KR101054773B1 - Manufacturing Method of TPI Type Ultra High Strength Steel Sheet - Google Patents

Abstract

Twinning induced plasticity ultra-high strength steel sheet with appropriately controlled contents of carbon (C), silicon (Si), manganese (Mn), aluminum (Al), molybdenum (Mo), phosphorus (P) and sulfur (S) The manufacturing method to increase yield strength, tensile strength and elongation is introduced. Characteristic of this manufacturing method is to induce the bond between the dislocation and the twin to promote the generation of sub-crystal grains in the austenite matrix grain recovery recovery heat treatment in the middle of cold rolling at 200 ~ 220 ℃, then at 700 ~ 850 ℃ Annealing heat treatment is performed for a short time to refine the grain size to 2-3㎛.

Twisted steel, grain, refinement, heat treatment

Description

Manufacturing Method of TPP Type Ultra High Strength Steel Sheet {METHOD FOR MANUFACTURING TWINNNING INDUCED PLASTICITY TYPE ULTRA-HIGH STRENGTH STEEL SHEET}

The present invention relates to a method for manufacturing a TWIP type ultra high strength steel sheet, and to a method for producing both yield strength, tensile strength, and elongation of a TWIP type ultra high strength steel sheet useful for vehicle body parts.

In general, the high tensile steel sheet widely applied as a material for vehicle body parts is mainly made of steel sheets having physical properties of tensile strength of 590 to 780 MPa, yield strength of 270 to 350 MPa, elongation of 25 to 35% and plastic strain ratio of 0.9 to 1.2. have. As the steel sheet became high tension, the thickness of the high tensile steel sheet was made and used in consideration of phenomena such as bursting and wrinkles caused by lack of elongation at the time of press forming parts and required side rigidity.

However, even if the elongation of the high tensile strength steel sheet is sufficiently secured, it is often difficult to form the high tensile strength steel sheet as the parts are complicated and multifunctional, and thus, it is necessary to improve the properties of the high tensile strength steel sheet with the development of mold technology.

Therefore, carbon and manganese (Mn) are based on carbon (C) and manganese (Mn), so that ultra high tensile strength and high elongation can be ensured, while carbon (C) is 0.15 to 0.30% by weight, silicon (Si) 0.01 to 0.03% by weight, and manganese ( Mn) 15-25 wt%, aluminum (Al) 1.2-3.0 wt%, phosphorus (P) 0.020 wt% or less, sulfur (S) 0.001-0.002 wt%, residual iron (Fe) and other unavoidable impurities In addition, TWIP (Twin Induced Plasticity) type ultra-high strength steel sheet, which is characterized in that the base structure of the steel is formed of austenite phase, has been attempted to meet the demand for weight reduction of the vehicle body. (KR 1020070018416)

However, in spite of the increasing demand for high impact stiffness despite the breakthrough properties of the steel plate material and the demand for it to be used as a body part having a complicated shape, the TWIP (Twin Induced Plasticity) type super It was necessary to improve both the yield strength, the tensile strength, and the elongation of the high strength steel sheet. This is because, when the elongation is low, a problem arises in that the defective rate increases during molding of the product.

To this end, the addition of various alloying elements was required, but the direction of reducing alloying elements has been recently researched due to the preparation of countermeasures against soaring raw material prices and the demand for eco-friendly materials. There was a need to develop ways to improve.

The present invention has been proposed to solve the above problems and to meet the necessity, to provide a manufacturing method which can increase the elongation while increasing the yield strength and tensile strength of the TWIP (TWin Induced Plasticity) type ultra high strength steel sheet. For the purpose of

Method for producing a TWIP (Twin Induced Plasticity) type ultra high strength steel sheet according to the present invention for achieving the above object is carbon (C): 0.15 ~ 0.30% by weight, silicon (Si): 0.01 ~ 0.03% by weight, manganese (Mn) ): 15-25 wt%, aluminum (Al): 1.2-3.0 wt%, phosphorus (P): 0.020 wt% or less, sulfur (S): 0.001-0.002 wt%, residual iron (Fe) and other unavoidable impurities Cold rolling is made of four passes or more for the hot rolled steel sheet having a composition comprising a, but after three passes, the recovery heat treatment is performed at 200 ~ 220 ℃, after which annealing heat treatment is performed.

The recovery heat treatment is preferably performed for 5 to 6 minutes.

The annealing heat treatment is preferably performed for 5 to 6 minutes at 700 ~ 850 ℃.

