KR101999030B1 - Ultra thin hot rolled steel sheet having excellent isotropic properties and method of manufacturing the same - Google Patents

Abstract

In one embodiment of the present invention by weight, C: 0.020 to 0.080%, Mn: 0.05 to 0.50%, Al: 0.05% or less, Ca: 0.001 to 0.005%, the remaining Fe and other unavoidable impurities, the C , Mn, Al, and Ca satisfy the following relations 1 to 4, and the microstructure includes an area fraction of ferrite: 80% or more and pearlite: 20% or less, and the ferrite grain size variation (ΔG) according to the rolling direction is Provided is an ultra-thin hot rolled steel sheet excellent in isotropy satisfying the following relational formula 5 and its manufacturing method.
[Relationship 1] 5 ≤ [Al / Ca] ≤ 40
[Relationship 2] 20 ≤ 1000 [C + 0.04Mn] ≤ 85
[Relationship 3] 1 ≤ 1000 [C + 0.04Mn] / [Al / Ca] ≤ 12
[Relationship 4] 0.05 ≤ [C + 0.4Mn] ≤ 0.3
[Equation 5] ΔG = [(G _TD + G _RD ) / 2]-G _{45 ° D} ≤ 2.5㎛
(The contents of C, Mn, Al and Ca in the relations 1 to 4 are by weight, ΔG in the relation 5 is a deviation of the ferrite grain average size (FGS), G _TD is perpendicular to the rolling direction Direction FGS, G _RD is the horizontal direction FGS in the rolling direction, G _{45 ° D} represents the FGS in the 45 ° direction of the rolling direction.)

Description

ULTRA THIN HOT ROLLED STEEL SHEET HAVING EXCELLENT ISOTROPIC PROPERTIES AND METHOD OF MANUFACTURING THE SAME}

The present invention relates to an ultra-thin hot rolled steel sheet excellent in isotropy and a method of manufacturing the same.

In general, carbon steels are classified into high carbon steels, medium carbon steels, low carbon steels, and ultra low carbon steels, depending on the carbon content in the steel. Among these, low carbon steel usually means carbon steel having a carbon content of 0.3 wt% or less. The low carbon steel may be manufactured in a hot rolling process or a cold rolling process, and is used in various industrial fields such as automobiles and home appliances, and thus has the highest yield among carbon steels.

Recently, parts manufacturing customers demand various product groups from steel companies according to various requirements of consumers, and in particular, automobile companies are constantly demanding weight reduction of automobile bodies in order to regulate CO ₂ emission and improve fuel efficiency worldwide. There is an increasing demand for the development of thinner, lower cost new steel grades that are more competitive in price than other competitors.

On the other hand, in Patent Literature 1, the plate thickness is 3 to 20 mm and the component composition is weight%, C: 0.3% or less (excluding 0%), Si: 0.5% or less (excluding 0%), Mn: 0.2 to 1%, P : 0.05% or less (excluding 0%), S: 0.05% or less (excluding 0%), Al: 0.01 ~ 0.1%, N: 0.008 ~ 0.025%, balance iron and inevitable impurities, solid solution N: 0.007% or more The content of C and N satisfies the relationship of 10C + N3.0, and the structure is less than 20% perlite in the area ratio of the entire structure, the balance is ferrite, and the average grain diameter of the ferrite is in the range of 3 to 35 μm. Introducing a method of manufacturing a hot rolled steel sheet having excellent cold workability and excellent surface hardness after processing, but there is a limit in manufacturing a hot rolled steel sheet of less than 2.0mm by the existing hot rolling process.

In the case of manufacturing low carbon steel in the existing hot rolling process, the slab (slab) having a thickness of 200 mm or more is usually manufactured through a reproducing process, and then hot rolled steel slabs are reheated, rough rolled, and finished rolled to produce hot rolled steel. In addition, the existing hot rolling is batch process, the top part is introduced into the rolling mill for every coil, and the tail part must exit the rolling mill, so thickness variation and scale quality are inferior to the top and tail parts. As a problem, it is difficult to manufacture a hot rolled steel sheet having a thickness of 1.4 mm or less. Therefore, in order to manufacture a low carbon steel (≤1.0mm) in the existing hot rolling mill, it is inferior in terms of price competition because it must go through a pickling / cold rolling and annealing process.

Therefore, while overcoming the problems of the conventional hot rolling, the ultra-thin hot rolled steel sheet and its manufacturing method which are excellent in price competition in order to satisfy the increasing requirements of customers and to meet the demand of light weight are developed. This is necessary.

Japanese Unexamined Patent Publication No. 2013-056658

One aspect of the present invention is to provide an ultra-thin hot rolled steel sheet excellent in isotropy and a method for producing the same by precisely controlling the rolling process conditions affecting the recrystallization in the component and the continuous continuous rolling process in the performance-rolling direct connection process.

In one embodiment of the present invention by weight, C: 0.020 to 0.080%, Mn: 0.05 to 0.50%, Al: 0.05% or less, Ca: 0.001 to 0.005%, the remaining Fe and other unavoidable impurities, the C , Mn, Al, and Ca satisfy the following relations 1 to 4, and the microstructure includes an area fraction of ferrite: 80% or more and pearlite: 20% or less, and the ferrite grain size variation (ΔG) according to the rolling direction is It provides an ultra-thin hot rolled steel sheet excellent in isotropy satisfying the following relational formula 5.

[Relationship 1] 5 ≤ [Al / Ca] ≤ 40

[Relationship 2] 20 ≤ 1000 [C + 0.04Mn] ≤ 85

[Relationship 3] 1 ≤ 1000 [C + 0.04Mn] / [Al / Ca] ≤ 12

[Relationship 4] 0.05 ≤ [C + 0.4Mn] ≤ 0.3

[Equation 5] ΔG = [(G _TD + G _RD ) / 2]-G _{45 ° D} ≤ 2.5㎛

(The contents of C, Mn, Al and Ca in the relations 1 to 4 are by weight, ΔG in the relation 5 is a deviation of the ferrite grain average size (FGS), G _TD is perpendicular to the rolling direction Direction FGS, G _RD is the horizontal direction FGS in the rolling direction, G _{45 ° D} represents the FGS in the 45 ° direction of the rolling direction.)

Another embodiment of the present invention includes, by weight, C: 0.020-0.080%, Mn: 0.05-0.50%, Al: 0.05% or less, Ca: 0.001-0.005%, remaining Fe and other unavoidable impurities, wherein C , Mn, Al and Ca are obtained by continuously casting molten steel satisfying the following relations 1 to 4 to obtain a thin slab; Rough rolling the thin slab to obtain a bar; Finish rolling the bar, in the first rolling mill is rolling at a rolling reduction of 50% or more at Ar ₃ + 100 ℃ or more, in the last rolling mill 20% or more in the temperature range of Ar _A50-F50 ~ Ar ₃ +30 ℃ Rolling at a rolling rate to obtain a hot rolled steel sheet; Air cooling the hot rolled steel sheet for 2.5 seconds or more, and then cooling to 20 to 100 ° C / s; And a step of winding the cooled hot rolled steel sheet from 500 ~ Ar _P -50 ℃, each of the steps provides a method of producing isotropic excellent, characterized in that is carried out continuously ultra-foil hot-rolled steel sheet.

