KR102325472B1 - Hot rolled steel sheet having excellent hole expansion property and method of manufacturing the same - Google Patents

Abstract

The present invention relates to a hot-rolled steel sheet having excellent hole expandability and a method for manufacturing the same, and more particularly, to a hot-rolled steel sheet having excellent hole expandability applicable to automobile wheels, rims and frames, and a method for manufacturing the same.
One embodiment of the present invention is by weight %, C: 0.045 to 0.10%, Mn: 0.40 to 1.4%, Al: 0.005 to 0.050%, Ca: 0.0005 to 0.0050%, Nb: 0.003 to 0.05%, balance Fe and others Contains unavoidable impurities, satisfies the following Relations 1 to 3, in area%, ferrite is 93.0 to 98.5%, and pearlite includes a microstructure of 1.5 to 7.0%, and the long/short axis ratio of the crystal grains of the ferrite is To provide a hot-rolled steel sheet with excellent hole expandability of 5.0 or less.
[Relational Expression 1] 0.20 ≤ C+Mn/5+5Nb ≤ 0.40
[Relational Expression 2] 3 ≤ Al/Ca ≤ 35
[Relation 3] 20 ≤ (Al/Ca)/(C+Mn/5+5Nb) ≤ 100
(However, in Relations 1 to 3, the unit of content of C, Mn, Nb, Al and Ca is weight %.)

Description

Hot-rolled steel sheet with excellent hole expandability and manufacturing method thereof

The present invention relates to a hot-rolled steel sheet having excellent hole expandability and a method for manufacturing the same, and more particularly, to a hot-rolled steel sheet having excellent hole expandability applicable to automobile wheels, rims and frames, and a method for manufacturing the same.

Hot-rolled steel sheets for automobiles are not only demanding a higher level of workability due to the tendency to increase strength for weight reduction of automobile molded products, but also require excellent weld fatigue characteristics in terms of automobile usage environment, and uniform parts in terms of part molding. It is already well known that good dimensional shape, ie, uniform material deviation, is required for fabrication. In order to increase the workability and strength of the steel sheet, it is generally manufactured by minimizing impurities in the steel and adding C, Si, Mn, Ti, Nb, Mo, V, and the like. However, since most of the above elements are elements that promote work hardening and reduce weldability, it is easy to deepen the anisotropy of the steel sheet immediately after hot rolling, and make the microstructure non-uniform to reduce workability and weldability, and during the manufacturing process, rolling in the hot rolling process It is easy to cause plate shape defects and plate breakage due to a sudden increase in load.

The prior art of such a hot-rolled steel sheet is as follows.

Patent Document 1 is characterized in providing a hot-rolled steel sheet having a yield strength of 50 ksi or more by adding small amounts of Ti and V to precipitate carbonitrides of these elements. Patent Documents 2 and 3 relate to a manufacturing technology of a hot-rolled steel sheet utilizing precipitation strengthening of these elements by adding Ti and Mo. In addition, Patent Document 4 is a technique for manufacturing a hot-rolled steel sheet made of ferrite and bainite by adding Ti and Nb and cooling it in three stages.

However, all of the above prior arts relate to a method of manufacturing in the existing hot-rolling process (batch mode), and it is difficult to avoid the problem that material deviation occurs greatly in the width and length directions. That is, as the rolling speed is inevitably accelerated at the tail in order to keep the finish rolling temperature constant in the conventional hot rolling process (Batch mode), there is a problem in that material deviations in the width and length directions occur greatly. In addition, there is a problem in that it is difficult to obtain a high hole expandability because the finish rolling temperature is high and the grain size of the final product is coarse.

Therefore, it is possible to overcome the material deviation problems that occur when manufacturing steel sheets in the existing hot-rolling (batch mode) process, while providing a 440 MPa-class thin film (0.6 to 2.3 mmt) hot-rolled steel sheet with high hole expandability and a method for manufacturing the same. development is required.

US Registered Patent Publication No. 5514227 Japanese Laid-Open Patent Publication No. 2002-322541 US Patent Publication No. 2003-0063996 Republic of Korea Patent Publication No. 2000-0043432

One aspect of the present invention is to provide a hot-rolled steel sheet having excellent hole expandability and a method for manufacturing the same by using the continuous rolling mode in the direct connection process of casting to rolling.

In addition, the subject of this invention is not limited to the above-mentioned content. The subject of the present invention will be understood from the overall content of the present specification, and those of ordinary skill in the art to which the present invention pertains will have no difficulty in understanding the additional subject of the present invention.

One embodiment of the present invention is by weight%, C: 0.045 to 0.10%, Mn: 0.40 to 1.4%, Al: 0.005 to 0.050%, Ca: 0.0005 to 0.0050%, Nb: 0.003 to 0.05%, balance Fe and others Contains unavoidable impurities, satisfies the following Relations 1 to 3, in area%, ferrite is 93.0 to 98.5%, and pearlite contains a microstructure of 1.5 to 7.0%, and the long/short axis ratio of the crystal grains of the ferrite is To provide a hot-rolled steel sheet with excellent hole expandability of 5.0 or less.

[Relational Expression 1] 0.20 ≤ C+Mn/5+5Nb ≤ 0.40

[Relational Expression 2] 3 ≤ Al/Ca ≤ 35

[Relation 3] 20 ≤ (Al/Ca)/(C+Mn/5+5Nb) ≤ 100

(However, in Relations 1 to 3, the unit of content of C, Mn, Nb, Al and Ca is weight %.)

Another embodiment of the present invention is, by weight%, C: 0.045-0.10%, Mn: 0.40-1.4%, Al: 0.005-0.050%, Ca: 0.0005-0.0050%, Nb: 0.003-0.05%, balance Fe and others obtaining a thin slab by continuously casting molten steel containing unavoidable impurities and satisfying the following Relations 1 to 3; roughing the thin slab to obtain a bar; Finish rolling the bar so that the exit temperature of the finish rolling is 740 ~ 870 ℃ to obtain a hot-rolled steel sheet; After air-cooling the hot-rolled steel sheet for 1.5 to 6.0 seconds, and then cooling to 560 to 690 °C at a cooling rate of 20 to 80 °C / sec, and winding the hole, wherein each step is continuously performed. A method for manufacturing a hot-rolled steel sheet having excellent expandability is provided.

[Relational Expression 1] 0.20 ≤ C+Mn/5+5Nb ≤ 0.40

[Relational Expression 2] 3 ≤ Al/Ca ≤ 35

[Relation 3] 20 ≤ (Al/Ca)/(C+Mn/5+5Nb) ≤ 100

(However, in Relations 1 to 3, the unit of content of C, Mn, Nb, Al and Ca is weight %.)

According to one aspect of the present invention, by appropriately controlling the alloy composition and manufacturing conditions, it is possible to provide a hot-rolled steel sheet excellent in hole expandability and a method for manufacturing the same by using the continuous rolling mode in the direct rolling-rolling process. In addition, since the reheating process in the conventional hot-rolling mill can be omitted, energy saving and productivity improvement can be achieved. In addition, through the thin slab playing method, it is possible to use steel obtained by melting scraps such as scrap iron in an electric furnace, thereby increasing the recyclability of resources.

