KR20140130258A

KR20140130258A - HIGH STRENGTH COLD-ROLLED STEEL SHEET FOR AUTOMOBILE WITH EXCELLENT FORMABILITY and Bendability and METHOD OF MANUFACTURING THE STEEL SHEET

Info

Publication number: KR20140130258A
Application number: KR20130047920A
Authority: KR
Inventors: 한영철; 한성수
Original assignee: 현대제철 주식회사
Priority date: 2013-04-30
Filing date: 2013-04-30
Publication date: 2014-11-10

Abstract

Disclosed are a high strength cold-rolled steel sheet for an automobile with a high strength of tensile strength of 780 or more MPa and excellent bendability and formability, and a method for manufacturing the same. According to the present invention, the method for manufacturing the high strength cold-rolled steel sheet for an automobile includes: a hot-rolling step for forming a hot-rolled steel by reheating, hot-rolling, cooling, and coiling a steel slab including 0.05-0.15 wt% of C, 0.1-1.0 wt% of Si, 2.0-2.5 wt% of Mn, 0.01 or less wt% of P, 0.003 or less wt% of S, 0.01-1.0 wt% of Al, 0.2-0.5 wt% of Cr, 0.05-0.08 wt% of Mo, 0.001 or less wt% of N, and remainder consisting of iron (Fe) and inevitable impurities; a cold-rolling step for forming a cold-rolled steel by cold-rolling the hot-rolled steel after pickling the hot-rolled steel; a heat treatment step for heat treating the cold-rolled steel at a temperature of 780-820°C; and an isothermal heat treatment step for performing isothermal heat treatment on the heat-treated steel at a temperature of 250-300°C after cooling the heat-treated steel under a temperature of 350°C. After the isothermal heat treatment step, the steel has a tensile strength (Ts) of greater than or equal to 780 MPa, a yield ratio (YR) of 0.5-0.7, an elongation of greater than or equal to 18%, and a bending property of a bending angle of 90° and the ratio of thickness to bending radius of 1.5 or less.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high strength cold rolled steel sheet for automobiles having excellent bendability and moldability, and a method for manufacturing the same. BACKGROUND ART < RTI ID = 0.0 >

The present invention relates to a cold-rolled steel sheet and a method of manufacturing the same, and more particularly, to a cold-rolled steel sheet and a method of manufacturing the same, which are capable of simultaneously securing excellent tensile strength (TS) To a high strength cold rolled steel sheet and a manufacturing method thereof.

Currently, automotive parts are becoming increasingly stronger in order to improve fuel efficiency. The steel sheet for manufacturing such an automobile part may include an intermediate steel sheet such as IF (Interstitial Free) steel sheet, Mild steel sheet, IF-HSS (Interstitial Free High Strength Steel), BH (Bake Hardening) , DP (Dual Phase) steel plate, and TRIP (Transformation Induced Plasticity) steel plate.

Fig. 1 schematically shows the mechanical properties according to the kind of the steel sheet.

Referring to FIG. 1, it can be seen that the tensile strength TS and the elongation EL are generally in inverse proportion. Particularly, in the case of a steel sheet having a tensile strength (TS) of 780 MPa or more, a sufficient elongation can be secured only in an extremely limited case. In addition, in order to improve the bending property, the yield strength (YS) or the yield ratio (YR) is inevitably increased, which negatively affects the elongation.

The normal tensile strength 780 MPa steel sheet has a limited application in various molding modes due to low elongation and low bending properties. Therefore, development of a high strength steel sheet having a tensile strength of 780 MPa or more and excellent both in bendability and moldability is required.

A related prior art is Korean Patent Laid-Open No. 10-2006-0115313 (published on November 11, 2006), which discloses a cold-rolled steel sheet having excellent moldability and a manufacturing method thereof.

It is an object of the present invention to provide a method for manufacturing a high strength cold rolled steel sheet for automobiles capable of simultaneously securing an excellent bending property and a formability while having a tensile strength (TS) of 780 MPa or more through control of an alloy component and a heat treatment process after cold rolling .

Another object of the present invention is to provide a high strength cold rolled steel sheet for automobiles having a high strength of 780 MPa or higher in tensile strength and exhibiting an elongation ratio (EL) of 18% or more and a bendability R (corner curvature radius) .