[Relationship 1] 5 ≤ [Al / Ca] ≤ 40

[Relationship 2] 20 ≤ 1000 [C + 0.04Mn] ≤ 85

[Relationship 3] 1 ≤ 1000 [C + 0.04Mn] / [Al / Ca] ≤ 12

[Relationship 4] 0.05 ≤ [C + 0.4Mn] ≤ 0.3

(The contents of C, Mn, Al, and Ca in the above relations 1 to 4 are by weight.)

According to the present invention, it is possible to provide an ultra-thin hot rolled steel sheet excellent in isotropy using a continuous continuous rolling mode in a performance-rolling direct connection process.

In addition, since the hot rolled steel sheet manufactured by the present invention can produce ultra-thin hot rolled steel sheet only by the hot rolling process, in order to manufacture a low carbon steel (≤1.0mm), it is necessary to go through the pickling / cold rolling and annealing process. It is excellent in price competition against hot rolling. In addition, the top part is inserted into the rolling mill for each coil by continuous rolling and the tail part does not exit the rolling mill, so the thickness variation and scale quality are good at the top and tail parts. The structure is uniform (isotropic), so it is excellent in workability such as Cup Deep Drawing. In addition, the thin slab playing method can eliminate the reheating process in the existing hot milling, which can save energy and improve productivity. In addition, the thin slab playing method can be used to melt the scrap steel, such as scrap iron in the electric furnace can increase the recycling of resources.

1 is a schematic diagram of a facility for a performance-rolling direct connection process applicable to the production of hot rolled steel sheet of the present invention.
Figure 2 is another schematic diagram of the facility for the performance-rolling direct connection process applicable to the production of hot rolled steel sheet of the present invention.
Figure 3 is a graph showing the width direction tensile strength, yield strength and elongation distribution of Inventive Example 2 according to an embodiment of the present invention.
Figure 4 is a graph showing the width direction tensile strength, yield strength and elongation distribution of the conventional example according to an embodiment of the present invention.
5 is a microstructure photograph of Example 2 of the present invention observed with an optical microscope.
6 is a microstructure photograph of the invention example 2 according to an embodiment of the present invention observed by SEM.
Figure 7 is a microstructure photograph of a conventional example observed with an optical microscope according to an embodiment of the present invention.
8 is a graph showing the distribution of the major axis length of the ferrite grains of Inventive Example 2 according to an embodiment of the present invention.
Figure 9 is a graph showing the long axis length distribution of the ferrite grains of the conventional example according to an embodiment of the present invention.
10 is a graph showing the distribution of the ratio of the long axis and short axis length of the ferrite grains of Inventive Example 2 according to an embodiment of the present invention.
11 is a graph showing the distribution of the ratio of the long axis and the short axis length of the ferrite grains of the conventional example according to the embodiment of the present invention.
12 is a graph showing the change in high temperature tensile strength according to the temperature of Inventive Example 2 according to an embodiment of the present invention.
Figure 13 is a SEM micrograph of the inclusions of Inventive Example 2 according to an embodiment of the present invention.
14 is a SEM micrograph of the inclusions of Comparative Example 4 according to an embodiment of the present invention.

Hereinafter, the present invention will be described. First, the alloy composition of the present invention will be described.

C: 0.020 to 0.080 wt%

Carbon (C) is an essential element added to secure the strength of the steel. If the C content is less than 0.020% by weight it may be difficult to secure the strength targeted in the present invention. On the other hand, if the C content is more than 0.080% by weight, the apolytic reaction (L + Delta Ferrite → Austentite) occurs during the solidification of the molten steel to form a solidified cell having a non-uniform thickness, which may lead to an operation accident due to the leakage of the molten steel. Therefore, it is preferable that C content is 0.020 to 0.080 weight%.

Mn: 0.05-0.50 wt%

Manganese (Mn) is an austenite stabilizing element and is effective for strengthening solid solution. When the Mn content is less than 0.05% by weight, it may be difficult to secure the strength targeted in the present invention. On the other hand, when the Mn content is more than 0.50% by weight, weldability is lowered, MnS inclusions and center segregation occur, thereby lowering the ductility of the steel sheet and lowering the corrosion resistance. Therefore, it is preferable that Mn content is 0.05-0.50 weight%.

Al: 0.05 wt% or less

Aluminum (Al) may be added for deoxidation during steelmaking. On the other hand, aluminum (Al) in the steel reacts with nitrogen (N) to precipitate AlN, and in the manufacture of thin slabs, slag cracks may be caused under slab cooling conditions in which these precipitates precipitate, thereby reducing the quality of cast steel or hot rolled steel sheet. In addition, the high melting point of oxide is formed by reacting with O in the molten steel may cause nozzle clogging. Therefore, the content should be controlled as low as possible, preferably controlled to 0.05% by weight or less.

Ca: 0.001-0.005 wt%

Calcium (Ca) is an element that reacts with Al and O in molten steel to form spherical inclusions (12CaO.17Al ₂ O ₃ ) having a low melting point to prevent nozzle clogging and to facilitate inclusion separation. If the Ca content is less than 0.001% by weight, it is difficult to secure the effect. On the other hand, when the Ca content is more than 0.005% by weight, high melting point inclusions may be formed to promote nozzle clogging and casting interruption may occur, and large inclusions (> 50 μm) may be formed to be inferior in workability. Therefore, the content of Ca is preferably controlled to 0.001 to 0.005% by weight.

On the other hand, in the hot rolled steel sheet of the present invention, it is preferable that C, Mn, Al, and Ca satisfy the following relations 1 to 4.

[Relationship 1] 5 ≤ [Al / Ca] ≤ 40

When the Al / Ca ratio is less than 5 in relation 1, a high melting point inclusion (eg, CaO, 3CaOAl ₂ O ₃ ) is formed in molten steel as the Al content is low or the Ca content is high, thereby causing nozzle clogging. High risk On the other hand, when the [Al / Ca] ratio exceeds 40, the Al content is high and the Ca content is low, and the high melting point inclusions in the molten steel (eg CaOAl ₂ O ₃ , CaO ₂ Al ₂ O ₃ , CaO ₆ Al ₂ O ₃ , Al ₂ O ₃ ) is formed, and the inclusions are not spherical, so floating separation is disadvantageous and casting interruption may occur due to clogging of the nozzle. Therefore, in order to stably high speed ultra-thin hot rolled material having excellent isotropy, it is preferable to precisely control the Al / Ca ratio to satisfy the range of 5 to 40.

[Relationship 2] 20 ≤ 1000 [C + 0.04Mn] ≤ 85

In the relation 2, when 1000 [C + 0.04Mn] is less than 20, it is difficult to secure a target strength, and when it exceeds 85, an apolytic reaction (L + Delta Ferrite → Austentite) occurs to form a solidified cell having a non-uniform thickness. As a result, molten steel spills can occur and cause operational accidents.