1 is a schematic diagram of a facility for continuous casting-rolling direct connection process applicable to the production of hot-rolled steel sheet of the present invention.
Figure 2 is another schematic view of the equipment for the continuous casting-rolling direct connection process applicable to the production of the hot-rolled steel sheet of the present invention.
3 is a graph showing the relationship of Relations 1 to 3;
4 is a photograph of the inclusion of Inventive Example 1 according to an embodiment of the present invention observed by SEM.
5 is a photograph observed by SEM of the inclusions of Comparative Example 4 according to an embodiment of the present invention.
6 shows a ferrite grain size distribution measured using EBSD in Inventive Example 9 according to an embodiment of the present invention.

The new steel manufacturing process, the casting and rolling direct connection manufacturing process, has the advantage of reducing the temperature deviation in the width and length directions of the strip as it is controlled at constant velocity and isothermal temperature due to the characteristics of the process. It is a process with potential. The present inventors have completed the present invention with the knowledge that by using such a casting-rolling direct connection manufacturing process, a hot-rolled steel sheet having a small material deviation as well as excellent hole expandability can be provided.

EMBODIMENT OF THE INVENTION Hereinafter, one Embodiment of this invention is described. First, the alloy composition of the present invention will be described. The content of the alloy composition described below means wt% unless otherwise specified.

C: 0.045-0.10%

Carbon (C) is an important element added to secure strength in transformed steel. When the C content is less than 0.045%, it may be difficult to secure the strength targeted in the present invention. On the other hand, when the C content is more than 0.10%, it may be difficult to secure the target hole expandability and elongation because the strength is too high. Therefore, the C content is preferably 0.045 to 0.10%. The lower limit of the C content is more preferably 0.050%, and even more preferably 0.055%. The upper limit of the C content is more preferably 0.09%, and even more preferably 0.08%.

Mn: 0.40~1.4%

Manganese (Mn) suppresses the formation of ferrite and increases the strength of austenite by facilitating the formation of low-temperature transformation phases (pearlite and bainite, etc.). When the Mn content is less than 0.40%, it may be difficult to secure the strength targeted in the present invention. On the other hand, when the Mn content is more than 1.4%, segregation zones are formed inside and/or outside the thin slab and hot-rolled steel sheet, causing cracks to occur and propagate, thereby lowering the final quality of the steel sheet, and improving weldability and hole expandability. can make it inferior. Accordingly, the Mn content is preferably 0.40 to 1.4%. The lower limit of the Mn content is more preferably 0.45%, even more preferably 0.50%. The upper limit of the Mn content is more preferably 1.2%, even more preferably 1.0%.

Al: 0.005~0.050%

Aluminum (Al) is an element added for deoxidation during steelmaking. If the Al content is less than 0.005%, there may be no deoxidation effect, and if it exceeds 0.050%, it reacts with oxygen (O) in the molten steel to form an oxide (inclusion) with a high melting point, and nozzle clogging may occur. The shape of the inclusion may be sharp, which may make the hole expandability inferior. Accordingly, the Al content is preferably 0.005 to 0.050%. The lower limit of the Al content is more preferably 0.010%, even more preferably 0.015%. The upper limit of the Al content is more preferably 0.045%, and even more preferably 0.040%.

Ca: 0.0005-0.0050%

Calcium (Ca) reacts with Al and O in molten steel _{to form spherical inclusions (12CaO·17Al 2} O ₃ ) having a low melting point, thereby preventing nozzle clogging and facilitating separation of inclusions. When the Ca content is less than 0.0005%, it is difficult to secure the above effect. On the other hand, when the Ca content is more than 0.0050%, high melting point inclusions are formed, which promotes nozzle clogging, and thus casting interruption may occur, and large inclusions (>50 μm) are formed, which may make hole expandability inferior. . Accordingly, the Ca content is preferably 0.0005 to 0.0050%. The lower limit of the Ca content is more preferably 0.0010%, and even more preferably 0.0015%. The upper limit of the Ca content is more preferably 0.0045%, and even more preferably 0.0040%.

Nb: 0.003~0.05%

Niobium (Nb) is a precipitate-forming element such as NbC and NbCN, and is an element that improves strength by strengthening precipitation and refining grains. When the Nb content is less than 0.003%, the above-described effect is insufficient. On the other hand, when the Nb content is more than 0.05%, precipitates such as NbC and NbCN are excessively formed, and the quality of the edge portion of the slab and/or bar may be inferior due to deterioration of high-temperature ductility. Therefore, the Nb content is preferably 0.003 to 0.05%. The lower limit of the Nb content is more preferably 0.006%, and even more preferably 0.01%. The upper limit of the Nb content is more preferably 0.04%, and even more preferably 0.03%.

The remaining component of the present invention is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in the normal manufacturing process, this cannot be excluded. Since these impurities are known to anyone skilled in the art of manufacturing processes, all details thereof are not specifically mentioned in the present specification.

On the other hand, in the hot-rolled steel sheet of the present invention, it is preferable that C, Mn, Nb, Al and Ca among the above alloy components satisfy the following Relations 1 to 3, respectively, and through this, the hole expandability targeted by the present invention is excellent. Hot-rolled steel sheet can be manufactured. However, the content units of C, Mn, Nb, Al and Ca described in the following Relations 1 to 3 are weight %.

[Relational Expression 1] 0.20 ≤ C+Mn/5+5Nb ≤ 0.40

The above relation 1 is a component relation for securing the strength desired by the present invention. When the value of C+Mn/5+5Nb is less than 0.20, it is difficult to secure the strength targeted by the present invention, and when it exceeds 0.40, the elongation is lowered and cracks may occur during processing. Accordingly, the value of C+Mn/5+5Nb preferably ranges from 0.20 to 0.40. As for the lower limit of the value of the said C+Mn/5+5Nb, it is more preferable that it is 0.22, and it is still more preferable that it is 0.24. As for the upper limit of the value of the said C+Mn/5+5Nb, it is more preferable that it is 0.39, and it is still more preferable that it is 0.37.

[Relational Expression 2] 3 ≤ Al/Ca ≤ 35

Relation 2 is a component relation for ensuring high hole expandability through inclusion control and preventing casting interruption due to nozzle clogging. In relation (2), when the Al/Ca value is less than 3, high-melting point inclusions having a high Ca content, such as _{CaO, 3CaOAl 2} O _{3, etc., are formed in the molten steel, and there is a high risk of nozzle clogging.} On the other hand, when the Al/Ca value exceeds 35, high melting point inclusions having a high Al content such as _{CaOAl 2} O ₃ , CaO ₂ Al ₂ O ₃ , CaO ₆ Al ₂ O ₃ , Al ₂ O _{3 etc.} Since these inclusions are not spherical, floating separation is disadvantageous, and casting may be interrupted due to nozzle clogging, and hole expandability may also be inferior. Therefore, in order to stably and high-speed cast a thin hot rolled material with high hole fixability, the Al/Ca value preferably satisfies the range of 3 to 35. The lower limit of the Al/Ca value is more preferably 4, still more preferably 5. As for the upper limit of the value of the said Al/Ca, it is more preferable that it is 30, and it is still more preferable that it is 25.