According to an aspect of the present invention, there is provided a method for manufacturing a high strength cold rolled steel sheet for automobiles, which comprises 0.05 to 0.15% of C, 0.1 to 1.0% of Si, 2.0 to 2.5% of Mn, 0.01% The steel slab consisting of 0.003% or less of S, 0.01 to 1.0% of Al, 0.2 to 0.5% of Cr, 0.05 to 0.08% of Mo and 0.001% or less of N and the balance of Fe and unavoidable impurities is reheated A hot rolling step of forming hot rolled steel by hot rolling, cooling and winding; A cold rolling step of pickling the hot rolled steel and cold rolling to form a cold rolled steel; A heat treatment step of heat-treating the cold-rolled steel at 780 to 820 ° C; And a constant-temperature heat treatment step of cooling the heat-treated steel to 350 ° C. or lower and then performing a constant temperature heat treatment at 250 to 300 ° C. After the constant temperature heat treatment step, the steel has a tensile strength (TS) of 780 MPa or more, , A yield ratio (YR) of at least 18%, an elongation (EL) of at least 18%, and a bending R ratio to a thickness of not more than 1.5 times.

According to another aspect of the present invention, there is provided a high strength cold rolled steel sheet for automobiles comprising 0.05 to 0.15% of C, 0.1 to 1.0% of Si, 2.0 to 2.5% of Mn, 0.01% (Fe) and unavoidable impurities and having a tensile strength of not less than 780 MPa, a S content of not more than 0.003%, Al of 0.01 to 1.0%, Cr of 0.2 to 0.5%, Mo of 0.05 to 0.08%, N of 0.001% (TS), a yield ratio (YR) of 0.5 to 0.7, an elongation (EL) of 18% or more, and a bending R ratio to a thickness of 1.5 times or less.

The high strength cold rolled steel sheet for automobiles according to the present invention and the method for manufacturing the same according to the present invention have a tensile strength (TS) of 780 MPa or more, a yield ratio (YR) of 0.5 to 0.7, elongation (EL) of 18% It is possible to produce a high strength cold rolled steel sheet exhibiting a 90 ° bendability with a bending R ratio of 1.5 times or less to the thickness.

Fig. 1 schematically shows the mechanical properties according to the kind of the steel sheet.
2 is a process flow chart schematically showing a method of manufacturing a high strength cold rolled steel sheet according to an embodiment of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS The advantages and features of the present invention, and how to accomplish them, will become apparent with reference to the embodiments and drawings described in detail below. 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.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, a high strength cold rolled steel sheet for automobiles having excellent bendability and moldability according to a preferred embodiment of the present invention and a method for producing the same will be described in detail with reference to the accompanying drawings.

High strength cold rolled steel sheet for automobiles

The high strength cold rolled steel sheet for automobiles according to the present invention has a tensile strength (TS) of 780 MPa or more, a yield ratio (YR) of 0.5 to 0.7, an elongation (EL) of 18% or more and a 90 ° bendability .

For this purpose, the high strength cold rolled steel sheet for automobiles according to the present invention contains 0.05 to 0.15% of C, 0.1 to 1.0% of Si, 2.0 to 2.5% of Mn, 0.01% or less of P, 0.003% or less of S, Al: 0.01 to 1.0%, Cr: 0.2 to 0.5%, Mo: 0.05 to 0.08%, N: 0.001% or less, and the balance of iron (Fe) and unavoidable impurities.

Hereinafter, the content of each component contained in the high strength cold rolled steel sheet for automobiles having excellent bendability and formability according to the present invention and the reason for addition thereof will be described.

Carbon (C)

Carbon (C) is added to ensure strength of the steel. In addition, the carbon serves to stabilize the austenite phase according to the amount that is concentrated in the austenite phase.

The carbon (C) is preferably added in a content ratio of 0.05 to 0.15% by weight based on the total weight of the cold-rolled steel sheet. When the content of carbon (C) is less than 0.05% by weight, it is difficult to secure sufficient strength. On the contrary, when the content of carbon (C) exceeds 0.15% by weight, the strength is increased but the toughness and weldability are greatly deteriorated.