[Relationship 3] 1 ≤ 1000 [C + 0.04Mn] / [Al / Ca] ≤ 12

In the relation 3, when 1000 [C + 0.04Mn] / [Al / Ca] is less than 1, strength cannot be secured and nozzle clogging may occur. If it exceeds 12, nozzle clogging and apocalyptic reaction may occur, and solidified cells of non-uniform thickness may be formed, which may cause molten steel outflow and cause an operation accident.

[Relationship 4] 0.05 ≤ [C + 0.4Mn] ≤ 0.3

If [C + 0.4Mn] is less than 0.05 in the relation 4, it may be difficult to secure the target strength, and if it exceeds 0.3, it may be difficult to secure the target elongation.

The remaining component of the present invention is iron (Fe). However, in the conventional manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably mixed, and thus cannot be excluded. Since these impurities are known to those skilled in the art, all of them are not specifically mentioned in the present specification.

Meanwhile, the hot rolled steel sheet of the present invention may include Si, P, S, and N as impurity elements.

Si: 0.3 wt% or less

Silicon (Si) may reduce the surface quality by generating a red scale during hot rolling. Therefore, it is preferable that Si content is 0.3 weight% or less.

P: 0.050% by weight or less

Phosphorus (P) may segregate at grain boundaries and / or interphase boundaries to cause brittleness. Therefore, the content of P is preferably 0.050% by weight or less.

S: 0.01 wt% or less

Sulfur (S) as impurities may segregate during MnS non-metallic inclusions and performance solidification in steel, causing hot cracks. Therefore, the content should be controlled as low as possible, it is preferable to control to 0.01% by weight or less.

N: 0.010% by weight or less

Nitrogen (N) may be present as a solid solution in the steel to reduce elongation, thereby reducing workability and formability of the steel sheet. Therefore, the lower the nitrogen content, the more favorable the moldability. When the content of nitrogen exceeds 0.010% by weight, as described above, the solid solution nitrogen is increased to reduce the impact properties and elongation of the steel, and may inhibit the weld toughness. Therefore, it is preferable that N content is 0.010 weight% or less.

In addition, the hot-rolled steel sheet of the present invention is one or more selected from the group consisting of Nb, V, Ti, Mo, Cu, Cr, Ni, Zn, Se, Sb, Zr, W, Ga, Ge and Mg as a tramp element The sum may include 0.3 wt% or less. The tramp element is an impurity element derived from ferroalloy or scrap used as a raw material in the steelmaking process, ladle and tundish refractories, and when the total is more than 0.3%, the surface cracks and hot rolling of the thin slab. The surface quality of a steel plate can be reduced.

In the hot-rolled steel sheet of the present invention, the microstructure preferably includes ferrite: 80% or more and pearlite: 20% or less in area fraction. When the ferrite area fraction is less than 80%, it is difficult to secure a target elongation due to the relatively high pearlite fraction, and when the pearlite area fraction is more than 20%, strength may be increased to make it difficult to secure an elongation. .

In addition, in the hot rolled steel sheet of the present invention, in order to secure excellent isotropy, it is preferable that the ferrite grain size deviation (ΔG) along the rolling direction satisfies the following relational expression (5). The small ferrite grain size variation along the rolling direction means that it has an isotropic structure. The variation of the grain size (FGS) may be defined as in Equation 5 below.

[Equation 5] ΔG = [(G _TD + G _RD ) / 2]-G _{45 ° D} ≤ 2.5㎛

(DELTA G in the above relation 5 is the deviation of the ferrite grain average size (FGS), G _TD is FGS in the vertical direction of the rolling direction, G _RD is FGS, G _{45 ° D} in the horizontal direction of the rolling direction FGS in the 45 ° direction of the rolling direction.)

In the above equation 5, if the ferrite grain size (FGS) deviation exceeds 2.5 μm, isotropic tissue is not obtained, and the edges of the product are formed on the edges of the product after molding, resulting in waste of materials. The drawing problem may occur due to the severe problem and the partial thickness difference.

It is preferable that the average size of the ferrite grains is 2 to 10 µm. If the size of the ferrite grains exceeds 10㎛ it may be difficult to secure the target strength and workability. The finer the ferrite grain, the better the strength and workability. However, in order to control it to 2 μm or less, a precipitate forming element such as Ti and Nb must be added, thereby increasing the manufacturing cost.

The ferrite grains preferably have a long axis / short axis average ratio of 1.0 to 3.0. If the average ratio of the long axis / short ratio of the ferrite grains exceeds 3.0, it may be difficult to secure the target strength and workability, and inferior deep drawing processability may be obtained because an isotropic structure cannot be obtained. As for the long-axis / short-axis average length ratio of the said ferrite crystal grain, it is more preferable that it is 1.0-2.5.

The ferrite grains preferably have a 50% or more share of the grains having an average ratio of long axis / short axis of 1.0 to 2.0. If the ferrite grains are less than 50%, it is difficult to secure the target strength and workability, and deep drawing is not obtained. Machinability may be inferior.

At this time, the Al-Ca-based inclusions included in the hot-rolled steel sheet of the present invention preferably have an average size of 10 µm or less. If the average size of the Al-Ca-based inclusions exceeds 10 μm, there may be a high possibility of cracking during processing.

The hot rolled steel sheet of the present invention may have a thickness of 1.4 mm or less, more preferably 1.0 mm or less, and even more preferably 0.9 mm or less.

The hot rolled steel sheet provided by the present invention has a tensile strength of 350 MPa or more and an elongation of 30% or more. In addition, the ratio between the maximum height and the minimum height of the ear occurring at the edge of the product after the cup deep drawing molding may be 3.5% or less.

Hereinafter, the hot rolled steel sheet manufacturing method of the present invention will be described.

1 is a schematic diagram of a facility for a performance-rolling direct connection process applicable to the production of hot rolled steel sheet of the present invention. The ultra-thin hot rolled steel sheet having excellent isotropy according to one embodiment of the present invention may be produced by applying a performance-rolling direct connection facility as shown in FIG. 1. Performance-rolling direct connection equipment is largely composed of a continuous casting machine 100, rough rolling mill 400, the finishing rolling mill 600. The performance-rolling direct connection facility includes a high speed continuous casting machine 100 for producing a thin slab (a) having a first thickness, and a bar (b) having a second thickness thinner than the first thickness of the slab. Rough rolling mill 400 for rolling in a furnace, finishing mill 600 for rolling the bar of the second thickness into a strip (c) of the third thickness, and a winding machine 900 for winding the strip. In addition, rough mill scale breaker 300 (Rough Mill Scale Breaker, hereinafter 'RSB') in front of the roughing mill 400 and finishing mill scale breaker (500) (Fishing Mill Scale Breaker, hereinafter 'in front of the finishing mill 600) FSB ') can be additionally included, and the surface scale can be easily removed to produce PO (Pickled & Oiled) steel sheets having excellent surface quality when pickling hot rolled steel sheets in a later process. In addition, isothermal isothermal rolling is possible through the direct process of rolling to rolling, so that the steel plate width and longitudinal temperature deviation are very low, and precise cooling control is possible in ROT [Run Out Table (700)] (hereinafter referred to as "runout table"). It is possible to produce ultra-thin hot rolled steel sheet with excellent isotropy. The strips thus rolled and cooled are cut by the high speed shears 800 and wound by the winding machine 900 to be produced as products. On the other hand, in front of the finish rolling scale breaker 500 may be provided with a heater 200 for further heating the bar.

