KR101113666B1 - Ultra-high strength twip steel sheets and the method thereof - Google Patents

Abstract

An ultra-high-strength tipped steel sheet of austenitic matrix containing 15-25 wt% manganese is introduced. This twipe steel sheet has a recrystallized texture in which brass orientation is suppressed during the heat treatment after cold rolling and {3 5 2} <2 2 1> is developed as a kitchen table, and the average plastic strain ratio is at least 1.2 or more, preferably 1.5 or more. to be. The twipe steel sheet is excellent in workability and strength, and can be easily manufactured to complex body parts without cracking or bursting through press molding.

Twisted steel, twin, aggregate, brass bearing, annealing

Description

Ultra-high strength tipped steel plate and manufacturing method thereof {ULTRA-HIGH STRENGTH TWIP STEEL SHEETS AND THE METHOD THEREOF}

The present invention relates to a Twiping Induced Plasticity (TWIP) steel sheet and a method of manufacturing the same, in which slip and twin act simultaneously as a deformation mechanism during plastic deformation, and in particular, ultra high strength for vehicle parts having excellent plastic deformation ratio. The present invention relates to a tipped steel sheet and a method of manufacturing the same.

In general, high tensile steel sheets are widely used as materials for body parts, and 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 are mainly used.

However, as the steel sheet becomes high, the steel sheet thickness is increased in consideration of various problems caused by elongation such as bursting and wrinkles during component press molding and required component rigidity. In addition, even if the elongation is sufficiently secured, molding is often difficult due to the complexity of the parts and the multifunctionalization. Therefore, it is necessary to greatly increase the plastic deformation ratio with the development of mold technology.

In order to meet the above requirements, the applicant, in weight%, carbon 0.15-0.30%, silicon 0.01-0.03%, manganese 15-25%, aluminum 1.2-3.0%, phosphorus 0.020% or less, sulfur 0.001-0.002% , Has proposed a super high-strength twipe steel sheet for the body parts including the remaining iron (Korean Patent Publication No. 2007-0018416).

However, despite the breakthrough physical properties of the steel sheet proposed in the above patents, at this point in time when the demand for the application of complex body parts increases, it is necessary to further improve the plastic strain ratio, which is the largest factor on the formability.

The present invention has been proposed to solve the above problems, and an object of the present invention is to provide an ultra-high strength steel sheet and a method of manufacturing the same with improved plastic strain ratio.

In order to achieve the above object, the present invention, rather than propose a new twipe steel sheet composition as in Korean Patent Laid-Open No. 2007-0018416, in a given composition through the recrystallization texture control in the annealing process after cold rolling The purpose of this paper is to propose a method to significantly improve the plastic strain ratio.

Specifically, the ultra-high strength tipped steel sheet according to the present invention has a recrystallized texture in which brass orientation is suppressed and {3 5 2} <2 2 1> is developed as a kitchen table during the heat treatment after cold rolling, and the average plastic strain ratio is at least. It is characterized by being 1.2 or more, preferably 1.5 or more. Twisted steel sheet according to the present invention contains 15 to 25% by weight of manganese, and has an austenite matrix as in the conventional twipe steel sheet. Very partially martensite or ferrite may be present, but it can be said to have an austenite single phase.
In this description, the National Assembly, as it is, refers to the most developed orientation in the collective.

Almost all materials obtained or processed in nature are lumps of crystals called polycrystalline. Their crystallographic orientations are in most cases not disordered and certain orientations appear strongly. A material with an unordered crystal orientation is said to have an orientation, that is, an aggregate.

In general, austenitic-based metal plates such as twipe steel exhibit crystallographic textures consisting of copper bearings, goth bearings, brass bearings, S bearings and cubic crystal bearings. Their relative volume fraction affects the average plastic strain ratio. The orientation of the crystal in the plate produced by rolling is defined by the rolled plate surface and the rolling direction. In other words, the texture can be exhibited in the plane of the crystal placed in parallel with the rolled surface and in the direction of the crystal placed in parallel with the rolling direction. The face of the crystal is represented by the Miller index {hkl} and the direction is represented by <uvw>, for example, the copper orientation is {112} <111>, the goth orientation is {011} <100>, and the brass orientation is {112 } <110>, the S orientation may be indicated as {123} <634> and the cubic orientation as {001} <100>.