[Relation 3] 20 ≤ (Al/Ca)/(C+Mn/5+5Nb) ≤ 100

The above relation 3 is a component relation for securing the strength, high hole expandability, and stable high-speed castability that the present invention aims. When the (Al/Ca)/(C+Mn/5+5Nb) value is less than 20, it is difficult to secure the target elongation and hole expandability, and the high Ca content such as _{CaO, 3CaOAl 2} O _{3, etc. in the molten steel is high.} There is a high risk of nozzle clogging due to the formation of melting point inclusions. On the other hand, when the (Al/Ca)/(C+Mn/5+5Nb) value exceeds 100, CaOAl ₂ O ₃ , CaO ₂ Al ₂ O ₃ , CaO ₆ Al ₂ O ₃ , Al ₂ High melting point inclusions such as O ₃ are formed, but these inclusions are not spherical, so floating separation is disadvantageous, and casting may be stopped due to nozzle clogging, and hole expandability may also be poor. Therefore, in order to stably and high-speed cast a thin hot-rolled material with excellent target elongation and hole determination, it is necessary that the value of (Al/Ca)/(C+Mn/5+5Nb) satisfies the range of 20 to 100. desirable. As for the lower limit of the value of the said (Al/Ca)/(C+Mn/5+5Nb), it is more preferable that it is 22, and it is still more preferable that it is 25. As for the upper limit of the value of the said (Al/Ca)/(C+Mn/5+5Nb), it is more preferable that it is 90, It is more preferable that it is 80.

On the other hand, the hot-rolled steel sheet of the present invention may contain one or more selected from the group consisting of Si, P, S, N, Mg, Sn, Sb, Zn and Pb as a trap element in a total of 0.1 wt% or less. have. The tramp element is an impurity element originating from ferroalloy or scrap used as a raw material in the steelmaking process, ladle and tundish refractories, etc., and when the total exceeds 0.1% by weight, on the surface of the thin slab Cracks may occur, which may deteriorate the surface quality of the hot-rolled steel sheet.

The hot-rolled steel sheet of the present invention may have a microstructure in which ferrite is 93.0 to 98.5% and pearlite is 1.5 to 7.0% by area%. In addition, the microstructure may further include 5% or less of bainite. When the fraction of ferrite is less than 93.0%, it is difficult to secure the target elongation because the fraction of pearlite or bainite is relatively high, and when it exceeds 98.5%, it may be difficult to secure the target strength. In addition, when the fraction of pearlite is less than 1.5%, it is difficult to secure the target strength because the fraction of ferrite having relatively low strength is high, and when it exceeds 7.0%, it may be difficult to secure the target elongation. When the bainite content exceeds 5%, it may be difficult to secure a target elongation because the strength is increased.

It is preferable that the long axis/short axis ratio of the crystal grains of the ferrite be 5.0 or less. The long-axis/short-axis ratio of the grains shows the degree of isotropy of the grains, and the closer to the isotropy, the better in terms of securing high hole expandability. Therefore, in order to ensure high hole expandability, the long axis/short axis ratio of the crystal grains of the ferrite is preferably 5.0 or less, more preferably 4.5 or less, and still more preferably 4.0 or less.

In the microstructure of the hot-rolled steel sheet of the present invention, since ferrite is the main structure having a fraction of 93.0% or more, the grain size of the ferrite may affect the strength. The smaller the ferrite average grain size is, the more advantageous it is in terms of securing strength and hole expandability. However, in order to refine the ferrite average grain size to less than 1.0 μm, it is necessary to additionally input ferrous alloys such as Nb, Ti, Mo and V, which are effective for grain refinement, but this causes an increase in manufacturing cost, so that the average ferrite grain size is The grain size is preferably 1.0 μm or more. On the other hand, when the grain size of the ferrite exceeds 8.0 μm, it is difficult to secure strength, and the hole expandability may be significantly inferior. Therefore, the average grain size of the ferrite is preferably 1.0 ~ 8.0㎛. The lower limit of the average grain size of the ferrite is more preferably 1.5 μm, and even more preferably 2.0 μm. The upper limit of the average grain size of the ferrite is more preferably 7.5 μm, and even more preferably 7.0 μm.

On the other hand, as the number of fine ferrite grains increases, it is advantageous to secure high hole expandability. Therefore, in the ferrite, the fraction of grains having a grain size of 1 to 5 μm is preferably 25 area% or more, more preferably 27.5 area% or more, and 30 area % or more is more preferable.

The hot-rolled steel sheet of the present invention provided as described above may have a yield strength of preferably 350 MPa or more, more preferably 360 MPa or more, and even more preferably 370 MPa or more. In addition, preferably, it may have a tensile strength of 440 MPa or more, more preferably 450 MPa or more, and still more preferably 460 MPa or more. In addition, it may have an elongation of preferably 26% or more, more preferably 27% or more, and still more preferably 28% or more. In addition, the hot-rolled steel sheet of the present invention may have a hardness of 135 ~ 185Hv. It is more preferable that it is 140Hv, and, as for the lower limit of the said hardness, it is still more preferable that it is 145Hv. As for the upper limit of the said hardness, it is more preferable that it is 185Hv, and it is still more preferable that it is 180Hv. In addition, the hot-rolled steel sheet of the present invention may have a hole expansion ratio (HER) of 90% or more, more preferably 95% or more, and even more preferably 100% hole expansion ratio. In addition, the hot-rolled steel sheet of the present invention may have a three-point bending maximum angle of 120° or more, more preferably 125° or more, and even more preferably 130° or more. On the other hand, the three-point bending maximum angle refers to the bending of the material until a crack occurs in the material by placing the material on a die having holes formed at an arbitrary interval and pressing the material with a mold having a V shape at the end. means angle.

The hot-rolled steel sheet of the present invention may have a thickness of 0.6 to 2.3 mm, more preferably 0.7 to 2.2 mm, and even more preferably 0.9 to 2.0 mm.

Hereinafter, an embodiment of the method for manufacturing a hot-rolled steel sheet excellent in hole expandability of the present invention will be described.

1 is a schematic diagram of a facility for continuous casting-rolling direct connection process applicable to the production of hot-rolled steel sheet of the present invention. A hot-rolled steel sheet having excellent surface quality and small material variation according to an embodiment of the present invention can be produced by applying a continuous casting-rolling direct connection facility as shown in FIG. 1 . The continuous casting-rolling direct connection facility is largely composed of a continuous casting machine 100 , a roughing mill 400 , and a finishing mill 600 . The continuous casting-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 having a second thickness thinner than the first thickness for the thin slab. ) (b) a roughing mill 400 for rolling, a finishing mill 600 for rolling a bar having the second thickness into a strip (c) having a third thickness, and a winder 900 for winding the strip can do. In addition, in front of the roughing mill 400, the rough rolling scale breaker 300 (Roughing Mill Scale Breaker, hereinafter referred to as 'RSB') and the finishing mill 600 in front of the finishing rolling scale breaker 500 (Finishing Mill Scale Breaker, Hereinafter, also referred to as 'FSB'), it is easy to remove surface scale, so it is possible to produce PO (Pickled & Oiled) steel sheets and plated steel sheets with excellent surface quality when pickling hot-rolled steel sheets in the post process. In addition, the continuous casting-rolling direct connection process enables isothermal and constant velocity rolling, so the temperature deviation in the width/length direction of the steel sheet is remarkably low. It is possible to produce uniform ultra-high-strength hot-rolled steel sheet. The strip, which has been rolled and cooled in this way, is cut by the high-speed shearing machine 800 , and wound up by the winder 900 to produce a product. On the other hand, in front of the finish rolling scale breaker 500 may be provided with a heater 200 for additionally heating the bar.