Figure 2 is another schematic diagram of the facility for the performance-rolling direct connection process applicable to the production of hot rolled steel sheet of the present invention. The performance-rolling direct connection facility disclosed in FIG. 2 has the same configuration as the facility described in FIG. 1, but is provided with a heater 200 ′ that additionally heats the slab in front of the rough rolling mill 400 and the rough rolling scale breaker 300. In addition, it is easy to secure the slab edge temperature to lower edge defects, which is advantageous for securing the surface quality. In addition, a space equal to the length of one or more slabs is secured before rough rolling, and batch rolling is also possible.

The ultra-thin hot rolled steel sheet having excellent isotropy of the present invention can be produced in both the performance-rolling direct connection facilities disclosed in FIGS. 1 and 2.

EMBODIMENT OF THE INVENTION Hereinafter, one Embodiment of the manufacturing method of the hot rolled steel sheet of this invention is described in detail.

First, molten steel having the alloy composition described above is continuously cast to obtain a thin slab. At this time, the continuous casting is preferably performed at a casting speed of 4.5 ~ 8.5mpm (m / min). The reason why the casting speed is more than 4.5mpm is because the high speed casting and the rolling process are connected, so that a certain casting speed is required to secure the target rolling temperature. However, when the casting speed is low, there is a risk of segregation from the cast, and if this segregation occurs, it is difficult to secure the strength and workability, and the risk of material deviation in the width direction or the length direction increases. If it exceeds 8.5mpm, since the operation success rate can be reduced due to the instability of the molten steel, the casting speed is preferably in the range of 4.5 ~ 8.5mpm.

On the other hand, the thin slab is preferably a thickness of 80 ~ 120mm. When the thickness of the thin slab exceeds 120mm, not only high-speed casting is difficult, but also the rolling load increases during rough rolling, and when the thickness of the slab is less than 80mm, the temperature of the cast slab rapidly occurs, making it difficult to form a uniform structure. In order to solve this problem, it is possible to additionally install a heating device, but this is a factor to improve the production cost, it is desirable to exclude as possible. Therefore, the thickness of the thin slab is preferably controlled to 80 ~ 120mm.

After obtaining the thin slab may further comprise the step of removing the scale by spraying a coolant to the thin slab. For example, a surface mill of a thin slab can be removed to a thickness of 200 μm or less by spraying coolant of 50 or less at 50 to 350 bar pressure from a roughing mill scale breaker (RSB) nozzle. have. When the pressure is less than 50bar, a large number of arithmetic scales and the like remain on the surface of the thin slab, which may degrade the surface quality after pickling. On the other hand, if it exceeds 350 bar, the bar edge temperature may drop sharply, resulting in edge cracking.

Thereafter, the thin slab is roughly rolled to obtain a bar. In the rough rolling step, the continuous cast thin slab may be roughly rolled in a rough mill consisting of 2 to 5 stands. The control of the cumulative reduction rate during rough rolling plays a role in helping to obtain the isotropic structure and the ultra-thin hot rolled material targeted in the present invention. That is, the higher the reduction ratio during rough rolling, the more uniform the microscopic distribution of C, Mn, etc., and the smaller the temperature gradient in the width and thickness direction of the strip, which is very effective for obtaining an isotropic structure and a uniform material. When the cumulative reduction ratio is less than 70%, the above effects may not be sufficiently exhibited, and the rolling reduction resistance may increase due to an increase in the reduction ratio during finish rolling, thereby causing plate breakage. If it exceeds 90%, since the rolling deformation resistance is greatly increased and rough rolling may be unstable, the cumulative reduction ratio preferably has a range of 70 to 90%.

On the other hand, the surface temperature of the thin slab drawn during the rough rolling is preferably 900 ~ 1200 ℃. The surface temperature of the thin slab at the inlet side of the roughing mill is preferably in the range of 900 ~ 1200 ℃. If the surface temperature of the thin slab is less than 900 ° C there is a possibility that cracks occur in the bar edge portion during the rough rolling load increase and the rough rolling process, in this case may lead to edge defects of the hot-rolled steel sheet. If the slab surface temperature exceeds 1200 ° C., problems such as deterioration of the hot rolled surface due to the remaining hot rolled scale may occur. Therefore, the surface temperature of the thin slab preferably has a range of 900 ~ 1200 ℃.

Further, immediately after the rough rolling, the bar edge temperature is preferably 800 to 1100 ° C. When the temperature of the edge portion of the bar is less than 800 ° C., a large amount of AlN precipitates are generated, and thus there is a problem in that edge crack generation sensitivity is very high due to a decrease in high temperature ductility. On the other hand, if the temperature of the edge portion is more than 1100 ℃ has a disadvantage that the center temperature of the bar is too high to generate a large number of arithmetic scale, which results in inferior surface quality after pickling.

After the step of obtaining the bar may further comprise the step of removing the scale by spraying coolant to the bar. For example, before finishing rolling the bar, spray the cooling water below 50 ℃ with 50 ~ 350bar pressure from the Finishing Mill Scale Breaker (FSB) nozzle to make the surface scale to 15㎛ thickness Can be removed When the coolant injection pressure is less than 50 bar, the scale is insufficient to be removed, and a large amount of fusiform and scale scales are generated on the surface of the steel sheet after finishing rolling, resulting in inferior surface quality after pickling. On the other hand, when the cooling water injection pressure exceeds 350bar, it is difficult to secure the target finish rolling temperature, and as the rolling load increases, the sheet flow may decrease, causing an operation interruption.

The finish rolling may be performed in a finish rolling mill consisting of three to seven stands of the bar made in the rough mill. Since the rolling temperature and the rolling reduction rate between the stands in the finishing rolling mill greatly influence the rolling load fluctuations, the sheeting properties, and the recrystallization according to the phase transformation, precise control is required.

Finish rolling the bar, in the first rolling mill is rolling at a rolling reduction of 50% or more at Ar ₃ + 100 ℃ or more, in the last rolling mill 20% or more in the temperature range of Ar _A50-F50 ~ Ar ₃ +30 ℃ Rolling is carried out at a reduction ratio to obtain a hot rolled steel sheet. Ar _{A50 -F50} means a temperature at which the austenite and ferrite fractions are 50:50. The finish rolling can be performed in a finish rolling mill consisting of three to six stands of the bar made in the rough mill. Since the rolling temperature between the stands in the finishing rolling mill greatly affects the rolling load fluctuations and the plateability due to the phase transformation, precise control is required.

If the rolling temperature in the first rolling mill is less than Ar ₃ + 100 ° C. or the rolling reduction is less than 50% during the finish rolling, the temperature and plastic strain energy required to generate new grains between grains drawn in the rolling direction are small, and thus recrystallization. This may not happen sufficiently and it may be difficult to obtain isotropic tissue.