Metals with low Stacking Fault Energy (SFE), such as Twisted Steel, develop brass orientation, especially during cold rolling.The cold rolled tipped steel sheet is annealed according to the prior art (8 to 10 hours at 800 to 900 ° C). Annealing treatment results in a so-called continuous recrystallization with little change in the texture of the aggregates and annealing twins (ie, Σ3 grain boundaries). However, the average plastic strain ratio of the conventional annealed tweezers steel sheet is about 0.911, it is difficult to increase more than that. Thus, in the present invention, in order to improve the plastic strain ratio, the brass orientation is suppressed in the annealing process and {3 5 2} <2 2 1> is developed as a kitchen plate.

The ultra-high strength twipe steel sheet manufacturing method according to the present invention is characterized by performing an annealing treatment on a cold rolled tipped steel sheet (hereinafter, simply referred to as a "cold rolled steel sheet"). That is, firstly, a recovery heat treatment is performed in the first temperature section to reduce the Coincide Site Lattice (CSL) grain fraction, such as Σ3, where recrystallization does not easily occur during high-angle grain boundaries. The recrystallization heat treatment is performed in the second temperature section so that the next recrystallization is performed. In the twipe steel sheet obtained through the annealing treatment, {3 5 2} <2 2 1> is developed as a kitchen plate instead of the brass bearing, and the annealing twin is significantly reduced in comparison with the prior art. Further, such a tipped steel sheet may have a grain boundary fraction of Σ3 having a misorientation angle of 60 ° of 10% or less, a grain boundary fraction of 5 ° or less of the deviation angle of 50% or more, and a grain boundary fraction of 15 ° or more of a deviation angle of 40%. May be less than.
As is known, the CSL grain boundary is "matched", and the energy of the grain boundary is low, and the above? 3 grain boundary corresponds to the CSL grain boundary.
As is already known in textbooks, papers, and the like in the field of research on metal textures, the misorientation angle of 60 ° is the <111> direction of one grain and the <111> direction of the other grain when two grains are adjacent to each other. The angle between these parts shows the rotational relationship of 60 degrees, and the grain boundary in this case is represented by Σ3.

The cold rolled tipped steel sheet, in weight percent, 0.15 to 0.30% carbon, 0.01 to 0.03% silicon, 15 to 25% manganese, 1.2 to 3.0% aluminum, phosphorus 0.020% or less, sulfur 0.001 to 0.002%, containing the remaining iron It may have a composition. The recovery heat treatment may be performed at 230 to 280 ° C. for 20 to 30 minutes, and the recrystallization heat treatment may be performed at 700 to 820 ° C. for 6 to 8 hours.

On the other hand, the cold rolled tipped steel sheet is preferably manufactured as follows. The slab obtained by dissolving a raw material composition in a converter and continuously casting is hot-rolled at 1300-1100 degreeC. Then, after cooling by slow cooling at a rate of 60 ° C./sec or less to 900 to 600 ° C. to prevent martensite from occurring, the hot rolled coil thus manufactured is cold rolled at a 20 to 30% reduction rate per pass over 5 to 7 passes. do.

As described above, the ultra-high-strength tipped steel sheet according to the present invention has an excellent average plastic deformation ratio, and thus, it is possible to easily manufacture a vehicle body part having a complicated shape without cracking or bursting through press molding.

In addition, the ultra-high-strength twipe steel sheet according to the present invention is excellent in both strength and formability, and can reduce the thickness of the steel sheet, thereby satisfying the stiffness and workability required for the vehicle body parts, thereby reducing vehicle weight.

Hereinafter, with reference to the accompanying drawings looks at the ultra-high strength steel sheet and the manufacturing method according to another embodiment of the present invention.

Ultra high strength twipe steel sheet according to the present invention is a high manganese steel sheet containing 15 to 25wt% manganese. Specifically, the ultra-high strength tipped steel sheet according to the present invention may have a composition (see Table 1 below) of the tipped steel sheet proposed in Korean Patent Laid-Open Publication No. 2007-0018416. The reason for limiting the content of each alloy component is not different from that described in the specification of the above patent.

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

In order to confirm the properties of the ultra-high strength twipe steel sheet according to the present invention, the cold-rolled twipe steel sheet having the composition shown in Table 2 below was subjected to annealing treatment under various conditions, and then experiments were conducted to measure the average plastic strain ratio.

ingredient C Si Mn Al P S Fe Content (wt%) 0.22 0.03 21 2.0 0.01 0.001 Remainder

The cold rolled tipped steel sheet used in the experiment was manufactured as follows. First, the slab obtained by dissolving the composition having the composition shown in Table 2 in a converter and continuously casting was started hot rolling at 1300 ℃ and finished the final hot rolling at 1100 ℃. And, after slowly cooling to 900 ~ 600 ℃ at a rate of 40 ℃ / sec was wound, and the hot rolled coil thus produced was cold rolled over 7 passes. Cold rolling was carried out under plane strain conditions of deformation of up to 30% reduction per pass.

