JP7186291B2 - Hot-rolled steel sheet and its manufacturing method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a hot-rolled steel sheet and a manufacturing method thereof, and more particularly, to a hot-rolled steel sheet manufactured using a continuous rolling mode in a continuous casting-rolling direct process and a manufacturing method thereof.

Due to the tightening of international environmental regulations and tightening of automobile fuel efficiency regulations, it is necessary to increase the strength and reduce the weight of automobile bodies. Ultra-high-strength hot-rolled steel sheets, which are used for bumper reinforcements and door impact beams, which are the body reinforcements of most automobiles, require high strength and are roll-forming. Excellent bending workability and little material variation are required. In order to satisfy these physical properties, steel sheets for automobile structural members are basically composed of a combination of ferrite, bainite, martensite and tempered martensite phases. It is classified into DP (Dual Phase) steel, TRIP (Transformation Induced Plasticity) steel, Complex Phase steel, MART steel, etc. according to the composition ratio of these phases.

This kind of steel is mainly applied to parts that require high energy absorption capacity in the event of a vehicle collision, such as members, pillars, bumper reinforcement materials, and sill sides. In addition to the tensile strength of , it must have a high elongation. However, it is difficult to avoid the reduction in elongation associated with ensuring ultra-high strength in such steels. Since it is necessary to go through a new process such as HPF (Hot Press Forming), which is the processing process used, there is a drawback that the manufacturing cost increases.
On the other hand, much research and development has been carried out to provide ultra-high-strength steel with a tensile strength of 1.2 GPa or higher, which is used as a vehicle body reinforcing material part for automobiles. There are ~5.

In Patent Document 1, the weight ratio of the chemical components is C: 0.15 to 0.20%, Si: 0.3 to 0.8%, Mn: 1.8 to 2.5%, Al: 0.02 ~0.06%, Mo: 0.1-0.4%, Nb: 0.03-0.06%, S: 0.02% or less, P: 0.02% or less, N: 0.005% After adding the following and homogenizing the aluminum-killed steel containing elements that are unavoidably contained during steel production at 1050 to 1300°C, finish hot rolling is performed at 850 to 950°C, which is just above the Ar ₃ transformation point. Then, a step of hot rolling and coiling at 550 to 650 ° C., a step of cold rolling the steel sheet at a cold reduction of 30 to 80%, and then a step of continuous annealing at _A3 temperature or higher, and this The steel plate is first slowly cooled to 600 to 700 ° C., and secondly to 350 to 300 ° C. at a cooling rate of -10 to -50 ° C./sec, and then gradually cooled between 350 and 250 ° C. Disclosed is a method for producing ultra-high strength cold-rolled steel sheets with a tensile strength of 1.2 GPa grade for automobile bumper reinforcements, including the step of holding for 1 minute or longer.

In Patent Document 2, Cr: One or two or more of 0.3% or less, Mo: 0.3% or less, and Ni: 0.3% or less have a strength of 1180 to 1400 MPa, and the bending/strain of the steel plate is 10 mm or less. Disclosed is a method for producing cold-rolled steel sheet with good shape. In addition, after quenching the steel sheet at a high temperature using a continuous annealing heat treatment equipment, it is over-aged at a temperature range of 150 to 200 ° C. After normal water cooling (quenching), tempering treatment is performed to reduce plate shape defects ( It discloses that deformation in the width direction of the steel plate can be improved.

In Patent Document 3, in weight %, C: 0.1 to 0.6%, Si: 1.0 to 3.0%, Mn: 1.0 to 3.5%, Al: 1.5% or less and After heating a cold-rolled steel sheet containing Cr: 0.003 to 2.0% at a temperature of Ac ₃ to Ac ₃ +50 ° C., cooling at a cooling rate of 3 ° C./s or more, (Ms - 100 ° C.) ~ By maintaining a constant temperature within the Bs (bainite start temperature) range, the phase fraction of retained austenite before working is 10% or more, the length of the austenite grain is 1 micron or more in the short axis, and the average axial ratio (long A method for producing a 1470 MPa class ultra-high strength cold-rolled steel sheet with a tensile strength of 5 or more and hydrogen embrittlement resistance is introduced.