When the finish rolling is _carried out at a temperature of less than Ar _{A50 -F50} in the final rolling mill, the austenitic fraction may not be sufficient, which may make it difficult to deposit fine ferrite between the austenite grains, and recrystallization may not occur sufficiently to obtain an isotropic structure. It can be difficult. When rolling over Ar ₃ + 30 ° C., grain growth may occur excessively after recrystallization, and thus, it may be difficult to secure a target strength. In addition, when the rolling reduction in the last rolling mill is less than 20%, the plastic deformation energy required for the formation of new grains is small, and dynamic recrystallization may not occur sufficiently, so that it may be difficult to obtain an isotropic structure.

On the other hand, the plate speed in the last rolling mill is preferably 300 ~ 800mpm (m / min). In the finishing rolling mill, the sheet speed of the final rolling may be directly connected to the casting speed and the thickness of the final hot rolled product, wherein the thickness of the hot rolled steel sheet may be 1.4 mm or less, more preferably 1.0 mm or less. When the last rolling speed is more than 800mpm, operation accidents such as plate breakage may occur, and isothermal isothermal rolling is difficult, so that uniform temperature is not secured, material deviation may occur, and time required for recrystallization is insufficient. It can be difficult to obtain isotropic tissue. On the other hand, if it is less than 300mpm, the last rolling speed may be too slow, causing problems in mass balance and heat balance, making it difficult to continuously roll continuously.

Thereafter, the hot rolled steel sheet is cooled by air, and then cooled to 20 to 100 ° C / s. When the cooling rate is less than 20 DEG C / sec, the ferrite grains are coarsened, and it is difficult to obtain pearlite structure, which makes it difficult to obtain desired strength. On the other hand, when it exceeds 100 ° C / sec is promoted pearlite transformation can be difficult to secure the target elongation, and the occurrence of the ear that occurs after the cup deep drawing (Cup Deep Drawing) may be severe.

The air cooling time is preferably 2.5 seconds or more. If the air cooling time is less than 2.5 seconds, it may be difficult to obtain isotropic structures because new grains generated during finish rolling may not sufficiently undergo dynamic recrystallization and grain growth.

Then, the cooled hot rolled steel sheet is wound at 500 ~ Ar _P -50 ℃. When the coiling temperature exceeds Ar _P- 50 ° C., the ferrite grains are coarsened, and it is difficult to obtain a pearlite structure, which makes it difficult to obtain desired strength. On the other hand, when the temperature is less than 500 ° C., the perlite transformation may be promoted, and thus it may be difficult to secure a target elongation, and the occurrence of an ear occurring after cup deep drawing may be severe.

On the other hand, after the winding step, it may further comprise a step of pickling the wound hot rolled steel sheet. In the present invention, since the scale can be sufficiently removed in the thin slab and bar descaling step, it is possible to obtain a Pickled & Oiled (PO) steel sheet having excellent surface quality even with a general pickling treatment. Therefore, the pickling treatment which can be used in the present invention is not particularly limited since it can be applied as long as it is a treatment method generally used in the hot acid pickling process.

On the other hand, the method for producing a hot rolled steel sheet according to the present invention is a continuous rolling mode used in a performance-rolling direct connection process, characterized in that each of the above-described processes are performed continuously.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, it is necessary to note that the following examples are only for illustrating the present invention in more detail, and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and the matters reasonably inferred therefrom.

(Example 1)

After preparing the molten steel having the alloy composition of Table 1, by applying a play-rolling direct connection process to continuously cast the molten steel at a casting speed of 6.8mpm to obtain a 90mm thick slab, the thin slab was prepared in Table 2 Under the condition, a hot rolled steel sheet (Hot Rolled, HR) was manufactured. However, in the conventional example 1, after casting a slab having a thickness of 250 mm in the existing hot-milling, multi-stage cooling [900 ℃ finish rolling → shear cooling (250 ℃ / sec) in the existing batch process under the manufacturing conditions described in Tables 2 and 3 ) → 1-2 seconds air cooling → after stage cooling (250 ℃ / sec)] to make 3.2mm thick hot rolled steel sheet, cold rolled at 75% rolling rate and then annealed (annealing temperature: 803 ℃, intermediate cooling temperature) : 650 ° C., final cooling temperature: 430 ° C.) to a 0.8 mm thick cold rolled steel sheet (Cold Rolled, CR). In addition, the ferrite transformation start temperature (Ar ₃ ), austenite (A) and ferrite (F) in the following Table 2, the temperature (Ar _{A50 -F50} ) and the pearlite transformation start temperature (Ar _P ) is 50:50 Calculations were made using commercial thermodynamic software JmatPro-v9.1.

For the inventive examples and comparative examples prepared as described above, whether the nozzle was clogged or plate fracture occurred, the microstructure area fraction, the ferrite grain size deviation (ΔG) according to the rolling direction, the average size of the ferrite grains, the long axis / short axis of the ferrite grains After measuring the length ratio and the mechanical properties, it is shown in Table 4 and Table 5.

The area fractions of ferrite (F) and pearlite (P) are 1/4 of specimen thickness in the rolling vertical (TD), horizontal (RD), and 45 ° (45 ° D) directions using an optical microscope. It was set as the average value of the area fraction obtained by measuring 10 views with a 500x magnification.

The grain size of ferrite was measured at 500x magnification by 10x at 1/4 point of specimen thickness in rolling vertical (TD), horizontal (RD) and 45 ° (45 ° D) directions using an optical microscope. It was set as the average value of the crystal grain size measured using. In addition, the degree of uniformity (isotropic, isotropic) of the grain size of the invention examples and comparative examples was evaluated by measuring the deviation of the average size of the ferrite grains according to the rolling direction as shown in relational equation 5.

[Equation 5] ΔG = [(G _TD + G _RD ) / 2]-G _{45 ° D} <2.5㎛

(DELTA G in the above relation 5 is the deviation of the ferrite grain average size (FGS), G _TD is FGS in the vertical direction of the rolling direction, G _RD is FGS, G _{45 ° D} in the horizontal direction of the rolling direction FGS in the 45 ° direction of the rolling direction.)

Tensile strength showed the value measured by taking the JIS No. 5 specimen in the rolling horizontal direction from the strip.

