WO2013039272A1

WO2013039272A1 - Thin cold-rolled steel plate having high strength and high formability, and preparation method thereof

Info

Publication number: WO2013039272A1
Application number: PCT/KR2011/006866
Authority: WO
Inventors: 이병호; 윤정봉; 김정철; 김성환
Original assignee: 주식회사 포스코
Priority date: 2011-09-16
Filing date: 2011-09-16
Publication date: 2013-03-21
Also published as: CN103797143B; CN103797143A; JP2014530296A; JP5826398B2

Abstract

The present invention relates to a thin cold-rolled steel plate used in home appliances and the like, and a preparation method thereof, and provides a thin cold-rolled steel plate with high strength and high formability, and a preparation method thereof. The present invention relates to an thin cold-rolled steel plate with high strength and high formability, comprising 0.15-0.25 wt% of carbon (C), 1.5-2.5 wt% of manganese (Mn), 0.1-1.0 wt% of silicon (Si), 0.01-0.05 wt% of titanium (Ti), 5-30 ppm of boron (B), and a balance of Fe and other impurities, wherein the tissue comprises 70-100 vol% of bainite, and 0-30 vol% of ferrite, and a preparation method thereof. The thin steel plate provided by the present invention has high strength and high formability, and thus can be effectively used in thin cold-rolled products and the like having high strength requiring high strength of 300 HV or higher on the basis of HV 500g in addition to parts supporting the strength of a chassis such as a notebook, an LCD monitor, an LCD, PMP or LED TV, and the like.

Description

【Specification】

[Name of invention]

Ultra-thin cold rolled steel sheet having high strength and high formability and its manufacturing method

The present invention relates to an ultra-thin cold rolled steel sheet used in home appliances and the like, and more particularly, to an ultra-thin galvanized steel sheet having high strength and high formability, and a method of manufacturing the same.

Background Art

In the case of steel materials used in conventional home appliances, most tend to use general low carbon steel series, so formability is considered an important factor, and strength aspects are not considered. Particularly, in the case of steel of grade EDDQ (Excellent Deep Drawing Quality) or higher, which requires high formability, the strength is not increased to a specific value by focusing on moldability. However, the most important keywords in the low cost, high fuel efficiency, slim, etc. of the product group mainly using natural steel sheets such as automobiles and home appliances are ultrathin and high strength. In other words, the use of ultra-thin products can reduce the total weight of the steel used in the product, and if the total weight of the steel used in one product is reduced, the cost can be realized and a thinner product can be made. The design also has the advantage of being diversified. Thus, ultrathin, high strength can have the effect of three stone trillion. Thus, many studies have been conducted for the development of ultrathin products having high strength and high formability in recent years. These studies are largely 1) strengthening the structure (transformation) using transformations generated during the steel sheet manufacturing process, 2) solid solution strengthening to control the components that can be employed in the steel, and 3) precipitation strengthening to increase the strength by distributing precipitates. ) Finally, through annealing The recrystallized steel sheet may be further divided into work reinforcement, which causes work hardening by second rolling. The prior art is classified into two types. 1) DR (Double Reducing; Rolling) type process and 2) D abbreviation type process which does not use secondary rolling. In other words, the transformation, solid-solution strengthening, precipitation strengthening, and the like may be classified into the DR process type and the DR omitted process according to the presence or absence of secondary rolling. Among them, in the case of the DR process-type process in which the strength is increased by using secondary rolling, defects such as dislocations in steel are inevitably generated due to the increase in strength due to the secondary rolling. While gradually increasing, the elongation is drastically reduced, so it is difficult to use it in an extremely hard part. For example, steel sheets using secondary rolling have most elongation levels of less than 2 to 3% and crack in the rolling direction due to the deterioration of formability due to the low elongation and the influence of the rolled grain generated during secondary rolling. This situation has been formed. Dividing these prior arts into carbon contents in steel, the ultra low carbon steels having a carbon content of generally 0.01 wt or less, the low carbon steels having a carbon content of 0.01 <wt% C <0.1, and 0.1 <wt% C <0.25 It can be divided into carbon-based bicarbonate steel, and high carbon steel having a carbon content of 0.25wt% or more. Looking at the prior art, the ultra low carbon steel is mainly used as a steel sheet for cans, and the conventional technique for this is to reduce the reduction ratio of the secondary reduction, and to improve the strength by controlling the content of Mn (JP1995-274558) and its The improved patent (JP1997-216980) etc. which adjust a reduction ratio for workability improvement, etc. are mentioned. In addition, the same steel sheet was subjected to high temperature strength by using solid solution strengthening and precipitation strengthening of Mn, P and TiC columns. Patents for improving (JP2002-307898, JP2002-201574) and the like have also been proposed. However, in the case of ultra-low carbon steel, there is a limit in strength and the elongation falls to a very low level during the secondary rolling to improve the strength, and there is a problem in producing a high formability and a high strength product. In addition, most of the high-strength steel sheet of low carbon steel is used as a black plate (BP) for cans, and the prior art is to use high nitrogen steel and low pressure of DRM (Double Reducing Mi 11) using DRM low pressure. Technique (JP1990-052642), technology of increasing the content of Mn, continuous lubrication rolling, secondary rolling (JP1996-239734), technology using the effect of overaging treatment (JP1997-040883), using fast tissue Technology (JP2006-074140) and the like. However, even in these prior arts, the low carbon steel has a low strength level, and even if the strength level is high, it requires a high angle of incidence that is difficult to realize in a general continuous annealing process, or the final elongation range obtained is lower than a target range. There is a limit. In addition, in the case of high carbon steel of more than 0.2wt, most of the high strength of the initial stage is not only difficult to roll down in PCM, but also difficult to leveling work for shape control after rolling, which is not applied to the ultrathin ductile material. In recent years, these concepts have been combined to solidify and strengthen the base structure by using P in the medium-heavy steel sheet, and to control the base structure as a two-phase structure of ferrite + pearlite and to control the secondary rolling to 10% or less. Steel plates have been developed that maximize the combination of strength and elongation (KR2009-0084530). In particular, this patent utilizes all of the above-described solid solution strengthening, texture control, and work hardening using the secondary rolling process, and the strength level is higher than that of other technologies (YS> 650 MPa). A method of providing an ultra-thin steel sheet having excellent moldability has been proposed. However, these patents have a problem that the process is complicated by using secondary rolling, and although the rolling amount is small, dislocations are generated by the effect of rolling, resulting in a difference in formability in the rolling direction and the rolling vertical direction.