Silicon (Si)

Silicon (Si) acts as a deoxidizer in the steel, and as a ferrite stabilizing element, it inhibits carbide formation in ferrite and contributes to securing strength and elongation.

The silicon (Si) is preferably added at a content ratio of 0.1 to 1.0% by weight based on the total weight of the cold-rolled steel sheet. If the content of silicon (Si) is less than 0.1% by weight, the effect of adding silicon can not be obtained properly. On the contrary, when the content of silicon (Si) exceeds 1.0% by weight, a large amount of oxides such as Mn ₂ SiO _{4 is} formed on the surface to deteriorate the plating property, and the carbon equivalent is increased to lower the weldability.

Manganese (Mn)

Manganese (Mn) contributes to the improvement of strength of steel by strengthening solid solution and increasing ingotability.

The manganese (Mn) is preferably added at a content ratio of 2.0 to 2.5% by weight based on the total weight of the cold-rolled steel sheet. When the content of manganese (Mn) is less than 2.0% by weight, the effect of addition thereof is insufficient, and it is difficult to secure a tensile strength of 780 MPa or more. On the contrary, when the content of manganese (Mn) exceeds 2.5% by weight, a manganese band structure is formed and segregation increases sharply, which deteriorates the workability of the steel and raises the carbon equivalent (Ceq)

In (P)

Phosphorus (P) contributes to improving the strength of steel by strengthening employment. However, when the content of phosphorus (P) is over 0.01% by weight, the possibility of occurrence of low-temperature embrittlement increases, which is not suitable for high strength steel sheet for automobile body structure.

Therefore, in the present invention, the content of phosphorus (P) is limited to a content ratio of 0.01% by weight or less based on the total weight of the cold-rolled steel sheet.

Sulfur (S)

Sulfur (S) reacts with manganese (Mn) to form precipitates of fine MnS to improve processability. However, when the content of sulfur (S) exceeds 0.003% by weight in the cold-rolled steel sheet according to the present invention, the amount of sulfur (S) dissolved therein is too large to greatly reduce ductility and formability, There is a concern. Therefore, in the present invention, the content of sulfur (S) is limited to 0.003% by weight or less based on the total weight of the cold-rolled steel sheet.

Aluminum (Al)

Aluminum (Al) is added for the purpose of deoxidation in steelmaking, and nitrogen (N) and nitride AlN are precipitated during hot rolling to suppress grain growth.

The aluminum (Al) is preferably added in a content ratio of 0.01 to 1.0% by weight based on the total weight of the cold-rolled steel sheet. When the content of aluminum (Al) is less than 0.01% by weight, it is difficult to exert the deoxidation effect properly. On the other hand, when the content of aluminum (Al) exceeds 1.0% by weight, inclusions are formed excessively during steelmaking and casting operations, and the elongation value is lowered.

Chromium (Cr)

Chromium (Cr) contributes to the improvement of steel strength by strengthening solid solution and increasing ingotability.

The chromium (Cr) is preferably added at a content ratio of 0.2 to 0.7% by weight based on the total weight of the cold-rolled steel sheet. When the content of chromium (Cr) is less than 0.2% by weight, the effect of the addition is insufficient, and it is difficult to secure a tensile strength of 780 MPa or more. On the contrary, when the content of chromium (Cr) exceeds 0.7% by weight, surface oxides are formed during the heat treatment to deteriorate the plating ability and increase the carbon equivalent, thereby deteriorating the weldability.

Molybdenum (Mo)

Molybdenum (Mo) contributes to the improvement of steel strength through strengthening of solid solution and increase of ingotability.

The molybdenum (Mo) is preferably added in an amount of 0.05 to 0.08% by weight based on the total weight of the cold-rolled steel sheet. When the content of molybdenum (Mo) is less than 0.05% by weight, the effect of the addition is insufficient and it is difficult to secure a strength of 780 MPa or more in tensile strength. On the other hand, when the content of molybdenum (Mo) exceeds 0.08% by weight, the amount of martensite which secures strength increases to decrease toughness. Also, the higher the cost, the higher the cost of the steel, which is disadvantageous to production.