On the other hand, after drawing molding, there are problems such as unevenness at the edges of the product, resulting in excessive waste of materials and partial thickness change, resulting in poor drawing processing and material properties depending on the direction of the material. Deep drawing tests were conducted to quantitatively assess the problems caused by other anisotropy. The degree of anisotropy was evaluated by the ratio of the difference between the maximum and minimum height of the ear occurring at the edge of the cup.

division Alloy composition (% by weight) Relationship 1 Relationship 2 Relationship 3 Relationship 4 C Mn Al Ca Inventive Steel 1 0.040 0.20 0.031 0.0025 12.4 48 3.9 0.12 Inventive Steel 2 0.045 0.41 0.028 0.0021 13.3 61.4 4.6 0.209 Invention Steel 3 0.068 0.19 0.025 0.0026 9.6 75.6 7.9 0.144 Inventive Steel 4 0.071 0.22 0.031 0.0025 12.4 79.8 6.4 0.159 Comparative Steel 1 0.095 0.45 0.021 0.0031 6.8 113 16.7 0.275 Comparative Steel 2 0.056 0.75 0.025 0.0035 7.1 82 11.5 0.316 Comparative Steel 3 0.051 0.19 0.056 0.0008 70 58.6 0.8 0.127 Inventive Steel 5 0.042 0.21 0.032 0.0015 21.3 50.4 2.4 0.126 Comparative Steel 4 0.045 0.22 0.015 0.0058 2.6 53.8 20.8 0.133 Comparative Steel 5 0.009 0.03 0.025 0.0031 8.1 9.8 1.2 0.017 Conventional Steel 0.041 0.21 0.025 0.0005 50 49.4 1.0 0.125 [Relationship 1] 5 ≤ [Al / Ca] ≤ 40
[Relationship 2] 20 ≤ 1000 [C + 0.04Mn] ≤ 85
[Relationship 3] 1 ≤ 1000 [C + 0.04Mn] / [Al / Ca] ≤ 12
[Relationship 4] 0.05 ≤ [C + 0.4Mn] ≤ 0.3

division Steel grade no. Bar thickness
(mm) Hot rolled sheet thickness
(mm) Finish rolling First rolling mill Last rolling mill Ar ₃
(℃) Rolling temperature
(℃) Rolling reduction
(%) Ar _{A50 -F50}
(℃) Rolling temperature
(℃) Rolling reduction
(%) Inventive Example 1 Inventive Steel 1 16 0.8 885 1020 58 870 875 28 Inventive Example 2 Inventive Steel 2 15 0.8 870 1005 56 855 861 27 Inventive Example 3 Invention Steel 3 15 0.8 868 1008 58 850 860 28 Inventive Example 4 Inventive Steel 4 16 0.8 865 1006 58 845 855 26 Comparative Example 1 Comparative Steel 1 15 0.8 845 986 54 785 800 29 Comparative Example 2 Comparative Steel 2 15 0.8 850 996 60 830 840 28 Comparative Example 3 Comparative Steel 3 15 0.8 885 1022 58 860 865 28 Inventive Example 5 Inventive Steel 5 15 0.8 880 1025 62 870 878 27 Comparative Example 4 Comparative Steel 4 15 0.8 875 1015 60 860 865 29 Comparative Example 5 Comparative Steel 5 15 0.8 903 1045 64 906 915 28 Conventional example Conventional Steel 45 3.2 884 1125 46 868 900 19

division Steel grade no. Cooling Winding Air cooling time (seconds) Cooling rate (℃ / sec) Ar _p (℃) Winding temperature (℃) Inventive Example 1 Inventive Steel 1 3.5 25 700 633 Inventive Example 2 Inventive Steel 2 3.8 28 695 620 Inventive Example 3 Invention Steel 3 3.7 26 700 621 Inventive Example 4 Inventive Steel 4 3.8 32 695 615 Comparative Example 1 Comparative Steel 1 3.1 25 695 625 Comparative Example 2 Comparative Steel 2 3.8 26 675 605 Comparative Example 3 Comparative Steel 3 4.0 31 700 623 Inventive Example 5 Inventive Steel 5 3.8 25 700 632 Comparative Example 4 Comparative Steel 4 3.8 34 695 625 Comparative Example 5 Comparative Steel 5 3.9 31 700 628 Conventional example Conventional Steel Multi-stage cooling 698 626

division Steel grade no. Nozzle
catch
Whether Breaking
Occur
Whether ferrite
Fraction
(area%) Pearlite
Fraction
(area%) ferrite
Grain
Average size
(△ G) Ferrite grains
Average size
(Μm) Average ratio of major and minor lengths of ferrite grains Inventive Example 1 Inventive Steel 1 × × 91 9 1.25 6.35 2.2 Inventive Example 2 Inventive Steel 2 × × 87 13 1.35 6.25 2.3 Inventive Example 3 Invention Steel 3 × × 90 10 1.20 6.38 2.6 Inventive Example 4 Inventive Steel 4 × × 89 11 1.25 6.41 2.4 Comparative Example 1 Comparative Steel 1 × - Stop casting with molten steel spill Comparative Example 2 Comparative Steel 2 × × 76 24 1.36 6.45 2.5 Comparative Example 3 Comparative Steel 3 ○ - Stop casting due to clogged nozzle Inventive Example 5 Inventive Steel 5 × × 89 11 1.26 6.41 2.3 Comparative Example 4 Comparative Steel 4 ○ - Stop casting due to clogged nozzle Comparative Example 5 Comparative Steel 5 × × 96 4 1.28 6.51 2.5 Conventional example Conventional Steel × × 90 10 2.59 7.26 3.5

division Steel grade no. % Share of ferrite grains with long and short average ratios between 1.0 and 2.0 The tensile strength
(MPa) Elongation
(%) Maximum of Earing
Height and minimum
% Of height difference Inventive Example 1 Inventive Steel 1 65 387 39 2.79 Inventive Example 2 Inventive Steel 2 62 420 34 2.56 Inventive Example 3 Invention Steel 3 63 397 37 2.51 Inventive Example 4 Inventive Steel 4 61 403 26 2.85 Comparative Example 1 Comparative Steel 1 Stop casting with molten steel spill Comparative Example 2 Comparative Steel 2 59 461 29 3.69 Comparative Example 3 Comparative Steel 3 Stop casting due to clogged nozzle Inventive Example 5 Inventive Steel 5 63 389 38 2.80 Comparative Example 4 Comparative Steel 4 Stop casting due to clogged nozzle Comparative Example 5 Comparative Steel 5 60 335 45 2.15 Conventional example Conventional Steel 40 389 38 11.01

As can be seen from Tables 1 to 5, in the case of Inventive Examples 1 to 5 satisfying both the alloy composition, the component relations 1 to 4, and the manufacturing conditions proposed by the present invention, the target microstructure characteristics without nozzle clogging and plate breaking Satisfies all, it can be seen that the excellent mechanical properties to obtain the present invention, that is, the tensile strength of 350MPa or more, elongation of 30% or more. In addition, in the case of the invention examples 1 to 5 it can be seen that the ratio between the maximum height and the minimum height of the ear (earning) is significantly lower than the conventional example produced in the conventional hot-drain mill is excellent in isotropy.

On the other hand, Comparative Examples 1 to 5 do not satisfy one or more of the alloy composition, component relations 1 to 5 proposed by the present invention, the casting stoppage due to leakage of molten steel or clogging of the nozzle occurs, or due to differences in the microstructure fraction, etc. It can be seen that the tensile strength, the elongation, and the earing characteristics are not satisfied.

Conventional example is produced in the existing hot rolling, it can be seen that the ratio of the difference between the maximum height and the minimum height of the ear (Earing) is very high isotropic.

On the other hand, Figure 3 and Figure 4 is a graph showing the distribution of the tensile strength, yield strength and elongation distribution of the invention example 2 and the conventional example, respectively, as can be seen from the results of the invention, in the case of invention example 2 is hot rolled, It can be seen that it has a uniform tensile property equivalent to that of the cold rolled material produced by cold rolling and annealing at.