[Detailed Description of the Invention]

[Technical problem]

One aspect of the present invention is to provide an ultra-thin carbon steel sheet having high strength and high formability and a method of manufacturing the same.

Another aspect of the present invention is to provide a method capable of producing an ultra-thin steel sheet having high strength and high formability without performing secondary rolling by appropriately controlling the steel composition and manufacturing conditions.

Technical Solution

According to one aspect of the invention, by weight ¾, carbon (C): 0.15-0.25%, manganese (Mn): 1.5-2.5%, silicon (Si): 0.1-1.0%, titanium (Ti): 0.01-0.05 %, Boron (B): ultrathin with high strength and high formability, containing 5-30 ppm, balance Fe and other impurities, and its structure comprising 70 100 vol.¾ of bainite and 0-30 μο1 ·% above ferrite Provide a natural steel sheet. According to another aspect of the invention, in weight%, carbon (C): 0.15 to 0.25%, manganese (Mn): 1.5 to 2. Wo, silicon (Si): 0.1 to 1.0%, titanium (Ti): 0.01 to 0.05%, boron (B): 5-30 ppm, after heating the steel slab containing the balance Fe and other impurities, hot-rolled and rolled at Ar ₃ silver or higher and wound at 500 ~ 80CTC, the hot rolled steel sheet 50 ~ after cold rolling with a reduction ratio of 90%, the cold-rolled steel sheet in a continuous annealing line 750 ~ 850 ^o at least 30 seconds at the annealing temperature in C, and then, 250 maintain - to a temperature range of 450 ^o C 10-50 ^o Provided is a method for producing an ultra-thin cold rolled steel sheet having a high strength and high formability after being swept at a angular velocity of C / sec, maintained at this temperature for 50 seconds or more. Advantageous Effects

As described above, since the ultra-thin steel sheet provided by the present invention has high strength and high formability, it is a notebook, a liquid crystal display (LCD) monitor and an LCD, a portable multimedia player (PMP), and an luminescent diode (LED). ) It can be effectively used for high strength ultrathin leaded products that require high strength over 300 HV as well as chassis supporting strength parts such as TV.