Annealing conditions for the cold-rolled twipe steel sheet prepared as described above are as shown in Table 3 below. As shown in Table 3, in Examples 1 to 9 according to the present invention, a binary annealing heat treatment clearly distinguished into a recovery heat treatment and a recrystallization heat treatment was performed. Specifically, the recovery heat treatment for Examples 1 to 9 was carried out for 30 minutes at a temperature of 230 ~ 280 ℃, recrystallization heat treatment was carried out for 8 hours at a temperature of 700 ~ 820 ℃. In Comparative Examples 1 to 9, annealing was performed at 800 to 900 ° C. for 8 to 10 hours according to the prior art, and Comparative Examples 10 to 25 were subjected to dual annealing as in the present invention, but the heat treatment temperature and time conditions were as follows. Was different from the conditions according to the invention. Specifically, the recovery heat treatment of Comparative Examples 10-25 was carried out for 5 minutes or 40 minutes at a temperature 230 ~ 280 ℃, recrystallization treatment was carried out for 7 hours or 9 hours at a temperature of 650 ℃ or 900 ℃. For reference, the recovery heat treatment and the recrystallization heat treatment were each performed in a batch furnace.

division Recovery heat treatment Recrystallization heat treatment Average
Plastic deformation ratio Temperature (℃) Minutes Temperature (℃) Time (h) Example 1 230 30 700 8 1.61 Example 2 230 30 770 8 1.71 Example 3 230 30 820 8 1.74 Example 4 250 30 700 8 1.62 Example 5 250 30 770 8 1.68 Example 6 250 30 820 8 1.68 Example 7 280 30 700 8 1.74 Example 8 280 30 770 8 1.75 Example 9 280 30 820 8 1.88 Comparative Example 1 - - 800 8 0.83 Comparative Example 2 - - 800 9 0.83 Comparative Example 3 - - 800 10 0.81 Comparative Example 4 - - 850 8 0.90 Comparative Example 5 - - 850 9 0.89 Comparative Example 6 - - 850 10 0.89 Comparative Example 7 - - 900 8 0.91 Comparative Example 8 - - 900 9 0.88 Comparative Example 9 - - 900 10 0.88 Comparative Example 10 230 5 650 7 0.77 Comparative Example 11 230 5 650 9 0.77 Comparative Example 12 230 40 650 7 0.81 Comparative Example 13 230 40 650 9 0.80 Comparative Example 14 230 5 900 7 0.78 Comparative Example 15 230 5 900 9 0.78 Comparative Example 16 230 40 900 7 0.66 Comparative Example 17 230 40 900 9 0.64 Comparative Example 18 280 5 650 7 0.83 Comparative Example 19 280 5 650 9 0.78 Comparative Example 20 280 40 650 7 0.86 Comparative Example 21 280 40 650 9 0.81 Comparative Example 22 280 5 900 7 0.82 Comparative Example 23 280 5 900 9 0.76 Comparative Example 24 280 40 900 7 0.73 Comparative Example 25 280 40 900 9 0.71

The average plastic strain ratio (R value) was calculated after applying 50% tensile strain in accordance with the KS standard for the tweezers annealed under the conditions described in Table 3 above. The average plastic strain ratio is calculated after measuring the plastic strain ratios in the 0 °, 45 ° and 90 ° directions with respect to the rolling direction, respectively. As a result, as shown in Table 3, the tweezers steel sheet according to Examples 1 to 9, the average plastic strain ratio (R value) is shown as more than 1.5, Comparative Examples 1 to 25 did not exceed 1.0.

With reference to Figures 1 to 6 concretely compare the Example and Comparative Example. 1 to 3 are each an Orientation Distribution Function (ODF) graph showing the aggregate structure of the tipped steel sheet according to Comparative Example 7, a plastic strain ratio (R) graph for each angle with respect to the rolling direction, and for each deviation angle Grain boundary distribution graph. 4 to 6 respectively corresponding to FIGS. 1 to 3 are graphs related to the tipped steel sheet according to the eighth embodiment.