Figure 2 is another schematic view of the equipment for the continuous casting-rolling direct connection process applicable to the production of the hot-rolled steel sheet of the present invention. The continuous casting-rolling direct connection facility disclosed in FIG. 2 has the same configuration as the facility disclosed in FIG. 1 , but a heater 200 ′ for additionally heating the slab in front of the rough rolling mill 400 and the rough rolling scale breaker 300 is provided. As a result, it is easy to secure the temperature of the slab edge part, and the occurrence of edge part defects is lowered, which is advantageous for securing the surface quality. In addition, since a space equal to the length of one or more slabs is secured before the roughing mill, batch rolling is also possible.

First, a thin slab is obtained by continuously casting molten steel having the aforementioned alloy composition. During the continuous casting, the average casting speed may be 4.5 to 7.5mpm (m/min). The reason why the casting speed is set to 4.5mpm or higher is that high-speed casting and rolling process are connected, and a casting speed of a certain level or higher is required to secure the target rolling temperature. However, if the casting speed is slow, there is a risk of segregation from the cast slab, and when such segregation occurs, it is difficult to secure strength and bending properties, and the risk of material deviation in the width direction or the length direction increases. If it exceeds 7.5mpm, since the operation success rate may be reduced due to instability of the molten steel molten steel, the casting speed is preferably in the range of 4.5 to 7.5mpm. The lower limit of the casting speed is more preferably 5.0mpm, even more preferably 5.5mpm. The upper limit of the casting speed is more preferably 7.0mpm, and the upper limit is more preferably 6.5mpm.

The thin slab may have a thickness of 75 to 120 mm. When the thickness of the thin slab exceeds 120 mm, not only high-speed casting is difficult, but also the rolling load increases during rough rolling. In order to solve this problem, it is possible to additionally install a heating facility, but since this becomes a factor to improve the production cost, it is preferable to exclude it as much as possible. Therefore, the thickness of the thin slab is preferably controlled to 75 ~ 120mm. The lower limit of the thickness of the thin slab is more preferably 80 mm, even more preferably 85 mm. The upper limit of the thickness of the thin slab is more preferably 115mm, even more preferably 110mm, and most preferably 100mm.

Then, the thin slab from which the scale is removed is rough rolled (Rough rolling Mill, RM) to obtain a bar. The rough rolling step may be performed by rough rolling the continuously cast thin slab in a rough rolling mill consisting of 2 to 5 rolling mills.

During the rough rolling, the reduction ratio in the last rolling mill may be 22 to 50%. When the reduction ratio is less than 22%, static recrystallization does not occur at high temperature, and it is difficult to secure high hole expandability due to the coarse grain size in the final product. This is so severe that cracks may occur. Therefore, it is preferable that the reduction ratio in the last rolling mill during the rough rolling is 22 to 50%. The lower limit of the reduction ratio in the last rolling mill during the rough rolling is more preferably 24%, more preferably 26%. The upper limit of the reduction ratio is more preferably 48%, still more preferably 46%.

It is preferable that the exit temperature in the last rolling mill during the rough rolling is 960 ~ 1200 ℃. When the exit temperature in the last rolling mill during rough rolling is less than 960° C., static recrystallization may not sufficiently occur, and the edge portion temperature of the bar is low, so precipitates such as NbC, NbCN, and AlN are excessively precipitated. cracks may occur. When the exit temperature of the last rolling mill exceeds 1200° C. during rough rolling, a large amount of scale may be generated on the surface of the bar, resulting in poor surface quality. Therefore, it is preferable that the exit temperature in the last rolling mill during the rough rolling is 960 ~ 1200 ℃. The lower limit of the exit temperature in the last rolling mill during the rough rolling is more preferably 980 ℃, even more preferably 1000 ℃. The upper limit of the exit temperature in the last rolling mill during the rough rolling is more preferably 1180 ℃, even more preferably 1160 ℃.

The bar obtained through the rough rolling preferably has a thickness of 10 to 24 mm. If the thickness of the bar is less than 10mm, plate breakage may occur due to an increase in rolling load during finish rolling due to a drop in surface temperature, and if it exceeds 24mm, it may be difficult to manufacture a thin hot-rolled steel sheet of 2.3mm or less. Therefore, it is preferable to control the thickness of the bar to be 10-24mm. The lower limit of the thickness of the bar is more preferably 12 mm, even more preferably 14 mm. The upper limit of the thickness of the bar is more preferably 22 mm, even more preferably 20 mm.

Finish rolling the bar so that the exit temperature of the finish rolling is 740 ~ 870 ℃ (Finishing) to obtain a hot-rolled steel sheet;

Thereafter, the bar is finish-rolled by a rolling mill (FM) to obtain a hot-rolled steel sheet. The finish rolling can be performed, for example, in a finish rolling mill comprising 3 to 6 stands. During the finish rolling, it is preferable to finish rolling the bar so that the finish rolling exit temperature is 740 to 870° C. to obtain a hot-rolled steel sheet. When the finish rolling exit temperature is less than 740° C., the austenite fraction during finish rolling is low, so that the pearlite structure to be obtained is not sufficiently obtained, and thus it is difficult to secure the target strength. If the finish rolling exit temperature exceeds 870 ℃, the grains may become coarse and high strength may not be obtained, and the bending properties may be inferior. Therefore, it is preferable that the exit temperature during the finish rolling is 740 ~ 870 ℃. The lower limit of the finish rolling exit temperature is more preferably 750°C, and even more preferably 760°C. The upper limit of the finish rolling exit temperature is more preferably 860°C, and even more preferably 850°C.

On the other hand, in the finish rolling of the bar, it is preferable to control the reduction ratio and temperature during the finish rolling because the reduction rate and temperature affect the dynamic recrystallization and affect the grain size of the final product. At the time of finish rolling with a finishing mill consisting of 3 to 6 stands, the rolling reduction in the intermediate rolling mill is preferably 16 to 44%. The reason for controlling the rolling reduction in the intermediate rolling mill is that when the bar is deformed at a temperature directly above the phase transformation temperature (Ar3), it is possible to further promote the ferrite phase transformation to form finer ferrite grains. When the rolling reduction in the intermediate rolling mill during the finish rolling is less than 16%, the deformation amount is small and dynamic recrystallization may not occur well. have. Therefore, it is preferable that the rolling reduction in the intermediate rolling mill during the finish rolling is 16 to 44%. The lower limit of the rolling reduction in the intermediate rolling mill during the finish rolling is more preferably 18%, and even more preferably 20%. The upper limit of the rolling reduction in the intermediate rolling mill during the finish rolling is more preferably 42%, more preferably 40%. On the other hand, the intermediate rolling mill referred to in the present invention means rolling mills located between the first rolling mill and the last rolling mill except for the first and last mills in the finishing mill consisting of 3 to 6 stands, and more preferably, when rolling, the bar is Ar3 It means a rolling mill that is rolled at the temperature closest to the direct phase. On the other hand, since the temperature of the bar during the rolling can be measured through a temperature sensor or the like, a person skilled in the art can designate an intermediate temperature device without difficulty.