[Brief Description of Drawings]

1 is an optical tissue picture of the invention and the comparative material outside the scope of the present invention according to the present invention, Figure 1 (a) shows a tissue picture of the invention material, Figure 1 (b) shows a tissue picture of the comparative material Indicates.

Figure 2 is (b) of Figure shows the organization picture of ^'an electron microscope photograph showing the tissue material invention with different magnification three (a) of Fig. 2 is a magnification of 1000 times ^(χ ΙΟΟΟ) consistent with the present invention, 2 An organization photograph at a magnification of 2000 times (X 2000) is shown, and FIG. 2C shows an organization photograph at a magnification of 5000 times (X 5000).

[Best form for implementation of the invention]

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail. In the present invention, in order to obtain low temperature transformation structure even at a low cooling rate, high hardenability is obtained through the removal of expensive alloying elements such as Nb, Mo, and Ti, which are generally added to steel, and the content control of Mn and B in relatively low cost steels. Slower cooling rate by securing For example, it is possible to form low-temperature transformation tissue during annealing even at a rate of 30 ^° C./sec or less, which is usually the angle of incidence during annealing in a general continuous annealing furnace (CAL).

The present invention steel is characterized in that the tissue comprises a ferrite of 70 ^~ 100 vol.% Bainite and 0 ~ 30vol.% Of, and bainite structure are so obtained that tissue under normal cooling rates in the Martensite steel Compared to the manufacturing process And, there is an advantage in excellent workability and formability. In addition, the steel sheet of the present invention has a hardness of 300 HV or more, but does not perform secondary rolling, even though the hardness of the present invention is higher than that of the high-strength ultrathin material using secondary rolling, which has a hardness of HV500g and 200-250HV, even if the secondary rolling is not performed. In addition, the anisotropy characteristic with respect to the various rolling directions shown at the time of secondary rolling is also not shown. Hereinafter, the steel composition of the present invention will be described (weight ¾).

The C is preferably contained 0.15% or more in order to control the structure to ensure the striking strength in the production of ultra-thin cold-rolled steel sheet, but the amount of carbide precipitation, considering the workability of the steel sheet, considering the possibility of rolling between rolling and deterioration of shape, It is preferable to limit the upper limit of the content to 0.25% as a cause of the mailing inhibition inhibition. The Mn lowers the Ar ₃ temperature and also improves the hardenability at the time of desorption, thereby delaying the formation of pearlite such as pearlite at low cooling rates, thereby allowing the formation of bainite phase at the normal dexterity rate. ， Also, as an essential ingredient added to prevent the redness brittleness of the impurity S, it is preferable to add more than 1.5% to exhibit this effect, but the content is 2.5% in consideration of the rolling properties of the slab, brittleness of the slab, etc. It is preferable to adjust to the following. The Ar ₃ temperature is an inverse transformation temperature for forming an austenite pool for causing transformation during the continuous annealing process. The B is a major element to improve the hardenability with Mn to form the bainite phase in spite of the general cornering speed during annealing heat treatment, when the content is less than 5ppm, the effect is not expected, than 30ppm When excessively excessively formed grain boundary boron-based precipitates adversely affect the properties of the steel, the content is preferably limited to 5 ~ 30ppm. Ti is an element added to more reliably obtain the effect of B, and is used to suppress the formation of boron nitride formed by the combination of N and B remaining in the steel. Add to act as a scavenger. Therefore, the content of Ti is preferably limited as determined by ^"in proportion to the content of the N remaining steel, a 0.01 ~ 0.05¾>. Si is an element that plays a role of deoxidizer and solid solution strengthening, but when the content exceeds 1.0%, crack brittleness problem occurs. The C, it is preferable to satisfy a relation of Mn and the product of the B content of 1.13 - 1 lead ^{4 <wt% C? Wt%} Mn? Wt% B <1.875 * 10- 3. If multiplication 1.875 * 10 - greater than ³ of the content is provided with the fear that the embrittlement occurs, and the rolling property is degraded, it is less than 1.13 * ^10-4 is raised off the Ar ₃ temperature curing ability and bainite is sufficiently formed It is difficult. In addition to the above components, Al, P and S may be included. Preferably, up to Al ^ 0.06%, P and S may be included up to 0.03%, respectively. Cold rolled steel sheet of the present invention is 70 ~ 100 \ ^. 1. > Bainite and ferrite from 0 to 30 vol.¾. Since the bainite structure can obtain its structure at a general cooling rate, it has less warpage during manufacture compared to martensitic steels, thereby improving workability and formability. The steel sheet structure of the present invention may contain up to 30 vol.% Ferrite. The ferrite is a structure that serves to secure the ductility of the steel may include up to 30 vol.¾). In the cold rolled steel sheet, the number of cracks that can be observed by the eye at the corner portion of the steel pipe in the r = 0 L-bending molding test is preferably 2 or less per unit m. Hereinafter, the manufacturing conditions of the present MmN mild steel sheet will be described. In the present invention, the steel slab formed as described above is heated, hot-rolled and rolled at an unknown temperature of Ar ₃ , and wound at 500 to 800 ^° C.