5 and 6 are microstructure photographs of Example 2 observed with an optical microscope and SEM, respectively, and FIG. 7 is a microstructure photograph of a conventional example observed with an optical microscope. As can be seen from Figures 5 to 7, the microstructure of Inventive Example 2 is composed of ferrite (F, white) and pearlite (P, black), the main structure of Inventive Example 2 from the high ferrite tissue fraction It can be seen that it is ferrite. In addition, inventive example 2 has a fine ferrite grain size compared to the conventional example, and as shown in Tables 4 and 5, it can be seen that the grain size variation is also small and excellent in isotropy.

8 and 9 are graphs showing the ferrite grain size distribution of Inventive Example 2 and Conventional Example, respectively. As can be seen from Figures 8 and 9, in the case of Inventive Example 2 it can be seen that there is a ferrite of 0.5 ~ 27.5㎛, in particular, ferrite grains of 1 ~ 10㎛ are mainly distributed. On the other hand, as can be seen through Figure 9, in the case of the conventional example, there is a ferrite of 1 ~ 42.5㎛, it can be seen that the size of the ferrite grains compared to the invention example 2.

10 and 11 are graphs showing the distributions of the ratios between the major and minor axis lengths of the ferrite grains of Inventive Example 2 and Conventional Example, respectively. Here, the length ratio between the long axis and the short axis is taken at 5 times magnification at a magnification of 500 times at the point of specimen thickness in the rolling vertical (TD), horizontal (RD), and 45 ° (45 ° D) directions, and Image-Plus Pro The software measures the major and minor lengths of the ferrite grain size and converts them into ratios. As can be seen from Figures 10 and 11, in the case of Inventive Example 2, the long axis / short axis length ratio is distributed in the vicinity of 1 ~ 2.0, in the case of the conventional example 1 there are many crystal grains having a long axis / short axis length ratio of 3.5 or more It can be confirmed that, after all, Inventive Example 2 is superior to the conventional example isotropic.

12 is a graph showing a change in high temperature tensile strength according to the temperature of the invention example 2. As can be seen from Figure 12, it can be seen that the intensity change is severe according to the temperature change and pearlite, ferrite and austenite transformation. Therefore, in order to produce an ultra-thin hot rolled material and produce a hot rolled steel sheet having excellent isotropy, it is suggested that the side temperature and the final rolling mill temperature are very important during finishing rolling.

13 and 14 are SEM micrographs of the inclusions of Inventive Example 2 and Comparative Example 4, respectively. The inclusions were analyzed by Electrolysis Extraction Separation (ESSA), in which an electrolyte (AA solution) was placed inside a shell consisting of a cathode and an anode, and the specimen was used as an anode. The precipitates remaining after electrolysis were collected & filtered to analyze the inclusion shape and components by SEM / EDX. As can be seen from Figure 13, Inventive Example 2 satisfies the Ca, Al content and the relations 1 and 3 presented in the present invention, the Ca-Al-based inclusions have a spherical shape. In addition, the average size of inclusions was 5.8 micrometers. On the other hand, as can be seen through Figure 14, in the case of Comparative Example 4 does not satisfy the Ca content, the Al / Ca ratio of the component relational formula 1 and the relational formula 3 and the inclusion of Al ₂ O ₃ is present alone, It can be seen that the shape of the inclusions is not spherical, which causes nozzle clogging and casting stoppage.

(Example 2)

After preparing molten steel having the alloy composition of Inventive Steel 1 of Table 1, the molten steel was continuously cast at a casting speed of 6.8mpm by applying a play-rolling direct connection process to obtain a thin slab of 90 mm thickness, and the thin slab was 16 mm thick. After the rough rolling of the bar, the bar was manufactured by hot rolled steel sheet (Hot Rolled, below HR) of 0.8 mm thickness under the manufacturing conditions shown in Table 6. The hot rolled steel sheet manufactured as described above was subjected to nozzle clogging / plate breakage, microstructure area fraction, ferrite grain size deviation (ΔG) according to the rolling direction, average size of ferrite grains, ferrite grain long axis / short length ratio, and mechanical properties. After the measurement, the results are shown in Tables 7 and 8 below. In addition, the said physical property measurement was performed on the conditions similar to Example 1.

division Finish rolling Cooling Winding First rolling mill Last rolling mill Air cooling
time
(second) Cooling rate
(℃ / s) Ar _p
(℃) Winding
Temperature
(℃) Ar ₃
(℃) Rolling
Temperature
(℃) Rolling reduction
(%) Ar _{A50 -F50}
(℃) Rolling
Temperature
(℃) Rolling reduction
(%) Inventive Example 6 885 1025 58 870 880 58 3.7 27 700 635 Inventive Example 7 1005 58 871 58 3.4 26 625 Comparative Example 6 970 58 820 58 3.5 26 635 Comparative Example 7 1025 40 882 40 3.6 28 615 Comparative Example 8 998 58 840 58 3.8 25 625 Comparative Example 9 1010 54 880 54 3.7 25 630 Comparative Example 10 1015 62 882 62 2.1 26 628 Inventive Example 8 1020 60 882 60 4.0 65 560 Inventive Example 9 1015 56 879 56 2.9 87 530 Comparative Example 11 1022 58 885 58 3.8 125 480

division Nozzle
catch
Whether Breaking
Occur
Whether ferrite
Fraction
(area%) Pearlite
Fraction
(area%) ferrite
Grain
Average size
(△ G) Ferrite grains
Average size
(Μm) Long axis / short length of ferrite grain
Average ratio Inventive Example 6 × × 91 9 1.25 6.35 2.2 Inventive Example 7 × × 90 10 1.35 6.45 2.3 Comparative Example 6 × × 90 10 2.61 6.65 3.6 Comparative Example 7 × × 91 9 2.75 6.78 3.8 Comparative Example 8 × × 89 11 2.59 6.69 3.6 Comparative Example 9 × × 90 10 2.81 6.89 4.0 Comparative Example 10 × × 90 10 2.79 6.88 3.9 Inventive Example 8 × × 86 14 1.41 6.30 2.1 Inventive Example 9 × × 84 16 1.36 6.32 2.3 Comparative Example 11 × × 74 26 1.56 6.55 2.9

division % Share of ferrite grains with long and short average ratios between 1.0 and 2.0 The tensile strength
(MPa) Elongation
(%) Maximum of Earing
Height and minimum
% Of height difference Inventive Example 6 65 387 39 2.79 Inventive Example 7 64 390 38 2.81 Comparative Example 6 41 378 38 5.52 Comparative Example 7 38 375 39 4.75 Comparative Example 8 45 379 40 3.89 Comparative Example 9 35 376 39 6.25 Comparative Example 10 40 374 40 5.31 Inventive Example 8 61 422 33 2.72 Inventive Example 9 63 427 32 2.98 Comparative Example 11 56 480 27 3.85

As can be seen through Tables 6 to 7, the invention examples 6 to 9 satisfying all of the first rolling mill entrance temperature, the reduction ratio and the final rolling mill entry temperature, the reduction ratio and cooling rate, the coiling temperature proposed by the present invention It can be seen that both the microstructure and the mechanical properties are satisfied.