It is preferable that the reduction ratio in the final rolling mill during the finish rolling is 6 to 26%. When the rolling reduction in the last rolling mill during the finish rolling is less than 6%, the deformation amount is small and the dynamic recrystallization effect is small. Therefore, it is preferable that the reduction ratio in the last rolling mill during the finish rolling is 6 to 26%. The lower limit of the reduction ratio in the final rolling mill during the finish rolling is more preferably 8%, and even more preferably 10%. The upper limit of the reduction ratio in the final rolling mill during the finish rolling is more preferably 24%, more preferably 22%.

It is preferable that the average sheet-threading speed in the final rolling mill during the finish rolling is 150 to 550mpm. During the finish rolling, the sheet-threading speed in the final rolling mill may be directly related to the casting speed and the thickness of the final hot-rolled steel sheet. When the average sheet-threading speed in the last rolling mill is more than 550mpm, an operation accident such as plate breakage may occur, and uniform temperature is not ensured due to isothermal and constant velocity rolling difficult, resulting in material and thickness deviation. On the other hand, in the case of less than 150mpm, the rolling speed is too slow, and there is a problem in the mass balance and the heat balance, and it may be difficult to perform continuous rolling. Therefore, it is preferable that the average sheet-threading speed in the final rolling mill during the finish rolling is 150 to 550 mpm (m/min). The lower limit of the average sheet-threading speed in the final rolling mill during the finish rolling is more preferably 200mpm (m/min), and even more preferably 250mpm (m/min). The upper limit of the average sheet-threading speed in the final rolling mill during the finish rolling is more preferably 500mpm (m/min), and even more preferably 450mpm (m/min). It is preferable that the deviation of the sheet-threading speed in manufacturing one strip during the finish rolling is 25% or less.

Thereafter, the hot-rolled steel sheet is air-cooled for 1.5 to 6.0 seconds, then cooled to 560 to 690° C. at a cooling rate of 20 to 80° C./sec, and then wound. The air cooling time until the strip manufactured by the finish rolling immediately before ROT (Run out Table) cooling is the time required for recrystallization of the microstructure, and it is preferable to control the time to 1.5 to 6.0 seconds or less. If the air cooling time is less than 1.5 seconds, the time required for recrystallization is not sufficient, so it is difficult to obtain an isotropic structure and thus the hole expandability may be inferior. The fraction of pearlite that affects Therefore, the air cooling time is preferably 1.5 to 6.0 seconds. The lower limit of the air cooling time is more preferably 2.0 seconds, and still more preferably 2.5 seconds. The upper limit of the air cooling time is more preferably 5.5 seconds, and still more preferably 5.0 seconds.

When the cooling rate is less than 20 °C/sec, the cooling rate is too slow to satisfy the target pearlite fraction, so the strength may be low, and when the cooling rate exceeds 80 °C/sec, the fraction of bainite and pearlite As this increases, it may be difficult to secure a target elongation. Therefore, the cooling rate is preferably 20 ~ 80 ℃ / sec. The lower limit of the cooling rate is more preferably 25°C/sec, and even more preferably 30°C/sec. The upper limit of the cooling rate is more preferably 75°C/sec, and even more preferably 70°C/sec.

If the coiling temperature is less than 560 ℃, pearlite and bainite transformation is excessively promoted, and it may be difficult to secure a target elongation. It may be inferior, and the grains may be coarse and the hole expandability may be inferior. Therefore, the coiling temperature is preferably 560 ~ 690 ℃. The lower limit of the coiling temperature is more preferably 570°C, and even more preferably 580°C. The upper limit of the coiling temperature is more preferably 680°C, and even more preferably 670°C.

Meanwhile, after the winding step, the step of pickling the wound hot-rolled steel sheet may be further included, and after the acid treatment step, the step of plating the acid-treated hot-rolled steel sheet may be further included. PO (Pickled & Oiled) material and plating material can be obtained through the pickling and plating treatment. In the present invention, since scale can be sufficiently removed in the thin slab and bar scale removal step, PO material and plating material having excellent surface quality can be obtained even with general pickling treatment and plating treatment. Therefore, in the present invention, any method generally used in the hot-rolling pickling process and the plating process is applicable, so the pickling process and the plating method are not particularly limited.

Hereinafter, the present invention will be described in more detail through examples. However, it is necessary to note that the following examples are only intended to illustrate 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 matters reasonably inferred therefrom.

(Example 1)

After preparing molten steel having an alloy composition shown in Table 1, a hot-rolled steel sheet having a thickness of 1.6 mm was manufactured under the manufacturing conditions shown in Tables 2 and 3 by applying a casting-rolling direct connection process. After the hot-rolled steel sheet was pickled to obtain a PO material, the microstructure and mechanical properties were measured, and the results are shown in Tables 4 and 5 below.

Microstructure (ferrite, pearlite, and bainite fractions, ferrite grain size, ferrite major/short axis ratio) was obtained by randomly photographing 5 locations at 1/4t of the thickness of the specimen at a magnification of 500 times using an optical microscope, and then Image- Measurements were made using Plus Pro software and then reported as average values.

Tensile properties (yield strength (YS), tensile strength (TS), and elongation (EL)) were measured by taking a specimen in the rolling direction (L direction) and processing it into a tensile specimen according to JIS No. . Specimens were sampled at regular intervals from 7 places in the width direction of the strip.

Hole Expansion Ratio (HER, Hole Expansion Ratio) is determined by punching a hole with a diameter of 10.8 mm in the specimen and pushing a cone through the hole to determine the diameter of the expanded hole immediately before cracks occur in the circumferential part of the hole. It was expressed as an average value calculated as a percentage of the diameter (10.8 mm). Specimens were sampled at regular intervals from 5 places in the width direction of the strip.

The hardness was measured 5 times by applying a load of 500 gf to 1/4 t (t=thickness) using a Vickers hardness machine and described as an average value.

The maximum three-point bending angle was measured as an average value after three measurements for each specimen in accordance with the standard of VDA (Verband Der Automobilindustrie).

Kang type No. Alloy composition (wt%) C Mn Al Ca Nb Equation 1 Equation 2 Equation 3 Invention lecture 1 0.061 0.82 0.031 0.0025 0.015 0.30 12.4 41.3 Invention lecture 2 0.06 0.79 0.035 0.002 0.016 0.30 17.5 58.7 Invention lecture 3 0.07 0.80 0.035 0.002 0.014 0.30 17.5 58.3 Invention lecture 4 0.074 0.87 0.025 0.0025 0.018 0.34 10.0 29.6 Invention River 5 0.06 0.89 0.031 0.0029 0.015 0.31 10.7 34.2 Invention lecture 6 0.059 0.81 0.031 0.0029 0.014 0.29 10.7 36.7 Invention Lesson 7 0.06 0.82 0.031 0.0021 0.015 0.30 14.8 49.4 Invention lecture 8 0.06 0.82 0.034 0.0028 0.015 0.30 12.1 40.6 Comparative lecture 1 0.035 0.45 0.033 0.0029 0.013 0.19 11.4 59.9 Comparative lecture 2 0.045 0.1 0.025 0.0025 0.018 0.16 10.0 64.5 Comparative lecture 3 0.05 0.5 0.031 0.0028 0.001 0.16 11.1 71.4 Comparative lecture 4 0.071 1.0 0.085 0.0011 0.015 0.35 77.3 223.3 Comparative steel 5 0.078 1.15 0.065 0.0015 0.016 0.39 43.3 111.7 Comparative lecture 6 0.081 0.95 0.032 0.0004 0.017 0.36 80.0 224.7 [Equation 1] C+Mn/5+5Nb
[Formula 2] Al/Ca
[Formula 3] (Al/Ca)/(C+Mn/5+5Nb)