In the present invention, the steel slab heating temperature is not particularly limited, but the heating temperature of the steel slab is preferably limited to 1100 ° C. or more in order to secure the stability of the hot rolling finish temperature. The hot rolling finish temperature is preferably limited to the Ar ₃ temperature or more, for the purpose of rolling in the austenite single phase region. More preferred hot rolling finish temperature is Ar ₃ ~ 950 ^° C.

In the hot finish rolling, the reduction ratio and the cornering condition are not particularly limited. The winding silver is preferably limited to 500 ^° C. or more in order to obtain intermetallic rolling properties, but is preferably limited to 800 ° C. or less to prevent grain coarsening.

Although the thickness of the hot rolled steel sheet is not particularly limited, for example, 1.0-3.0 kPa is preferable. In the present invention, do not add a large amount of precipitation-reinforced elements, the winding temperature is controlled to more than 500 ° C. does not form a hard structure during hot rolling, the hot-rolled final strength is not very high, PCM (Pickling & Cold Rolling Mill) during rolling Can reduce the rolling load. Next, after hot rolling the hot rolled steel sheet as described above at a reduction ratio of 50 ~ 90% 넁 Maintain the rolled steel sheet for more than 30 seconds at the annealing temperature of 750 ~ 850 ° C in the continuous annealing line, and then keep the temperature of 10 ~ 50 ^o C / sec until 250 ~ 450 ° C. An ultra-thin cold rolled steel sheet having high strength and high formability is produced by angling at an angular velocity, holding (overkill) at this temperature for 50 seconds or more, and then performing continuous annealing. The intermetallic rolling reduction rate during intermetallic rolling is to determine the thickness of the final material in the present invention, and is preferably limited to 50 to 90¾. If the cold reduction rate is less than 50%, it is difficult to secure the target thickness, and if it exceeds 90%, there is a problem of inferior rollability ᅳ In the case of the annealing degree is less than 750 ° C. Inverse transformation into austenite Is not sufficiently generated, heat buckle is likely to occur when it exceeds 850 ^o C. If the holding time is less than 30 seconds, since reverse transformation to austenite does not occur sufficiently, the holding time is preferably limited to 30 seconds or more.

When exceeding the above cooling-stop temperature (overaging temperature) is less than 250 ° C or 450 ^o C is not sufficiently formed is bainite, it is limited to the nyaenggak stop temperature (overaging temperature) is 250 ~ 450 ° C desirable.

And when the cooling rate is less than 10 ^o C / sec, the pearlite may be formed, if the cooling rate exceeds 50 ^o C / sec Martensite may be formed, the angular rate is 10 ~ 50 ° C / It is preferable to limit to sec.

Preferred excitation speed is 10-30 ° C./sec. If the holding time (over aging time) is less than 50 seconds, bainite is not formed sufficiently, and the holding time (over aging time) is preferably limited to 50 seconds or more. The moving speed of the steel sheet during the continuous annealing is preferably limited to 100 ~ 500m / min to produce a fine bainite (bainite) phase.

In the present invention, it is a material that can cause reverse transformation of the austenite phase at 750 ~ 850 ° C when annealing through active ingredient control, 250 ~ 450 ° in the state that is not transformed into a tissue such as ¾ light at the time of the austenite phase wetting Cooling to the temperature range of C, and maintaining at this temperature to produce bainite transformation to form a low-temperature transformation structure in the steel to produce a high strength ultra-thin steel sheet. The steel sheet produced as described above has a structure of 70 to 100 vol. ^ Contains bainite and ferrites from 0-30 vol.S¾. In the cold rolled steel sheet, the number of cracks that can be visually observed in the corner portion of the steel sheet in the r = 0 L-bending molding test is preferably 2 or less per unit m. Although the thickness of the said mild steel sheet is not specifically limited, For example, 0.5 mm or less is preferable.