According to the manufacturing method of the TWIP-type super high strength steel sheet as described above, it is possible to increase the yield strength and tensile strength, and at the same time increase the elongation, thereby significantly lowering the component molding failure rate.

Hereinafter, with reference to the accompanying drawings looks at the manufacturing method of the TWIP type ultra high strength steel sheet according to a preferred embodiment of the present invention.

Above TWIP type ultra high strength steel sheet, carbon (C): 0.15 ~ 0.30% by weight, silicon (Si): 0.01 ~ 0.03% by weight, manganese (Mn): 15-25% by weight, aluminum (Al): 1.2 ~ 3.0% %, Phosphorus (P): 0.020% by weight or less, sulfur (S): 0.001 to 0.002% by weight, steel sheet composition containing the remaining amount of iron (Fe) and other unavoidable impurities.

In the method of manufacturing a TWIP type ultra high strength steel sheet according to an embodiment of the present invention, the steel sheet composition having the composition and content as described above is dissolved in a converter and continuously cast, followed by hot rolling at 1100 to 1300 ° C., followed by 600 to 900 ° C. Winding step is the same as the existing manufacturing method.

After performing a cold rolling consisting of a total of five passes, the recovery heat treatment is performed at 200 ~ 220 ℃ after three passes. After the annealing heat treatment is performed at 700 ~ 850 ℃. In this case, the recovery heat treatment is preferably performed for 5 to 6 minutes, and the annealing heat treatment is also preferably performed for 5 to 6 minutes.

Here, the recovery heat treatment is performed to induce the bond between the dislocation and the twin so that the generation of subgrains in the austenite matrix can be promoted. Specifically, the bond between dislocation and twin is induced through a recovery heat treatment performed after three passes of cold rolling, and the dislocation and twin combined through cold rolling of four passes and five passes are generated as sub-crystals in the austenite grains. Induced.

The cold rolling consisting of the five passes is preferably carried out at a rate of 20-30% reduction per pass, which is similar to the conventional cold rolling of 5-7 passes having a reduction ratio of about 30% per pass. Cold rolling at this rolling rate is a method that is applied not only to tweezers but also to materials.

 Unlike the general steel sheet, the deformation mechanism of the TWIP type ultra high strength steel sheet is twin and slip simultaneously. Therefore, it was confirmed by the inventor's research that a recovery heat treatment process should be introduced in the middle of cold rolling to refine the grain. Therefore, the recovery heat treatment is introduced after three passes during the cold rolling, which is performed in five passes as described above.

If the recovery heat treatment is performed after 1 pass or 2 passes of cold rolling, the desired purpose cannot be achieved due to undeveloped sub-crystal grains, and if the recovery heat treatment is performed after 4 passes, misorientation between the developed granules It is preferable to perform a recovery heat treatment after three passes of cold rolling because the) has a low angle and cannot achieve the desired target properties during annealing heat treatment.

On the other hand, the cold annealing introduced into the intermediate heat treatment after the annealing heat treatment for a short time at 700 ~ 850 ℃ is to refine the grain size to 2 ~ 3㎛. By making the crystal grains fine in this manner, the elongation can be increased.

Hereinafter, the present invention will be described in more detail through comparison with Examples.

Examples and comparative examples of the present invention were prepared by the TWIP type ultra high strength steel sheet according to the composition and content of the following Table 1 and measured the mechanical properties through a tensile test, an optical microscope and an image analyzer, EBSD (Electron Back Scattered Diffraction The grain size was analyzed through the measurement. The results through such measurement and analysis are shown in Table 2 below.

Example is the slab prepared by melting in the converter and continuous casting start the hot rolling at 1300 ℃ and finally rolled at 1100 ℃, after cooling at a rate of 40 ℃ / sec from 900 ℃ to 600 ℃ and wound up, After 5 passes, cold rolling is carried out at a rolling reduction of 30% or less for each pass, but after 3 passes, heat treatment is performed at 200 to 220 ° C. for 5 minutes, and the remaining 2 passes are completed to complete 5 passes. By using the annealing heat treatment for 5 minutes at 700 ~ 850 ℃ to refine the grains.

On the other hand, the comparative example is generally according to the manufacturing method, specifically up to hot rolling is the same as the above embodiment, cold rolling is carried out cold rolling consisting of a total of five passes at a rolling rate of less than 30% per pass. The annealing heat treatment was performed at 850 ℃ for 8-10 hours using a furnace.