division Kang type No. Park Slav
thickness
(mm) casting speed
(mpm) rough rolling bar thickness
(mm) Last rolling mill rolling reduction (%) Last rolling mill exit temperature (℃) Invention Example 1 Invention lecture 1 90 6.5 36 1100 16 Invention Example 2 Invention lecture 2 90 6.5 36 1095 16 Invention example 3 Invention lecture 3 90 6.5 36 1090 16 Invention Example 4 Invention lecture 4 90 6.5 36 1105 16 Invention Example 5 Invention River 5 90 6.5 36 1106 16 Invention example 6 Invention lecture 6 90 6.5 36 1110 16 Invention Example 7 Invention Lesson 7 90 6.5 36 1101 16 Invention Example 8 Invention lecture 8 90 6.5 36 1098 16 Comparative Example 1 Comparative lecture 1 90 6.5 36 1106 16 Comparative Example 2 Comparative lecture 2 90 6.5 36 1099 16 Comparative Example 3 Comparative lecture 3 90 6.5 36 1105 16 Comparative Example 4 Comparative lecture 4 90 6.5 36 1109 16 Comparative Example 5 Comparative steel 5 90 6.5 36 1098 16 Comparative Example 6 Comparative lecture 6 90 6.5 36 1105 16

division Kang type No. finish rolling hot rolled steel
thickness
(mm) air cooling time
(candle) cooling rate
(℃/sec) winding temperature
(℃) middle
rolling mill rolling reduction
(%) Last rolling mill rolling reduction
(%) Last rolling mill exit temperature
(℃) Last rolling mill plate speed
(mpm) Invention Example 1 Invention lecture 1 30 21 820 349 1.6 3.6 50 607 Invention Example 2 Invention lecture 2 30 21 825 352 1.6 3.6 50 600 Invention example 3 Invention lecture 3 30 21 821 358 1.6 3.6 50 605 Invention Example 4 Invention lecture 4 30 21 819 355 1.6 3.6 50 610 Invention Example 5 Invention River 5 30 21 818 353 1.6 3.6 50 607 Invention example 6 Invention lecture 6 30 21 827 356 1.6 3.6 50 609 Invention Example 7 Invention Lesson 7 30 21 831 358 1.6 3.6 50 608 Invention Example 8 Invention lecture 8 30 21 835 354 1.6 3.6 50 607 Comparative Example 1 Comparative lecture 1 30 21 821 362 1.6 3.6 40 638 Comparative Example 2 Comparative lecture 2 30 21 829 357 1.6 3.6 40 635 Comparative Example 3 Comparative lecture 3 30 21 827 356 1.6 3.6 50 609 Comparative Example 4 Comparative lecture 4 30 21 819 350 1.6 3.6 50 610 Comparative Example 5 Comparative steel 5 30 21 820 351 1.6 3.6 50 620 Comparative Example 6 Comparative lecture 6 30 21 819 359 1.6 3.6 50 620

division Kang type No. Nozzle
catch
Whether Microstructure (area%) Ferrite average
grain
size
(μm) Fraction of ferrite grains with a grain size of 1 to 5 μm (area%) ferrite grains
Long/Short Ratio ferrite pearl light bainite Invention Example 1 Invention lecture 1 × 94.9 4.6 0.5 5.3 37.2 2.5 Invention Example 2 Invention lecture 2 × 94.0 4.8 1.2 5.5 38.5 2.6 Invention example 3 Invention lecture 3 × 94.5 5.1 0.4 5.7 37.1 2.7 Invention Example 4 Invention lecture 4 × 95.1 4.9 0 5.2 37.8 2.9 Invention Example 5 Invention River 5 × 94.8 5.2 0 5.3 37.9 2.4 Invention example 6 Invention lecture 6 × 94.7 5.3 0 5.7 36.9 2.0 Invention Example 7 Invention Lesson 7 × 95.2 4.8 0 5.5 37.9 2.1 Invention Example 8 Invention lecture 8 × 95.3 4.7 0 5.2 39.5 2.5 Comparative Example 1 Comparative lecture 1 × 98.9 1.1 0 6.8 28.3 2.8 Comparative Example 2 Comparative lecture 2 × 99.1 0.9 0 7.1 26.8 3.1 Comparative Example 3 Comparative lecture 3 × 97.5 2.5 0 8.5 13.7 4.1 Comparative Example 4 Comparative lecture 4 ○ 94.2 5.8 0 5.6 41.2 3. Comparative Example 5 Comparative steel 5 ○ 93.8 6.2 0 5.1 42.3 13.0 Comparative Example 6 Comparative lecture 6 ○ 95.7 4.3 0 6.0 40.2 3.4

division Kang type No. yield strength
(MPa) tensile strength
(MPa) elongation
(%) Hardness
(0.5Kgf, Hv) hole expansion rate
(%) 3 point bend
maximum angle
(°) Invention Example 1 Invention lecture 1 393 481 29.0 161 128 135 Invention Example 2 Invention lecture 2 391 487 29.2 164 131 135 Invention example 3 Invention lecture 3 390 489 29.1 167 129 137 Invention Example 4 Invention lecture 4 389 481 28.9 162 127 138 Invention Example 5 Invention River 5 399 491 29.1 167 125 139 Invention example 6 Invention lecture 6 391 490 29.5 159 129 140 Invention Example 7 Invention Lesson 7 390 494 29.6 161 128 141 Invention Example 8 Invention lecture 8 391 497 29.0 159 128 143 Comparative Example 1 Comparative lecture 1 345 428 35.8 131 131 142 Comparative Example 2 Comparative lecture 2 314 415 36.2 129 135 145 Comparative Example 3 Comparative lecture 3 325 420 36.0 141 80 130 Comparative Example 4 Comparative lecture 4 410 520 27.1 175 65 118 Comparative Example 5 Comparative steel 5 412 524 26.5 178 72 125 Comparative Example 6 Comparative lecture 6 391 491 28.5 167 86 127

As shown in Tables 1 to 5, Inventive Examples 1 to 8, which satisfy both the alloy composition, Relational Expressions 1 to 3, and the manufacturing conditions proposed by the present invention, secure the microstructure targeted by the present invention, and the present invention It can be seen that the desired mechanical properties such as tensile properties and hole expandability are secured. On the other hand, Comparative Examples 1 to 6, which satisfy the manufacturing conditions proposed by the present invention but do not satisfy the alloy composition and Relations 1 to 3, do not satisfy the microstructure targeted by the present invention, or have tensile properties and hole expandability. It can be seen that they are not satisfied. In particular, in the case of Comparative Examples 4 to 6, it can be seen that the nozzle clogging occurred during continuous casting as the Relations 2 and 3 were not satisfied.