As described above, the present invention promotes bainite transformation during continuous annealing without increasing the initial strength by using expensive Mo, Nb, Ti, etc. and alloys such as Mn and B, which are relatively low-cost. By using this method, it is possible to obtain the ultimate strength and formability in the continuous annealing line without performing secondary rolling.

The present invention is carried out a rapid angle of 50 ^° C / sec or more to cause transformation in low coal series Compared with the prior art, such as utilizing martensitic tissue, it is possible to overcome the low formability characteristic of martensite tissue at a similar level of strength, and to prevent buckling caused by shear deformation. Have In addition, the present invention has the advantage that it is possible to obtain a low temperature high strength transformation structure without the effect of the addition of expensive alloys or fast wetting speed by lowering the wetting speed during transformation in the continuous annealing process to a general continuous annealing furnace (CAL) level cooling rate. In addition, the present invention has a good molding characteristics during L-bending, which is a deformation mode of a general high-strength ultrathin material because it does not perform secondary rolling, and its yield strength (YR) value is not achieved because secondary rolling is not performed. Has a high advantage. Hereinafter, the present invention will be described in more detail with reference to Examples.

(Example 1)

The steel having the composition of Table 1 is hot rolled (heating temperature: 1250 ° C., finish rolling temperature: 900 ° C, hot rolled steel sheet thickness: 2.7 kPa and winding temperature: 600 ^° C), and then to the manufacturing conditions of Table 2 After cold rolling (rolling rate of primary hot rolling: 89%, thickness: 0.3mm), and then annealed to the manufacturing conditions of Table 3 below, yield strength and total elongation, hardness and formability (cracks during L-bending) Occurrence), yield strength and total elongation are shown in Table 2, hardness in Table 4, and moldability evaluation results (cracking) are shown in Table 5, respectively. On the other hand, the amount of transformation according to the annealing conditions of the invention steel was measured, and the results are shown in Table 6 below. In Table 2 below, in the case of Comparative Materials A and B, the yield strength and the elongation according to the secondary reduction ratio were shown, and in the case of the Inventive Materials, secondary rolling was not performed immediately after continuous annealing. Yield strength and elongation of the state are shown. Table 5 below shows the results of the test of the formability of the inventive steel and the comparative steel. In the L-bending experiment, die clearances are affected by the presence of cracks. A poor condition of 0 was assumed and 9Q degree L-bending experiment was performed using r = 0 bending. And the annealing condition of the invention steel was not able to obtain the target high strength for the invention steel made at 700 ^o C level (the result of low bainite fraction in the tissue due to low annealing temperature, which does not cause reverse transformation). test piece for the test was the experiment by limiting the annealing temperature to ^{750 o C, 780 ° C,} 800 o C. Experiment shot

Two times. Means the occurrence of the case stipulated that O In Table 5 the crack and, Δ is the surface which means the case ^did, a crack does not occur a crack occurs to the previous stage of necking (necking) has occurred, and, X is a clear crack did not occur (Table 6) shows the amount of bainite transformation at 350 ° C overaging using the results of dilatometer experiments to simulate the effect of annealing conditions on the phase transformation of the inventive steel. It is shown relatively. The normalized transformation length of the last term is the relative amount of austenite transformation to bainite at 350 ° C overaging temperature. In Tables 2 and 5, tissue B represents bainite, F represents ferrite, and P represents pearlite.

[Table 1]

[Table 2]

[Table 3]

Table 4

[Table 5]

[Table 6]