In Patent Document 4, in weight %, C: 0.10 to 0.27%, Si: 0.001 to 1.0%, Mn: 2.3 to 3.5%, Al: 1.0% or less ( 0%), Cr: 2% or less (excluding 0%), P: 0.02% or less (excluding 0%), S: 0.01% or less (excluding 0%), N: 0. 01% or less (excluding 0%), B: 0.005% or less (excluding 0%), Ti: 0.004 to 0.03%, Mo: 0.2% or less (excluding 0%), Nb : 0.05% or less (excluding 0%), the balance being Fe and other unavoidable impurities, cold-rolled strip was heated at a rate of 1 to 5 ° C./s [(Ac ₃ -90 ° C.) ~ (Ac ₃ ± 15 ° C.)], then primary cooling to a temperature range of 500 to 750 ° C. at a cooling rate of 1 to 3 ° C./s, and then cooling at a cooling rate of 3 to 50 ° C./s [ (Ms-120) to 460°C], and then maintain isothermal transformation for 6 to 500 seconds, or tensile after continuous annealing stage of slow cooling at a cooling rate of 1°C/s or less. A method for manufacturing cold-rolled steel sheets with a strength of 1.5 GPa is introduced.
However, when manufacturing according to Patent Documents 1 to 4, it is necessary to go through a cold rolling and annealing heat treatment (CAL, Continuous Annealing Line) process after hot rolling, so there is only a drawback that the manufacturing cost rises sharply. However, it suffers from relatively low tensile strength for application in automotive bumpers or reinforcements currently in commercial use.

Further, in Patent Document 5, in terms of weight %, C: 0.26 to 0.45%, Mn + Cr: 0.5 to 3.0%, Nb: 0.02 to 1.0%, 3.42N + 0.001 ≤ Ti in an amount satisfying Ti ≤ 3.42N + 0.5, and one or two of Si: 0.5% or less, Ni: 2% or less, Cu: 1% or less, V: 1% or less, and Al: 1% or less species or more, in some cases B: 0.01% or less, Nb: 1.0% or less, Mo: 1.0% or less, Ca: 0.001 to 0.005% containing one or two or more Disclosed is a method for producing an ultra-high-strength steel sheet having a tensile strength of 1.8 GPa by hot press forming a cold-rolled steel sheet.
When manufactured according to Patent Document 5, an ultra-high strength of 1.8 GPa can be secured, but the cold-rolled steel sheet needs to undergo a hot press forming step again, so the manufacturing cost is high. becomes even higher.
Therefore, the ultra-high strength hot-rolled steel sheet can not only replace conventional ultra-high strength cold-rolled steel sheets and hot-formed steels, but also can secure higher tensile strength and dramatically reduce manufacturing costs. And it is the actual situation that the development about the manufacturing method is calculated|required.

Korean Patent Application Publication No. 2004-0057777 Japanese Patent Application Publication No. 2007-100114 Korean Patent Application Publication No. 2008-0073763 Korean Patent Application Publication No. 2013-0069699 Korean Patent Application Publication No. 2008-0111549

An object of the present invention is to provide a hot-rolled steel sheet manufactured by using a continuous continuous rolling mode in a continuous casting-rolling direct connection process, and a method for manufacturing the same.
On the other hand, the subject of the present invention is not limited to the contents described above. The subject of the present invention can be understood from the overall contents of this specification, and a person having ordinary knowledge in the technical field to which the present invention belongs can understand the additional subject of the present invention. There is no difficulty in