On the other hand, in order to determine the annealing heat treatment temperature for applying to the embodiment of the present invention, the grain size was measured by heat-treating the TWIP type ultra high strength steel sheet from 600 ℃ to 920 ℃ for 5 minutes, as shown in FIG. Referring to Figure 1, it can be seen that the grain size is about 2 ~ 3㎛ in the section of 700 ℃ to 850 ℃.

Chemical composition (wt%) C Si Mn Al P S Fe 0.15
~ 0.30 0.01
~ 0.03 15.0
~ 25.0 1.20
~ 3.00 0.020
Below 0.001
~ 0.002 remain

Recovery heat treatment temperature, ℃ Recovery heat treatment time, min Recovery heat treatment trial pass Annealing Heat Treatment Temperature, ℃ Annealing Heat Treatment Time, Min Yield strength, MPa Tensile strength, MPa Elongation,% Average grain size, μm Example 1
200
5
3
700
5
580
1020
53
2.1 Example 2
200
5
3
750
5
580
1020
53.2
2.3 Example 3
200
5
3
800
5
560
992
52.1
2.5 Example 4
200
5
3
850
5
520
989
52.1
2.9 Example 5
220
5
3
700
5
592
1008
52.3
2.0 Example 6
220
5
3
750
5
590
1010
52.2
2.12 Example 7
220
5
3
800
5
577
998
52.8
2.6 Example 8
220
5
3
850
5
580
992
53.1
2.88 Comparative Example 1
-
-
-
850
480
510
978
48.2
6.83 Comparative Example 2
-
-
-
850
540
502
978
48.5
9.35 Comparative Example 3
-
-
-
850
600
490
950
48.8
12.1 Comparative Example 4
200
5
3
850
480
505
980
48
7.0 Comparative Example 5
200
5
3
850
540
493
960
48.2
11.1 Comparative Example 6
200
5
3
800
600
462
963
48.5
12.4 Comparative Example 7
220
5
3
850
480
499
942
46.5
8.3 Comparative Example 8
220
5
3
850
540
493
931
47.3
9.2 Comparative Example 9
220
5
3
800
600
460
922
48.1
12.4 Comparative Example 10
200
4
3
700
5
530
980
42.1
3.3 Comparative Example 11
200
4
3
850
5
510
977
43.2
4.2 Comparative Example 12
220
4
3
700
5
523
977
42.8
3.5 Comparative Example 13
220
4
3
850
5
499
963
44.6
3.9 Comparative Example 14
200
7
3
700
5
510
977
41.2
3.8 Comparative Example 15
200
7
3
850
5
503
973
40.2
4.1 Comparative Example 16
220
7
3
700
5
511
974
45.1
3.9 Comparative Example 17
220
7
3
850
5
482
958
42.6
4.7

On the other hand, it was confirmed that recrystallization does not occur at a temperature of less than 700 ℃, in this case separately confirmed that the elongation of the final product is less than 20%, annealing heat treatment at a temperature below 700 ℃ was excluded.

Looking at the results shown in Table 2, according to the embodiment of the present invention it can be seen that the yield strength compared to Comparative Examples 1 to 3 at least 30MPa or more increased up to about 100MPa, the elongation also increased by more than 3 ~ 4%. In general, the higher the strength, the lower the elongation. In the embodiment of the present invention, both the strength and the elongation can be increased due to the presence of the twin in the amorphous grains produced by the intermediate heat treatment.

On the other hand, as shown in Comparative Examples 4 to 9, even if the intermediate heat treatment during cold rolling in the same as the above embodiment can be seen that no effect at all when the annealing heat treatment conditions are the same as before.

In addition, as shown in Comparative Examples 10 to 17, it can be seen that the medium heat treatment time during cold rolling has the greatest effect of about 5 minutes.

On the other hand, the reason why the annealing heat treatment time is set to 5 minutes in the embodiment of the present invention is because recrystallization does not occur in the heat treatment time below that, and in this case, an increase in elongation cannot be expected.

By the way, when the annealing heat treatment time significantly exceeds 5 minutes, it can be expected to increase the elongation, but it is not preferable to cause excessive decrease in yield strength due to excessive growth of grains. This can be confirmed through Figure 2 showing the decrease in yield strength with time at 700 ℃.