3 is a graph showing the relationship of Relations 1 to 3, wherein the invention steels indicated in the invention area represent Inventive Examples 1 to 8, and Comparative Steels represent Comparative Examples 1 to 6. As can be seen from FIG. 3, it can be seen that the relational expressions 1 to 3 of the present invention must be satisfied in order to obtain the tensile properties and hole expandability targeted in the present invention.

4 is a photograph of the inclusion of Inventive Example 1 observed by SEM, and FIG. 5 is a photograph of the inclusion of Comparative Example 4 observed by SEM. As can be seen from FIGS. 4 and 5 , In Inventive Example 1 satisfying the Ca and Al contents and Relations 2 and 3 proposed by the present invention, a spherical Al-Ca-O-based inclusion is formed, but Comparative Example 4 is Al It can be seen that prismatic and irregular inclusions are formed as the content and Relations 2 and 3 are not satisfied.

(Example 2)

In order to examine the effect of manufacturing conditions on mechanical properties, molten steel having the alloy composition of Inventive Steel 1 described in Table 1 was prepared, and then a casting-rolling direct connection process was applied to 1.6 mm under the manufacturing conditions shown in Tables 6 and 7 below. A thick hot-rolled steel sheet was manufactured. After the hot-rolled steel sheet was pickled to obtain a PO material, the microstructure and tensile properties were measured, and the results are shown in Tables 8 and 9 below. The microstructure and mechanical properties were measured in the same manner as in Example 1. On the other hand, the Ar3 temperature of the invention steel was 865 ℃.

division Kang type No. Park Slav
thickness
(mm) casting speed
(mpm) rough rolling bar thickness
(mm) Last rolling mill rolling reduction (%) Last rolling mill exit temperature (℃) Invention Example 9 Invention lecture 1 90 6.5 36 1120 16 Invention example 10 Invention lecture 1 90 6.5 36 1130 16 Invention Example 11 Invention lecture 1 90 6.1 36 1060 17 Invention example 12 Invention lecture 1 90 6.5 38 1089 19 Comparative Example 7 Invention lecture 1 90 6.5 15 1107 22 Comparative Example 8 Invention lecture 1 90 6.5 36 950 16 Comparative Example 9 Invention lecture 1 90 6.5 38 1106 16 Comparative Example 10 Invention lecture 1 90 6.5 36 1099 12 Comparative Example 11 Invention lecture 1 90 6.5 40 1108 15 Comparative Example 12 Invention lecture 1 90 6.5 36 1099 16 Comparative Example 13 Invention lecture 1 90 6.5 36 1106 16 Comparative Example 14 Invention lecture 1 90 6.5 36 1109 16 Comparative Example 15 Invention lecture 1 90 6.5 38 1098 16 Comparative Example 16 Invention lecture 1 90 6.5 36 1109 16 Comparative Example 17 Invention lecture 1 90 6.5 36 1099 16 Comparative Example 18 Invention lecture 1 90 6.5 30 1085 18

division Kang type No. finish rolling hot rolled steel
thickness
(mm) air cooling time
(candle) cooling rate
(℃/sec) winding temperature
(℃) middle
rolling mill rolling reduction
(%) Last rolling mill rolling reduction
(%) Last rolling mill exit temperature
(℃) Last rolling mill plate speed
(mpm) Invention Example 9 Invention lecture 1 32 25 820 352 1.6 3.5 50 585 Invention example 10 Invention lecture 1 32 25 829 356 1.6 3.6 50 675 Invention Example 11 Invention lecture 1 34 20 825 349 1.6 3.8 50 607 Invention example 12 Invention lecture 1 25 18 821 352 1.6 2.5 50 608 Comparative Example 7 Invention lecture 1 25 15 820 350 2.0 2.8 50 609 Comparative Example 8 Invention lecture 1 40 25 828 325 1.6 4.0 55 601 Comparative Example 9 Invention lecture 1 15 20 831 352 1.6 3.6 50 599 Comparative Example 10 Invention lecture 1 30 5 835 353 1.6 3.5 50 600 Comparative Example 11 Invention lecture 1 30 20 730 362 1.6 2.5 40 590 Comparative Example 12 Invention lecture 1 20 10 890 357 2.2 3.5 35 651 Comparative Example 13 Invention lecture 1 30 20 827 356 1.6 1.0 50 611 Comparative Example 14 Invention lecture 1 28 18 821 420 1.4 1.2 50 610 Comparative Example 15 Invention lecture 1 30 20 820 351 1.6 3.5 10 700 Comparative Example 16 Invention lecture 1 30 20 815 359 1.6 3.5 10 687 Comparative Example 17 Invention lecture 1 30 20 819 359 1.6 3.5 65 545 Comparative Example 18 Invention lecture 1 28 18 835 345 2.0 3.5 105 565

division Kang type No. Microstructure (area%) ferrite
average
grain
Size (㎛) Fraction of ferrite grains with a grain size of 1 to 5 μm
(area%) ferrite grains
Long/Short Ratio ferrite pearl light bainite Invention Example 9 Invention lecture 1 93.5 5.4 1.1 4.3 40.8 2.7 Invention example 10 Invention lecture 1 97.1 2.9 0 7.2 26.5 3.2 Invention Example 11 Invention lecture 1 95.2 4.3 0.5 5.8 37.9 2.9 Invention example 12 Invention lecture 1 94.9 5.1 0 5.1 38.2 2.8 Comparative Example 7 Invention lecture 1 94.4 5.6 0 6.8 30.1 5.5 Comparative Example 8 Invention lecture 1 94.7 5.3 0 6.2 32.5 5.3 Comparative Example 9 Invention lecture 1 95.1 4.9 0 6 31.1 5.8 Comparative Example 10 Invention lecture 1 93.8 6.2 0 4.8 40.1 5.2 Comparative Example 11 Invention lecture 1 94.0 4.5 1.5 5 30.9 5.3 Comparative Example 12 Invention lecture 1 98.9 1.1 0 8.5 23.1 4.8 Comparative Example 13 Invention lecture 1 95.2 4.8 0 5.8 34.9 5.8 Comparative Example 14 Invention lecture 1 95.4 4.6 0 5.9 35.2 5.5 Comparative Example 15 Invention lecture 1 99.1 0.9 0 6.2 24.5 3.2 Comparative Example 16 Invention lecture 1 98.9 1.1 0 6.1 25.5 3.1 Comparative Example 17 Invention lecture 1 87.5 7.2 5.3 4.5 41.2 2.8 Comparative Example 18 Invention lecture 1 82.9 5.6 11.5 4.3 45.5 2.7

division Kang type No. yield strength
(MPa) tensile strength
(MPa) elongation
(%) hole expansion rate
(%) 3 point bend
maximum angle
(°) Invention Example 9 Invention lecture 1 420 500 27.5 137 129 Invention example 10 Invention lecture 1 373 463 35.2 95 145 Invention Example 11 Invention lecture 1 387 479 29.5 131 138 Invention example 12 Invention lecture 1 385 483 28.6 128 141 Comparative Example 7 Invention lecture 1 371 467 31.2 82 130 Comparative Example 8 Invention lecture 1 385 471 30.5 86 129 Comparative Example 9 Invention lecture 1 382 481 29.5 80 128 Comparative Example 10 Invention lecture 1 414 490 28.6 89 131 Comparative Example 11 Invention lecture 1 398 480 28.5 89 130 Comparative Example 12 Invention lecture 1 345 436 34.5 85 139 Comparative Example 13 Invention lecture 1 405 491 27 86 136 Comparative Example 14 Invention lecture 1 400 485 27.9 88 138 Comparative Example 15 Invention lecture 1 347 431 35.1 95 135 Comparative Example 16 Invention lecture 1 348 435 35.1 96 137 Comparative Example 17 Invention lecture 1 420 500 24.5 141 135 Comparative Example 18 Invention lecture 1 441 520 22.5 145 138

