
Cooling the hot rolled TWIP steel;
Warm-rolling and processing the cooled TWIP steel; And
And a step of winding the hot-rolled TWIP steel,
Warm-rolling and processing the cooled TWIP steel,
500 to < RTI ID = 0.0 > 700 C. < / RTI >
delete The method according to claim 1,
Wherein the heating is performed until the temperature reaches 1100 to 1200 占 폚.
The method according to claim 1,
Wherein the cooling is performed until the temperature reaches 500 to 700 ° C.
The method according to claim 1,
Wherein the coiling is performed at 500 DEG C or less.
The method of claim 3,
Wherein the heating is performed for 1 to 3 hours. The method according to claim 1,
Wherein the hot rolling is performed such that the reduction rate is 50% or more.
(TWIN) containing microcrystalline grains recrystallized by partial dynamic recrystallization, the remainder being Fe and inevitable impurities, and having a composition of Mn: 15 to 20%, C: 0.5 to 1.0%, Al: 0.5 to 2.0% -induced plasticity)
Further comprising coarse crystal grains which have not yet been recrystallized by the partial dynamic recrystallization,
In the partial dynamic recrystallization,
Heating TWIP (twinning-induced plasticity) steel;
Hot-rolling the heated TWIP steel;
Cooling the hot rolled TWIP steel;
Warm-rolling and processing the cooled TWIP steel; And
Winding the hot-rolled TWIP steel,
Warm-rolling and processing the cooled TWIP steel is performed at a temperature of 500 to 700 ° C.
TWIP steel plate.
9. The method of claim 8,
Wherein the TWIP steel sheet has a yield strength of 600 MPa or more. 9. The method of claim 8,
Wherein the TWIP steel sheet has an elongation of 25% or more. 9. The method of claim 8,
And the size of the fine crystal grains is 3 탆 or less. 12. The method of claim 11,
Wherein the volume fraction of the fine grains in the steel sheet is 10 to 20%.

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