The hot-rolled steel sheet of the present invention has C: 0.16 to 0.27%, Mn: 0.8 to 2.6%, Si: 0.05 to 0.3%, and Al: 0.05% by weight. % or less, Ti: 0.01 to 0.08%, B: 0.001 to 0.005%, Ca: 0.001 to 0.005%, N: 0.001 to 0.010%, the balance is Fe and other inevitable impurities, satisfying the following relational expressions 1 to 3, the total area fraction of martensite and autotempered martensite is 95% or more, and ferrite is 5% or less (0% A composite of M(X) (M = Ti, Nb, Si, Al, B, Mg, Cr, Ca, P, X = C, N) having a microstructure of 40 nm or less It is characterized by containing precipitates.
[Relationship 1]
16≤100(C+Mn/100+B/10)≤28
[Relational expression 2]
1≦[(Al/27)×(N/14)]/[(Ti/48)×(B/11)]≦14
[Relational expression 3]
0.05≦[(Al/27)×(N/14)]/[(Ti/48)×(B/11)]/100(C+Mn/100+B/10)≦0.66
(However, the contents of the alloy components described in the above relational expressions 1 to 3 are weight percent.)

The method for producing a hot-rolled steel sheet of the present invention has, in weight percent, C: 0.16 to 0.27%, Mn: 0.8 to 2.6%, Si: 0.05 to 0.3%, Al: 0.05% or less, Ti: 0.01 to 0.08%, B: 0.001 to 0.005%, Ca: 0.001 to 0.005%, N: 0.001 to 0.010%, The balance is composed of Fe and other unavoidable impurities, a step of continuously casting molten steel that satisfies the following relational expressions 1 to 3 to obtain a thin slab, a step of rough rolling the thin slab to obtain a bar, and a step of obtaining the bar. obtaining a hot-rolled steel _sheet by finish rolling so that the delivery-side temperature of the finish rolling is Ar ₃ +10° C. to Ar ₃ +60° C.; It includes a step of winding at Mf-50° C. or lower, and each of the above steps is performed continuously.
[Relationship 1]
16≤100(C+Mn/100+B/10)≤28
[Relational expression 2]
1≦[(Al/27)×(N/14)]/[(Ti/48)×(B/11)]≦14
[Relational expression 3]
0.05≦[(Al/27)×(N/14)]/[(Ti/48)×(B/11)]/
100(C+Mn/100+B/10)≦0.66
(However, the contents of the alloying components described in the above relational expressions 1 to 3 are weight percent.)

According to the present invention, the hot-rolled steel sheet of the present invention is produced by appropriately controlling the alloy composition and manufacturing conditions and utilizing the continuous continuous rolling mode in the continuous casting-rolling direct process to achieve the hot-rolled steel sheet and its manufacturing method. can provide. In addition, the hot-rolled steel sheet of the present invention can replace ultra- high-strength cold-rolled steel sheets and hot-formed steels, and has the effect of dramatically reducing manufacturing costs. At the same time, it is possible to use steel obtained by melting scraps such as old iron in an electric furnace through the thin slab continuous casting method, so that the reusability of resources can be improved.

1 is a schematic diagram of equipment for a continuous casting-rolling direct connection process applicable to the production of the hot-rolled steel sheet of the present invention. FIG. FIG. 4 is another schematic diagram 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. 4 is a graph showing values of relational expressions 1 and 2 regarding invention examples 1 to 15 and comparative examples 1 to 13 according to one embodiment of the present invention. 1 is a microstructure photograph of Invention Example 1 according to an example of the present invention observed with a scanning electron microscope (SEM). 1 is a microstructure photograph of Invention Example 1 according to an embodiment of the present invention observed with a transmission electron microscope (TEM), (a) showing lath and (b) showing carbide. 1 is a graph showing distributions of martensite and autotempered martensite of Example 1 according to an example of the present invention with respect to lath width. 1 is a photograph of a precipitate of Invention Example 1 according to an example of the present invention, observed with a transmission electron microscope (TEM). FIG. 10 is a photograph of a precipitate of Comparative Example 8 according to an example of the present invention observed with a transmission electron microscope (TEM); FIG.