Nitrogen (N)

Nitrogen (N) inhibits the elongation and deteriorates the formability of the steel. Therefore, the lower the content of nitrogen is, the better, but the cost of manufacturing steel can be greatly increased if the content is controlled to an excessively low content. Therefore, in the present invention, the content of nitrogen (N) is limited to 0.001% by weight or less based on the total weight of the cold-rolled steel sheet.

The cold-rolled steel sheet according to the present invention comprising the above alloy components may have a composite structure including ferrite, bainite and martensite in terms of microstructure. Such a composite structure can be formed by controlling the fraction of each phase by carbon, silicon, manganese, chromium, molybdenum or the like through heat treatment.

The cold rolled steel sheet according to the present invention has a tensile strength (TS) of 780 MPa or more, a yield ratio (YR) of 0.5 to 0.7, an elongation (EL) of 18% or more and a bending R ratio 90 ° bendability. More specifically, in the high-strength cold-rolled steel sheet according to the present invention, the ratio of bending R to thickness (radius of curvature of corner) is 0.6 to 0.8 times 90 ° bendability.

Manufacturing Method of High Strength Cold Rolled Steel Sheet for Automobile

2 is a process flow chart schematically showing a method of manufacturing a high strength cold rolled steel sheet according to an embodiment of the present invention.

Referring to FIG. 2, the illustrated cold rolled steel sheet manufacturing method includes a hot rolling step S110, a cold rolling step S120, a heat treatment step S130, and a constant temperature heat treatment step S140.

Hot rolling

In the hot rolling step (S110), the steel slab is reheated, hot rolled, cooled and wound to form hot rolled steel.

The steel slab in the semi-finished product state to be subjected to the hot rolling process in the method of manufacturing high-strength cold-rolled steel sheet according to the present invention comprises 0.05 to 0.15% of C, 0.1 to 1.0% of Si, 2.0 to 2.5% of Mn, (Fe) and unavoidable impurities in an amount of not more than 0.001%, S: not more than 0.003%, Al: 0.01 to 1.0%, Cr: 0.2 to 0.5%, Mo: 0.05 to 0.08%

At this time, the steel slab is produced through a continuous casting process after obtaining molten steel through a steelmaking process.

Reheating is performed to reuse the segregated components during casting and to homogenize the steel slabs. The reheating is preferably performed at a slab reheating temperature (SRT) of 1150 to 1250 ° C for 1 to 3 hours. If the reheating temperature is less than 1150 占 폚 or the reheating time is less than 1 hour, the segregated components may not be reused. On the other hand, when the reheating temperature exceeds 1250 占 폚 or the reheating time exceeds 3 hours, the austenite grain size increases and the crystal grains of the ferrite are coarsened, making it difficult to secure the strength.

The hot rolling is preferably carried out under FDT (Final Delivery Temperature): 800 to 900 ° C. If the finishing rolling temperature (FDT) is lower than 800 ° C, the rolling progresses in the ferrite region so that the rolling load increases sharply and the productivity deteriorates. In the rolling, an excessive potential is formed in the ferrite, So that uneven crystal grains are formed. On the other hand, when the finishing rolling temperature (FDT) exceeds 900 DEG C, the size of the ferrite crystal grains increases and the strength can be reduced.

The coiling after cooling is preferably carried out by cooling the coiling temperature to 500 to 600 占 폚, more preferably 540 占 폚 to 20 占 폚. If the coiling temperature (CT) is higher than 600 ° C, coiling will occur due to continuous phase transformation after winding, which increases the probability of causing defects in the post-process. On the other hand, if the coiling temperature (CT) is less than 500 占 폚, the ductility is lowered and the load during cold rolling is increased.

Cold rolling

In the cold rolling step (S120), the hot-rolled steel is subjected to pickling treatment, followed by cold rolling to form cold-rolled steel. At this time, the cold rolling reduction rate is preferably 40 to 70%. When the cold rolling reduction is less than 40%, the deformation effect of the hot-rolled steel is small. On the other hand, when the cold rolling reduction is more than 70%, not only the cost required for cold rolling increases, but also drawability is deteriorated and cracks are generated at the edges of the steel sheet, which may cause the steel sheet to break.

Heat treatment

In the heat treatment step (S 130), the cold-rolled steel is heat-treated at 780 to 820 ° C. This step is carried out for the purpose of controlling the austenite phase fraction by heat-treating the cold-rolled steel.