Therefore, the annealing heat treatment time is preferably 5 minutes to 6 minutes, which can be confirmed through the change in elongation shown in FIG. That is, referring to Figure 3, it can be seen that good elongation of 50% or more can be secured only in the heat treatment time of 5 minutes or more and less than 6 minutes.

On the other hand, Table 3 below is a part of the examples shown in Table 2 to confirm the change in physical properties of the TWIP type ultra high strength steel sheet after the intermediate heat treatment during cold rolling after the composition and content in the above embodiment And Comparative Examples are shown. Referring to Table 3, as shown in Comparative Examples 19 to 22 and Comparative Examples 23 to 26, it can be seen that performing the intermediate heat treatment after 4 passes or after 2 passes does not affect the physical properties at all. Therefore, the point of time of intermediate heat treatment is most preferably after three passes of cold rolling.

Recovery heat treatment temperature, ℃ Recovery heat treatment time, min Recovery heat treatment trial pass Annealing Heat Treatment Temperature, ℃ Annealing Heat Treatment Time, Min Yield strength, MPa Tensile strength, MPa Elongation,% Average grain size, μm Example 1
200
5
3
700
5
580
1020
53
2.1 Example 4
200
5
3
850
5
520
989
52.1
2.9 Example 5
220
5
3
700
5
592
1008
52.3
2.0 Example 7
220
5
3
800
5
577
998
52.8
2.6 Comparative Example 19
200
5
4
700
5
492
977
46.3
4.1 Comparative Example 20
200
5
4
850
5
488
976
46.3
3.9 Comparative Example 21
220
5
4
700
5
479
943
45.5
4.0 Comparative example
22
220
5
4
800
5
482
930
46.1
4.6 Comparative Example 23
200
5
2
700
5
490
975
47.1
3.7 Comparative Example 24
200
5
2
850
5
490
975
47.3
3.6 Comparative Example 25
220
5
2
700
5
483
950
43.5
4.2 Comparative Example 26
220
5
2
800
5
480
945
46.2
3.9

According to the manufacturing method of the TWIP type ultra high strength steel sheet according to the embodiment of the present invention as described above, the yield strength of the TWIP type ultra high strength steel sheet can be increased up to 100 MPa, and the elongation is increased by 3 to 4%. Elongation can be more than 50%, tensile strength can also be more than 980MPa. Therefore, it is possible to manufacture a TWIP-type ultra high strength steel sheet capable of complex molding while securing rigidity corresponding to the collision performance. Such matters can be confirmed through FIG. 4 showing the yield strength and the elongation of the Examples and Comparative Examples. Referring to FIG. 4, the yield strength and elongation data (A) of the embodiment indicate that the yield strength of the TWIP type ultra high strength steel sheet according to the embodiment is 520 to 592 MPa, and the elongation is 50% or more. On the contrary, the yield strength and elongation data (B) of the comparative example indicate that the yield strength of the TWIP type ultra high strength steel sheet according to the comparative example is 520 MPa or less, and the elongation is 50% or less.

While specific embodiments of the present invention have been illustrated and described, those of ordinary skill in the art may vary the present invention without departing from the spirit of the invention as set forth in the following claims. It is to be understood that modifications and variations are possible.

1 is a graph showing the grain size according to the annealing heat treatment time of TWIP type ultra high strength steel sheet.

Figure 2 is a graph showing the yield strength according to the annealing heat treatment time of the embodiment.

Figure 3 is a graph showing the elongation according to the annealing heat treatment time of the embodiment.

Figure 4 is a graph showing the yield strength and elongation of Examples and Comparative Examples.

<Description of the symbols for the main parts of the drawings>

A: Yield Strength and Elongation Data of Examples

B: Yield Strength and Elongation Data of Comparative Example

Claims (3)

Carbon (C): 0.15 to 0.30 wt%, Silicon (Si): 0.01 to 0.03 wt%, Manganese (Mn): 15 to 25 wt%, Aluminum (Al): 1.2 to 3.0 wt%, Phosphorus (P): 0.020 Less than or equal to weight, sulfur (S): 0.001 to 0.002% by weight, cold-rolled four or more passes for a hot-rolled steel sheet having a composition containing a residual amount of iron (Fe) and other unavoidable impurities, but 200 to 3 after three passes Recovery heat treatment is performed at 220 ℃ for 5 minutes, and then annealing heat treatment for 5-6 minutes at 700 ~ 850 ℃ manufacturing method of ultra-high strength TWIP (Twin Induced Plasticity) type steel sheet. delete delete

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