A hot-rolled steel sheet according to one embodiment of the present invention will be described below. First, the alloy composition of the present invention will be explained. Contents of the alloy compositions described below mean weight percent unless otherwise specified.

C: 0.16-0.27%
Carbon (C) is a very important element that increases strength by forming a microstructure into martensite during quenching after hot rolling. When the C content is less than 0.16%, the strength of martensite itself is low, and it may become difficult to secure the strength targeted in the present invention. On the other hand, when the C content exceeds 0.27%, there is a problem that the weldability and bending workability are deteriorated due to an excessive increase in strength. Therefore, the C content is preferably 0.16-0.27%. The lower limit of the C content is more preferably 0.17%, still more preferably 0.18%, and most preferably 0.19%. The upper limit of the C content is more preferably 0.26%, still more preferably 0.25%, and most preferably 0.24%.

Mn: 0.8-2.6%
Manganese (Mn) suppresses ferrite formation and increases strength by increasing the stability of austenite and facilitating the formation of low temperature transformation phases. If the Mn content is less than 0.8%, it may become difficult to secure the strength targeted in the present invention. On the other hand, when the Mn content exceeds 2.6%, segregation bands are formed inside or outside the continuous cast slab and hot-rolled steel sheet, or in all of them, and crack generation and propagation are induced. There is a risk that the final quality will be degraded and the weldability and bending workability will be inferior. Therefore, the Mn content is preferably 0.8-2.6%. The lower limit of the Mn content is more preferably 0.85%, still more preferably 0.90%, and most preferably 0.95%. The upper limit of the Mn content is more preferably 2.5%, still more preferably 2.4%, and most preferably 2.3%.

Si: 0.05-0.3%
Silicon (Si) is a useful element that can ensure strength without reducing the ductility of the steel sheet. In addition, it is an element that promotes the formation of martensite by promoting the formation of ferrite and the concentration of C in untransformed austenite. If the Si content is less than 0.05%, it is difficult to sufficiently secure the above effects. On the other hand, when the Si content exceeds 0.3%, red scale is formed on the surface of the steel sheet, and marks may remain on the surface of the steel sheet after pickling, resulting in deterioration of the surface quality. Therefore, the Si content is preferably 0.05-0.3%. The lower limit of the Si content is more preferably 0.06%, still more preferably 0.07%, and most preferably 0.08%. The upper limit of the Si content is more preferably 0.28%, still more preferably 0.26%, and most preferably 0.24%.

Al: 0.05% or less Aluminum (Al) is concentrated on the surface of the steel sheet and deteriorates the plateability, while suppressing the formation of carbides and increasing the ductility of the steel. Furthermore, aluminum (Al) in steel reacts with nitrogen (N) to precipitate AlN. There is a risk of degrading the quality of the steel sheet. Therefore, it is necessary to control the content as low as possible, preferably 0.05% or less. The Al content is more preferably 0.048% or less, still more preferably 0.046% or less, and most preferably 0.045% or less.

Ti: 0.01-0.08%
Titanium (Ti) is an element that increases the strength of steel as a precipitate and nitride forming element. In addition, Ti is an element that removes dissolved N by forming TiN near the solidification temperature and reduces the amount of precipitates such as AlN, thereby preventing the deterioration of high-temperature ductility and reducing the occurrence of edge cracks. . If the Ti content is less than 0.01%, excessive precipitation of fine AlN or BN precipitates will lead to a decrease in the ductility of the cast slab, degrading the quality of the slab. On the other hand, when the Ti content exceeds 0.08%, not only is it difficult to expect the effect of crystal grain refinement due to the formation of coarse TiN precipitates, but also the production cost increases. Therefore, the Ti content is preferably 0.01-0.08%. The lower limit of the Ti content is more preferably 0.012%, still more preferably 0.014%, and most preferably 0.016%. The upper limit of the Ti content is more preferably 0.07%, still more preferably 0.06%, and most preferably 0.05%.