The heat treatment temperature is preferably 780 to 820 占 폚. That is, the austenite and ferrite anomalous reverse treatment is carried out to secure the austenite and ferrite fraction, thereby contributing to the final strength and elongation. At this time, when the heat treatment temperature is lower than 780 캜, ductility is lowered. Conversely, when the heat treatment temperature is higher than 820 캜, it may be difficult to secure strength due to an increase in grain size.

Constant temperature heat treatment

In the constant-temperature heat treatment step (S140), the heat-treated steel is cooled to 350 DEG C or less and then subjected to a constant-temperature heat treatment at 250 to 300 DEG C.

In this step, the final material, that is, having a tensile strength of 780 MPa or more, has a yield ratio of 0.5 to 0.7 and a high elongation of 18% or more, and has a 90 ° bendability of 1.5 times or less the ratio of bending R (corner curvature radius) In order to secure the heat, the steel which has been heat-treated by using the gas containing 5 to 25% of hydrogen + nitrogen is cooled and subjected to the constant-temperature heat treatment.

At this time, it is preferable that the cooling end temperature is 350 DEG C or less, because if the cooling end temperature exceeds 350 DEG C, bainite transformation may be caused.

The heat treatment at a constant temperature is preferably performed at 250 to 300 ° C for 120 to 240 seconds. When the constant-temperature heat treatment temperature exceeds 300 ° C, or when the constant-temperature heat treatment time exceeds 240 seconds, the martensitic transformation is partially delayed, so that it is difficult to secure sufficient tensile strength. On the other hand, when the constant-temperature heat treatment temperature is less than 250 ° C or the constant-temperature heat treatment time is less than 120 seconds, the effect of heat treatment of martensite is small and the difference in hardness between phases is not reduced.

Although not shown in the figure, after the constant-temperature heat treatment step, the step of cooling to 150 to 200 ° C at a cooling rate of approximately 10 to 50 ° C / sec may be further performed.

The high strength cold rolled steel sheet produced in the above steps S110 to S140 has an abnormal structure including ferrite and martensite, wherein the ferrite has a cross-sectional area ratio of 70% or more, and martensite The second phase comprising has less than 30% cross-sectional area percent.

The high-strength cold-rolled steel sheet produced by the above process has a tensile strength (TS) of 780 MPa or more, a yield ratio (YR) of 0.5 to 0.7, an elongation (EL) of 18% or more, and a 90 ° bend Have sex. More specifically, in the high-strength cold-rolled steel sheet according to the present invention, the ratio of bending R to thickness (radius of curvature of corner) is 0.6 to 0.8 times 90 ° bendability.

Example

Hereinafter, the configuration and operation of the present invention will be described in more detail with reference to preferred embodiments of the present invention. It is to be understood, however, that the same is by way of illustration and example only and is not to be construed in a limiting sense.

The contents not described here are sufficiently technically inferior to those skilled in the art, and a description thereof will be omitted.

1. Manufacture of steel sheet

The steel slab having the composition shown in Table 1 was reheated at 1,200 DEG C for 2 hours and then subjected to finishing hot rolling at 895 DEG C, followed by cooling to 540 DEG C and winding. Thereafter, cold rolling was carried out at a reduction ratio of 50% after the pickling treatment.

Then, in order to examine the mechanical properties according to the heat treatment temperature, the anisotropic phase and the austenite single phase reverse annealing temperature (SS) shown in Table 2 were changed for each specimen, and a rapid heat treatment was performed at 300 deg. And the mechanical properties and bending properties were observed.

For all the specimens, the reversed phase and austenite single phase reverse annealing was carried out for 150 seconds, cooling was carried out for 55 seconds and annealing was carried out for 180 seconds.

[Table 1]

[Table 2]

2. Evaluation results of mechanical properties

Referring to Tables 1 and 2, when the cooling temperature was 400 ° C, the target tensile strength was not satisfied even at the heat treatment temperatures of 780 ° C, 800 ° C and 820 ° C. This is due to the decrease in the final martensitic transformation fraction due to the bainite transformation of austenite. When the cooling temperature was 300 ℃, the mechanical properties were satisfied at 780 ℃, 800 ℃ and 820 ℃, and the bending properties satisfied the target values. This is because the martensite partially softened due to the low-temperature heat treatment for 180 seconds after the formation of martensite upon cooling at 300 ° C, resulting in less difference in hardness between the martensite and the ferrite, resulting in excellent bendability do.