However, Comparative Example 6 is a rolling temperature in the first rolling mill, Comparative Example 7 is a rolling rate in the first rolling mill, Comparative Example 8 is a rolling temperature in the second mill, Comparative Example 9 is a rolling rate in the second mill, Comparative Example 10 is As air cooling time during cooling, Comparative Example 11 does not satisfy the cooling rate and the coiling temperature, it can be seen that the microstructure and mechanical properties of the present invention are not satisfied.

a: slab b: bar
c: strip
100: continuous casting machine 200, 200 ': heater
300: Roughing Mill Scale Breaker (RSB)
400: roughing mill
500: Finishing Mill Scale Breaker (FSB)
600: finish rolling mill 700: runout table
800: high speed shear 900: winder

Claims (18)

By weight, C: 0.020-0.080%, Mn: 0.05-0.50%, Al: 0.05% or less, Ca: 0.001-0.005%, remaining Fe and other unavoidable impurities,
The C, Mn, Al and Ca satisfy the following relations 1 to 4,
The microstructure comprises an area fraction of ferrite: 80% or more, pearlite: 20% or less,
Ferrite grain size deviation (ΔG) according to the rolling direction satisfies the following relational formula 5,
Ultra-thin hot rolled steel sheet with excellent isotropy with a thickness of 1.4 mm or less.
[Relationship 1] 5 ≤ [Al / Ca] ≤ 40
[Relationship 2] 20 ≤ 1000 [C + 0.04Mn] ≤ 85
[Relationship 3] 1 ≤ 1000 [C + 0.04Mn] / [Al / Ca] ≤ 12
[Relationship 4] 0.05 ≤ [C + 0.4Mn] ≤ 0.3
[Equation 5] ΔG = [(G _TD + G _RD ) / 2]-G _{45 ° D} ≤ 2.5㎛
(The contents of C, Mn, Al and Ca in the relations 1 to 4 are by weight, ΔG in the relation 5 is a deviation of the ferrite grain average size (FGS), G _TD is perpendicular to the rolling direction Direction FGS, G _RD is the horizontal direction FGS in the rolling direction, G _{45 ° D} represents the FGS in the 45 ° direction of the rolling direction.)
The method according to claim 1,
The hot rolled steel sheet includes Si, P, S, and N as impurity elements, and Nb, V, Ti, Mo, Cu, Cr, Ni, Zn, Se, Sb, Zr, W, Ga, Ge, and Mg as tramp elements. Ultra-thin hot rolled steel sheet excellent in isotropy comprising at least one selected from the group consisting of 0.3 wt% or less in total.
The method according to claim 1,
The ultra-thin hot rolled steel sheet having excellent isotropy of the average size of the ferrite grains is 2 ~ 10㎛.
The method according to claim 1,
The ferrite grains are ultra-thin hot rolled steel sheet excellent in isotropy having a long axis / short axis average ratio of 1.0 ~ 3.0.
The method according to claim 1,
The ferrite grains are ultra-thin hot rolled steel sheet having excellent isotropy with a 50% or more share of the grains having a long axis / short axis average ratio of 1.0 to 2.0.
The method according to claim 1,
The hot rolled steel sheet is an ultra-thin hot rolled steel sheet excellent in isotropy containing Al-Ca-based inclusions having an average size of 10㎛ or less.
delete The method according to claim 1,
The hot rolled steel sheet is an ultra-thin hot rolled steel sheet having an excellent isotropy having a tensile strength of 340 MPa or more and an elongation of 30% or more.
The method according to claim 1,
The hot rolled steel sheet is an ultra-thin hot rolled steel sheet excellent in isotropy having a ratio of the maximum height and the minimum height of the ear generated after cup deep drawing molding is 3.5% or less.
By weight, C: 0.020-0.080%, Mn: 0.05-0.50%, Al: 0.05% or less, Ca: 0.001-0.005%, remaining Fe and other unavoidable impurities, wherein C, Mn, Al and Ca Continuous casting of molten steel that satisfies the following relations 1 to 4 to obtain a thin slab;
Rough rolling the thin slab to obtain a bar;
Finish rolling the bar, in the first rolling mill is rolling at a rolling reduction of 50% or more at Ar ₃ + 100 ℃ or more, in the last rolling mill 20% or more in the temperature range of Ar _A50-F50 ~ Ar ₃ +30 ℃ Rolling at a rolling rate to obtain a hot rolled steel sheet;
Air cooling the hot rolled steel sheet for 2.5 seconds or more, and then cooling to 20 to 100 ° C / s; And
And winding the cooled hot-rolled steel sheet at 500 ~ Ar _P -50 ℃,
Wherein each step is a continuous ultra-thin hot rolled steel sheet manufacturing method, characterized in that is carried out continuously.
[Relationship 1] 5 ≤ [Al / Ca] ≤ 40
[Relationship 2] 20 ≤ 1000 [C + 0.04Mn] ≤ 85
[Relationship 3] 1 ≤ 1000 [C + 0.04Mn] / [Al / Ca] ≤ 12
[Relationship 4] 0.05 ≤ [C + 0.4Mn] ≤ 0.3
(The contents of C, Mn, Al, and Ca in the above relations 1 to 4 are by weight.)
The method according to claim 10,
The molten steel includes Si, P, S, and N as impurity elements, and Nb, V, Ti, Mo, Cu, Cr, Ni, Zn, Se, Sb, Zr, W, Ga, Ge, and Mg as tramp elements. A method for producing an ultra-thin hot rolled steel sheet excellent in isotropy, comprising at least one selected from the group consisting of a total of 0.3% by weight or less.
The method according to claim 10,
The continuous casting is a method of producing an ultra-thin hot rolled steel sheet excellent in isotropy performed at a casting speed of 4.5 ~ 8.5mpm (m / min).
The method according to claim 10,
The thin slab is a method of producing an ultra-thin hot rolled steel sheet excellent in isotropic thickness of 80 ~ 120mm.
The method according to claim 10,
After the step of obtaining the thin slab, spraying the cooling water to the thin slab at a pressure of 50 ~ 350bar further comprises the step of removing the scale to a thickness of less than 200㎛ ultra-thin hot rolled steel sheet having excellent isotropic.
The method according to claim 10,
The surface temperature of the thin slab drawn during the rough rolling is 900 ~ 1200 ℃, immediately after the rough rolling, the bar edge temperature is 800 ~ 1100 ℃ excellent manufacturing method of ultra-thin hot rolled steel sheet excellent in isotropy.
The method according to claim 10,
The cumulative rolling reduction during rough rolling is a method for producing an ultra-thin hot rolled steel sheet having excellent isotropy of 70 ~ 90%.
The method according to claim 10,
After the step of obtaining the bar, by spraying the cooling water at a pressure of 50 ~ 350bar to the bar further comprising the step of removing the scale to a thickness of less than 15㎛ ultra-thin hot rolled steel sheet having excellent isotropic.
The method according to claim 10,
The sheet rolling speed during the final rolling in the last rolling mill is a manufacturing method of ultra-thin hot rolled steel sheet excellent in isotropy is 300 ~ 800mpm (m / min).

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