B: 0.001 to 0.005%
Boron (B) is an element that increases the hardenability of steel. If the content is less than 0.001%, the above effect cannot be obtained, and if it exceeds 0.005%, the austenite recrystallization temperature is raised and the weldability is deteriorated. Therefore, it is preferable to limit the B content to 0.001 to 0.005%. The lower limit of the B content is more preferably 0.0012%, still more preferably 0.0014%, and most preferably 0.0016%. The upper limit of the B content is more preferably 0.0045%, still more preferably 0.0040%, and most preferably 0.0035%.

Ca: 0.001-0.005%
Calcium (Ca) reacts with Al and O in molten steel to form spherical inclusions (12CaO 17Al ₂ O ₃ ) with a low melting point, thereby preventing nozzle clogging and facilitating separation and floating of inclusions. is an element. When the Ca content is less than 0.001%, it is difficult to ensure the above effects. On the other hand, if the Ca content exceeds 0.005%, high-melting inclusions are formed, which may promote clogging of the nozzle and may interrupt casting, resulting in large inclusions (>50 μm ) may be formed to deteriorate the workability of the steel sheet. Therefore, it is preferable to control the Ca content to 0.001 to 0.005%. The lower limit of the Ca content is more preferably 0.0012%, still more preferably 0.0014%, and most preferably 0.0016%. The upper limit of the Ca content is more preferably 0.0045%, still more preferably 0.0040%, and most preferably 0.0035%.

N: 0.001 to 0.010%
Nitrogen (N) is an austenite stabilizing and nitride forming element. If the N content is less than 0.001%, the above effects become insufficient. On the other hand, when the N content exceeds 0.010%, it reacts with the forming elements of the precipitates to increase the effect of precipitation strengthening, but there is a possibility that the ductility may be suddenly lowered. Therefore, the N content is preferably 0.001 to 0.010%. The lower limit of the N content is more preferably 0.0012%, still more preferably 0.0014%, and most preferably 0.0016%. The upper limit of the N content is more preferably 0.009%, still more preferably 0.008%, and most preferably 0.007%.

The remaining component of the present invention is iron (Fe). However, unintended impurities from raw materials or the surrounding environment may inevitably be mixed in during normal manufacturing processes, and cannot be excluded. These impurities are known to anyone skilled in the normal manufacturing process, and therefore the full content thereof is not specifically referred to in this specification.

On the other hand, in the hot-rolled steel sheet of the present invention, C, Mn, B, Al, Ti and N among the above alloy components preferably satisfy the following relational expressions 1 to 3, respectively. Mechanical properties and excellent surface quality can be ensured. However, the contents of the alloy components described in the following relational expressions 1 to 3 are weight percent.
[Relationship 1]
16≤100(C+Mn/100+B/10)≤28
The above relational expression 1 is a component relational expression for ensuring the mechanical properties to be obtained by the present invention. If the value of the relational expression 1 is less than 16, it is difficult to ensure the strength targeted by the present invention, and if it exceeds 28, the elongation ratio may decrease and cracks may occur during processing. Therefore, it is preferable that the value of relational expression 1 is in the range of 16-28. The lower limit of the value of relational expression 1 is more preferably 17, even more preferably 18, and most preferably 19. The upper limit of the value of relational expression 1 is more preferably 27, even more preferably 26, and most preferably 25.

[Relational expression 2]
1≦[(Al/27)×(N/14)]/[(Ti/48)×(B/11)]≦14
The relational expression 2 is a component relational expression for securing the edge quality of the slab or bar to improve the surface quality of the finally obtained hot-rolled steel sheet. When the value of the relational expression 2 is less than 1, the Ti or B content is high or the Al or N content is low, and coarse Ti (C, N) and B (C, N) Excessive precipitation of precipitates may lead to a decrease in hot ductility and cracks at the edges of the slab or bar. On the other hand, when it exceeds 14, the Ti or B content is small, or the Al or N content is large, and AlN is excessively precipitated and the high temperature ductility is lowered, resulting in the formation of a slab or bar. Edge quality may deteriorate. Therefore, the value of relational expression 2 above preferably ranges from 1 to 14. The lower limit of the value of relational expression 2 is more preferably 1.1, even more preferably 1.2, and most preferably 1.3. The upper limit of the value of relational expression 2 is more preferably 13, still more preferably 12, and most preferably 11.