As shown in FIG. 1, the tweezer steel sheet according to Comparative Example 7 had a brass bearing developed in the kitchen, and as shown in FIG. 2, the average plastic strain ratio (R value) was about 0.91 not exceeding 1.0. As a result of analysis using an EBSD (Electron Back Scattering Diffraction) device, as shown in FIG. 3, the twipe steel sheet exhibited high density near the high-angle grain boundary, particularly, the deviation angle of 60 ° (Σ 3 grain boundary). The fraction of Σ3 grain boundaries is about 23%, and this increase in Σ3 grain boundaries indicates that annealing twins have increased. On the other hand, the fraction of the grain boundary of the deviation angle of 15 degrees or more appeared about 81.2%.

As shown in FIG. 4, in the tipped steel sheet according to Example 8, the brass orientation is significantly reduced (20 °, 70 °, 30 °), and {3 5 2} <2 2 1> is developed as a kitchen table, as shown in FIG. 5. The average plastic strain ratio (R value) was 1.754, which was increased by 190% or more as compared to the tipped steel sheet according to Comparative Example 7. As shown in FIG. 6, the density of the twisted steel sheet around 60 ° was significantly reduced, and the Σ3 grain boundary fraction was about 8%. This reduction in Σ3 grain boundaries means that the annealing twins have also been reduced. On the other hand, the fraction of low-angle grain boundary less than 5 ° of shift angle increased to about 52%, and the fraction of grain boundary above 15 ° of deviation angle was about 36.1%, less than 40%. Interestingly, no grain boundaries exceeding 60 ° of deviation shown in Comparative Example 7 were found in Example 8 (see FIGS. 3 and 6).

According to the above results, it can be seen that the density of Σ3 is reduced through the recovery heat treatment according to the present invention, and then the recrystallization heat treatment causes the movement of the remaining high-angle grain boundaries or crystal recrystallization to be activated. The reduced annealing twins, as in Example 8, later developed during certain processes, appearing to improve the machinability of the tipped steel sheet. The development of the {3 5 2} <2 2 1> texture according to the present invention, in a very simple manner, is to dualize the annealing treatment into recovery heat treatment and recrystallization heat treatment, and to optimize the temperature and time conditions of each heat treatment. It can be said that it is achieved by.

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 an example of an orientation distribution function (ODF) graph of a twipe steel sheet according to a comparative example,

Figure 2 is an example of the plastic strain ratio (R) graph for each angle with respect to the rolling direction of the twipe steel sheet according to a comparative example,

3 is an example of a grain boundary distribution graph for each deviation angle of a tipped steel sheet according to a comparative example;

Figure 4 is an example of the azimuth distribution function graph of the twipe steel sheet according to an embodiment of the present invention,

5 is an example of the angle-specific plastic strain ratio (R) graph with respect to the rolling direction of the twipe steel sheet according to an embodiment of the present invention,

6 is an example of a grain boundary distribution graph for each deviation angle of a tweezer steel sheet according to an embodiment of the present invention.

Claims (8)

By weight%, carbon 0.15 to 0.30%, silicon 0.01 to 0.03%, manganese 15 to 25%, aluminum 1.2 to 3.0%, phosphorus 0.020% or less, sulfur 0.001 to 0.002%, composition containing the remaining iron and austenitic matrix Has, An ultra-high strength tipped steel sheet having a recrystallized texture in which {3 5 2} <2 2 1> is developed into a kitchen plate by cold rolling and has an average plastic strain ratio of at least 1.2. delete The ultrahigh-strength tipped steel sheet according to claim 1, wherein a grain boundary fraction of Σ3 having a shift angle of 60 ° is 10% or less and a grain boundary fraction of a shift angle of 5 ° or less is 50% or more. The ultrahigh-strength tipped steel sheet according to claim 1, wherein a grain boundary fraction of a deviation angle of 15 ° or more is less than 40%. In the method of manufacturing a high strength tipped steel sheet having an improved average plastic strain ratio, Austenitic matrix having a composition containing, by weight, 0.15 to 0.30% carbon, 0.01 to 0.03% silicon, 15-25% manganese, 1.2 to 3.0% aluminum, 0.020% or less phosphorus, 0.001 to 0.002% sulfur, and remaining iron Anneal the cold rolled tweezers of the steel to develop a recrystallized texture with {3 5 2} <2 2 1> as the kitchen table. Recovery heat treatment at 230 ~ 280 ℃ to reduce the Σ3 grain boundary fraction and recrystallization heat treatment at 700 ~ 820 ℃ higher than the temperature of the recovery heat treatment to develop the recrystallized texture Ultra-high strength tipped steel sheet manufacturing method. delete The method of claim 5, wherein the recovery heat treatment is performed for 20 to 30 minutes. The method of claim 5, wherein the recrystallization heat treatment is performed for 6 to 8 hours.

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