As shown in Table 2, in the case of the inventive steel, after performing the secondary rolling of the comparative material B in the case of conditions B and C other than the condition A, it can be seen that they have the same or superior physical properties compared to the combination of yield strength and elongation. have. For example, in the case of Comparative Steel A, in order to obtain a yield strength of 630 MPa or more, secondary rolling of 40% or more must be performed, and the elongation obtained at this time is about 1.5%. In addition, in the case of comparative steel B, in order to obtain a similar level of yield strength, it is necessary to perform secondary rolling between 6 and 10¾>, and it can be seen that the elongation at that time is formed as high as about 6>. On the other hand, invented steels not subjected to the secondary rolling process exhibited characteristics in which the yield strengths exceeded 650 MPa and the elongation values exceeded 5.0 in the case of conditions B and C annealed at a temperature of 750 ^° C. or higher. On the other hand, in the case of the condition A of the invention steel, the annealing temperature is low, the yield strength can be seen that the ductility is less than 5% or less than that of the conditions B, C of the invention steel. On the other hand, in the case of the actual ultrathin material, the strength other than the yield strength due to the error in the yield strength Hardness is used a lot as a standard of measurement. As shown in Table 4, in the case of the inventive steel, it can be seen that the hardness value is significantly higher than that of the comparative steel in comparison with the yield strength value. This phenomenon is generally inferred from the fact that the hardness value is proportional to the tensile strength rather than the yield strength of the steel.In the case of the invented steel in comparison with the comparative steels A and B in which work hardening occurs through the secondary rolling, No hardening occurs because no hardening occurs, and the base structure itself is due to the strong bainite structure, and thus the yield ratio value itself is high. For example, in the case of comparative steels A and B, the tensile strength does not increase by more than 30 MPa compared to the yield strength in the tensile test.In the invention steel condition A, the yield strength is 483MPa, the tensile strength is 683MPa, and the yield steel condition B is the yield strength. Tensile strength of 949 MPa at 50 MPa, and condition C, yield strength of 1038 MPa at yield strength of 790 MPa, significantly higher than that of comparative steel near 700 MPa. This high tensile strength guarantees a high hardness value, and in the case of the actual ultrathin material, the effect will be greater. These properties are all due to the difference in the structure of the inventive and comparative steels.

In the case of the inventive steel, the hardness of the inventive steel is higher than that of the comparative steel due to the bainite microstructure of the inventive steel. In other words, the comparative steels had an abnormal structure of ferrite + pilite and the second rolling resulted in the decrease of elongation instead of the increase of the strength, whereas the invented steels did not undergo the second rolling, and thus the elongation of the structure itself. It can maintain the, and due to the microstructure characteristic has the characteristics of the original strength is high, there is a feature that can secure the physical properties equivalent to the comparative material.

As shown in Table 5, in the case of the invention steel, various angles in the case of 750 ° C annealing material Cracks occurred at all speeds and fractured, but at annealed paths above 780 ^o C

At low angles of incidence as low as 15 ° C / sec, it was found that the specimens were safe even at L-bending or worse folding. On the contrary, in the case of comparative material A, in the case of the test piece which performed the secondary rolling of about 40% to obtain the target strength, cracks were formed and fractured in the test piece after the secondary rolling and bending. Cracks did not occur in the test piece when the secondary rolling of less than or equal to% was applied, but cracks or necking occurred in the test piece when more secondary rolling was applied.

Therefore, in the case of the invention steel subjected to annealing at an annealing temperature of 780 ° C. or more and an angular rate of 15 ° C./sec. As shown in Table 6, the annealing time after annealing at annealing temperature and annealing time increases, but the amount of bainite transformation increases, but the annealing time, which was determined to be an important factor in causing reverse transformation, has little effect. It can be inferred that austenite reverse transformations occur in ferrites with holding times longer than 30 seconds at temperatures above C. On the other hand, the effect of the annealing temperature was found to be very large, it can be seen that as the annealing temperature increases the fraction transformed to bainite increases significantly.

In terms of this phase transformation, the faster the angle of annealing at 800 ° C, the better the formation of the bainite phase. However, due to the problems of continuous annealing, it is still possible to form sufficient bainite phase even at a low angle of about 20 ° C / sec. In this steel, the annealing conditions are annealing degree of more than 750 ^° C, the annealing angle is limited to 10-50 ° C / sec. As described above, in the case of the present invention, the additional process such as secondary rolling can be omitted compared to the comparative steels, the rolling direction forming characteristics are not excellent because the secondary rolling is not performed. The advantage of producing continuous annealing conditions, strength level is tensile strength. It has the advantage of high strength (TS) level of 900MPa or more.