As shown in Tables 6 to 9, the alloy compositions proposed by the present invention, Inventive Examples 9 to 12 satisfying all of Relations 1 to 3 and manufacturing conditions, secure the microstructure targeted by the present invention, and the present invention It can be seen that the desired mechanical properties such as tensile properties and hole expandability are secured. On the other hand, the alloy composition and relational formulas 1 to 3 proposed by the present invention are satisfied, but Comparative Examples 7 to 18, which do not satisfy the manufacturing conditions, do not satisfy the microstructure targeted by the present invention, or have tensile properties and hole expandability. It can be seen that they are not satisfied.

6 shows the ferrite grain size distribution measured using EBSD in Inventive Example 9. As can be seen from FIG. 6 , in the case of Inventive Example 9, it can be seen that the fraction of crystal grains having a grain size of 5 to 6 μm of ferrite is the highest, and the fraction of crystal grains having a grain size of 1 to 5 μm of ferrite is 25% It can be seen that the above is satisfied.

a: slab b: bar
c: strip
100: continuous casting machine 200, 200': heater
300: RSB (Roughing Mill Scale Breaker, rough rolling scale breaker)
400: rough rolling mill
500: FSB (Fishing Mill Scale Breaker, finish rolling scale breaker)
600: finishing mill 700: run-out table
800: high-speed shear 900: winder

Claims (19)

By weight, C: 0.045-0.10%, Mn: 0.40-1.4%, Al: 0.005-0.050%, Ca: 0.0005-0.0050%, Nb: 0.003-0.05%, the balance including Fe and other unavoidable impurities,
It satisfies the following Relations 1 to 3,
In terms of area%, ferrite is 93.0 to 98.5%, pearlite is 1.5 to 7.0%, and bainite contains a microstructure of 5% or less,
A hot-rolled steel sheet having excellent hole expandability in which the long/short axis ratio of the ferrite grains is 5.0 or less.
[Relational Expression 1] 0.20 ≤ C+Mn/5+5Nb ≤ 0.40
[Relational Expression 2] 3 ≤ Al/Ca ≤ 35
[Relation 3] 20 ≤ (Al/Ca)/(C+Mn/5+5Nb) ≤ 100
(However, in Relations 1 to 3, the unit of content of C, Mn, Nb, Al and Ca is weight %.)
The method according to claim 1,
The hot-rolled steel sheet has excellent hole expandability including at least one selected from the group consisting of Si, P, S, N, Mg, Sn, Sb, Zn and Pb as a trap element in a total of 0.1 wt% or less hot rolled steel sheet.
delete The method according to claim 1,
The average grain size of the ferrite is a hot-rolled steel sheet with excellent hole expandability of 1.0 ~ 8.0㎛.
The method according to claim 1,
The ferrite is a hot-rolled steel sheet with excellent hole expandability having a crystal grain size of 1 to 5 μm and a fraction of crystal grains of 25 area% or more.
The method according to claim 1,
The hot-rolled steel sheet has excellent hole expandability with yield strength: 350 MPa or more, tensile strength: 440 MPa or more, elongation: 26% or more, hardness: 135 to 185 Hv, hole expansion rate: 90% or more, three-point bending maximum angle: 120° or more hot rolled steel sheet.
The method according to claim 1,
The hot-rolled steel sheet is a hot-rolled steel sheet excellent in hole expandability having a thickness of 0.6 to 2.3 mm.
In wt%, C: 0.045 to 0.10%, Mn: 0.40 to 1.4%, Al: 0.005 to 0.050%, Ca: 0.0005 to 0.0050%, Nb: 0.003 to 0.05%, the remainder including Fe and other unavoidable impurities, and the following obtaining a thin slab by continuously casting molten steel satisfying Relations 1 to 3;
roughing the thin slab to obtain a bar;
Finish rolling the bar so that the exit temperature of the finish rolling is 740 ~ 870 ℃ to obtain a hot-rolled steel sheet;
After air-cooling the hot-rolled steel sheet for 1.5 to 6.0 seconds, and then cooling to 560 to 690 ° C at a cooling rate of 20 to 80 ° C / sec, and winding,
The method of manufacturing a hot-rolled steel sheet excellent in hole expandability, characterized in that each step is performed continuously.
[Relational Expression 1] 0.20 ≤ C+Mn/5+5Nb ≤ 0.40
[Relational Expression 2] 3 ≤ Al/Ca ≤ 35
[Relational Expression 3] 20 ≤ (Al/Ca)/(C+Mn/5+5Nb) ≤ 100
(However, in Relations 1 to 3, the content units of C, Mn, Nb, Al and Ca are weight %.)
9. The method of claim 8,
The molten steel is a hot rolled having excellent hole expandability including at least one selected from the group consisting of Si, P, S, N, Mg, Sn, Sb, Zn and Pb as a trap element in a total of 0.1 wt% or less A method for manufacturing a steel plate.
9. The method of claim 8,
During the continuous casting, the casting speed is 4.5 ~ 7.5mpm (m / min) method of manufacturing a hot-rolled steel sheet excellent in hole expandability.
9. The method of claim 8,
The thin slab is a method of manufacturing a hot-rolled steel sheet having excellent hole expandability having a thickness of 75 to 120 mm.
9. The method of claim 8,
A method of manufacturing a hot-rolled steel sheet having excellent hole expandability in which the reduction ratio in the final rolling mill during the rough rolling is 22 to 50%.
9. The method of claim 8,
A method of manufacturing a hot-rolled steel sheet having excellent hole expandability, wherein the exit temperature at the last rolling mill during the rough rolling is 960 to 1200°C.
9. The method of claim 8,
The bar is a method of manufacturing a hot-rolled steel sheet excellent in hole expandability having a thickness of 10 to 24 mm.
9. The method of claim 8,
A method of manufacturing a hot-rolled steel sheet having excellent hole expandability in which the reduction ratio in the intermediate rolling mill during the finish rolling is 16 to 44%.
9. The method of claim 8,
A method of manufacturing a hot-rolled steel sheet having excellent hole expandability in which the reduction ratio in the final rolling mill during the finish rolling is 6 to 26%.
9. The method of claim 8,
A method of manufacturing a hot-rolled steel sheet excellent in hole expandability, wherein the average sheet-threading speed in the final rolling mill during the finish rolling is 150 to 550 mpm
9. The method of claim 8,
After the winding, the method of manufacturing a hot-rolled steel sheet excellent in hole expandability further comprising the step of pickling the hot-rolled steel sheet.
19. The method of claim 18,
After the pickling treatment, the method for producing a hot-rolled steel sheet excellent in hole expandability further comprising the step of plating the pickling-treated hot-rolled steel sheet to obtain a plating material.

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