Further, as the heat treatment temperature rises, the austenite fraction in the abnormal phase increases, and the yield ratio can be increased at the same time. Therefore, it is a component system capable of adjusting the mechanical properties by advancing the heat treatment temperature at a high temperature to a part requiring a high porosity and bending property and proceeding to a low temperature heat treatment temperature at a part requiring a formability.

Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims. These changes and modifications may be made without departing from the scope of the present invention. Accordingly, the scope of the present invention should be determined by the following claims.

S110: Hot rolling step
S120: Cold rolling step
S130: heat treatment step
S140: Constant temperature heat treatment step

Claims

The steel sheet according to any one of claims 1 to 3, wherein the steel contains 0.05 to 0.15% of C, 0.1 to 1.0% of Si, 2.0 to 2.5% of Mn, 0.01% A hot rolling step of reheating, hot rolling, cooling and winding a steel slab composed of 0.05 to 0.08% of Mo, 0.006% or less of Mo, and remaining iron (Fe) and unavoidable impurities to form hot rolled steel;
A cold rolling step of pickling the hot rolled steel and cold rolling to form a cold rolled steel;
A heat treatment step of heat-treating the cold-rolled steel at 780 to 820 ° C; And
And a constant-temperature heat treatment step of cooling the heat-treated steel to 350 ° C or lower, followed by a constant-temperature heat treatment at 250 to 300 ° C,
After the constant temperature heat treatment step, the steel has a tensile strength (TS) of at least 780 MPa, a yield ratio (YR) of 0.5 to 0.7, an elongation (EL) of at least 18%, and a 90 ° bendability Of the cold-rolled steel sheet.
(Where the bend R is the radius of curvature of the corner).

The method according to claim 1,
In the hot rolling step,
The reheating
SRT (Slab Reheating Temperature): 1150 to 1250 占 폚 for 1 to 3 hours to manufacture a high strength cold rolled steel sheet.

The method according to claim 1,
In the hot rolling step,
The hot rolling
FDT (Final Delivery Temperature): 800 to 900 占 폚.

The method according to claim 1,
In the hot rolling step,
The cooling and winding
CT (Coiling Temperture): A method for manufacturing a high strength cold rolled steel sheet for automobile, characterized by cooling to 500 to 600 캜 and winding.

The method according to claim 1,
In the constant-temperature heat treatment step,
The constant temperature heat treatment
Wherein the annealing is performed for 120 to 240 seconds.

The method according to claim 1,
After the constant-temperature heat treatment step,
Further comprising a step of cooling the steel sheet to 150 to 200 占 폚 at a cooling rate of 10 to 50 占 폚 / sec.

The method according to claim 1,
The bending R ratio to thickness
Wherein the steel sheet has a 90 占 bendability of 0.6 to 0.8 times.
(Where the bend R is the radius of curvature of the corner).

The steel sheet according to any one of claims 1 to 3, wherein the steel contains 0.05 to 0.15% of C, 0.1 to 1.0% of Si, 2.0 to 2.5% of Mn, 0.01% , Mo: 0.05 to 0.08%, N: 0.001% or less and the balance of iron (Fe) and unavoidable impurities,
Characterized in that it has a tensile strength (TS) of 780 MPa or more, a yield ratio (YR) of 0.5 to 0.7, an elongation (EL) of 18% or more and a bending R ratio of 1.5 times or less, Steel plate.
(Where the bend R is the radius of curvature of the corner).

9. The method of claim 8,
The steel sheet
And has an abnormal structure including ferrite and martensite,
Wherein the ferrite has a cross-sectional area percentage of 70% or more, and the second phase containing martensite has a cross-sectional area ratio of less than 30%.

9. The method of claim 8,
The bending R ratio to thickness
Wherein the steel sheet exhibits a 90 占 bendability of 0.6 to 0.8 times.
(Where the bend R is the radius of curvature of the corner).