(Example ² )

The optical micrograph of the invention material of Example 1 and the comparative material B and the electron micrograph of the invention material were observed, and the result was shown in FIG. 1, and the electron micrograph is shown in FIG. Fig. 1 (a) shows a tissue photograph of the invention material, Fig. 1 (b) shows a tissue photograph of the comparative material, and Fig. 2 (a) shows a tissue photograph with a magnification of 1000 times (X 1000); 2 (b) shows a tissue photograph with a magnification of 2000 times ( ^χ 2000), and (c) of FIG.

Represent a tissue photograph of 5000 times (x 5000).

The invention material is prepared under the annealing condition C annealed at 800 ^° C., Comparative material B is produced after the secondary rolling of 14% after annealing. Referring to Figure 1, in the case of the invention and the comparative material can be seen a clear difference, in the case of the comparative material is a mixed abnormal tissue of pearlite and ferrite (black) expressed in black, in the case of the invention material It appears to have a single-phase tissue. In order to understand such a tissue characteristic, the structure of the invention was observed with a high magnification electron microscope in FIG. 2. The photograph of Figure 2 shows the magnification of 1000, 2000, 5000 on an electron microscope and is clearer than that of an optical photograph. It was found to have a typical bainite tissue in which carbides were formed inside the ferrite lath.

Claims

[Range of request]

[Claim 1]

In weight%, carbon (C): 0.15-0.25%, manganese (Mn): 1.5-2.5%, silicon (Si): 0.1-1.0%, titanium (Ti): 0.01-0.05%, boron (B): 5 ~ 30 ppm ₍ high strength and high formability steel sheet containing residual Fe and other impurities, and the structure of which is 70-100 vol.% Of bainite and 0-30 vol.% Of ferrite.

[Claim 2]

In the low-U, wherein the high-strength and high-performance forming, characterized by satisfying the relationship between the C, Mn, and a product of the B content of ^{1.13 * 10- 4? <Wt C} wt% Mn? Wt% B <1.875 * 10 "3 Ultra-thin steel sheet having a thickness.

[Claim 3]

The ultra-thin steel sheet having high strength and high formability according to claim 1 or 2, wherein the thickness of the molten steel sheet is 0.5 mm or less.

[Claim 4]

The steel sheet according to claim 1 or 2, wherein, in the r = 0 L-bending molding test of the steel sheet, the number of the stacks that can be visually observed at the corners thereof is 2 or less per unit m. Ultrathin thin steel sheet having high strength and high formability.

[Claim 5]

By weight%, carbon (C): 0.15-0.25%, manganese (Mn): 1.5-2.5%, silicon (Si): 0.1-1.0%, titanium (Ti): 0.01-0.05%, boron (B): 5 After heating the steel slab containing ~ 30ppm, balance Fe and other impurities, hot finish rolling above Ar ₃ temperature and winding at 500 ~ 800 ^o C, and then hot rolled steel sheet at 50 ~ 90% reduction ratio After rolling, the hot rolled steel sheet was maintained at an annealing temperature of 750 to 850 ^o C in a continuous annealing line for at least 30 seconds,

Angled at an angular velocity of 10 to 50 ^o C / sec until a temperature range of 250 to 450 ° C, maintained at this temperature for more than 50 seconds, and then subjected to extreme high strength and high formability. Manufacturing method

[Claim 6]

6. The high strength and high strength according to claim 5, wherein the product of (: ， Mn and B content satisfies a relationship of 1.13 * ¹⁰ — ⁴ <wt C? Wt% Mn? Wt% B <1.875 * 10— ³ . Method for producing ultrathin galvanized steel sheet having a form.

[Claim 7]

7. The method of claim 5 or 6, wherein the moving speed of the steel sheet during continuous annealing is 100 to 500 m / min.

[Claim 8]

According to claim 5 or 6, wherein the hot rolling finish temperature is Ar3 ~ 950 ° C, and the angular velocity is 10 ~ 30 ^o C / sec of the ultra-thin sintered steel sheet having high strength and high formability Manufacturing method. .

[Claim 9]

The method according to claim 5 or claim 6, wherein the hot rolled steel sheet has a thickness of 1.0 to 3.0 kPa, and the cold rolled steel sheet has a thickness of 0.5 kPa or less.

[Claim 10]

7. The cold rolled steel sheet according to claim 5 or 6, wherein the cold rolled steel sheet has a sheet number of not more than 2 creeks per unit m that can be observed by the corner at the time of r = 0 L—bending molding test. A method for producing an ultra-thin cold rolled steel sheet having high strength and high formability.