[Relational expression 3]
0.05≦[(Al/27)×(N/14)]/[(Ti/48)×(B/11)]/100(C+Mn/100+B/10)≦0.66
The above relational expression 3 is a component relational expression for ensuring the mechanical properties and excellent surface quality targeted by the present invention. If the value of the relational expression 3 is less than 0.05, it becomes difficult to secure the target strength, and if it exceeds 0.66, excessive precipitation of precipitates causes a decrease in hot ductility. There is a danger that cracks will form on the edges of the slab or bar. Therefore, it is preferable that the value of relational expression 3 is in the range of 0.05 to 0.66. The lower limit of the value of relational expression 3 is more preferably 0.06, still more preferably 0.08, and most preferably 0.10. The upper limit of the value of relational expression 3 is more preferably 0.62, still more preferably 0.58, and most preferably 0.56.

On the other hand, in the hot-rolled steel sheet of the present invention, the tramp element is selected from the group consisting of Nb, V, Ti, Mo, Cu, Cr, Ni, Zn, Se, Sb, Zr, W, Ga, Ge and Mg. More than one species can be included in the range up to a total of 0.1% by weight. The tramp element is an impurity element such as ferroalloy or scrap used as a raw material in the steelmaking process, ladle and tundish refractories, etc. When the total amount exceeds 0.1% However, there is a risk that cracks will occur on the surface of the thin slab, degrading the surface quality of the hot-rolled steel sheet.

The hot-rolled steel sheet of the present invention includes a microstructure in which the total area fraction of martensite and autotempered martensite is 95% or more and the ferrite is 5% or less (including 0%). is preferred. The above martensite and autotempered martensite structures are essential structures for obtaining the strength targeted by the present invention, and if the fraction is less than 95%, it is difficult to ensure the strength. In the present invention, ferrite can be contained in a range of 5% or less to ensure ductility. However, if the fraction exceeds 5%, although ductility increases, it is difficult to ensure strength. There is fear. On the other hand, the total fraction of martensite and autotempered martensite is more preferably 96% or more, still more preferably 97% or more, and most preferably 98% or more.

The main microstructures of the present invention are martensite and autotempered martensite, where the average width of the lath grains of said martensite and autotempered martensite affects strength and workability. For this reason, the average width of lath grains of martensite and autotempered martensite is preferably 1 μm or less with respect to the minor axis. If the average width of the lath grains of the martensite and autotempered martensite exceeds 1 μm, it may become difficult to ensure the target strength and workability. The narrower the average width of the lath grains of the martensite and autotempered martensite, the more advantageous it is for securing strength, but it is difficult to control it to less than 0.1 μm under normal cooling conditions. The lower limit of the average lath width of martensite and autotempered martensite is more preferably 0.12 μm, still more preferably 0.14 μm, and most preferably 0.16 μm. The upper limit of the average width of martensite and autotempered martensite is more preferably 0.9 μm, still more preferably 0.8 μm, and most preferably 0.7 μm.

The hot-rolled steel sheet of the present invention has composite precipitates of M(X) (M=Ti, Nb, Si, Al, B, Mg, Cr, Ca, P, X=C, N) having an average size of 40 nm or less. is preferably included. If the average size of the composite precipitates exceeds 40 nm, it may be difficult to ensure the strength effectively, and edge cracks may occur to deteriorate the edge quality. Although the smaller the average size of the composite precipitates, the more advantageous it is for securing strength, it is difficult to control the size to less than 5 nm under the production conditions of the present invention. The average size of the composite precipitates is more preferably 38 nm or less, still more preferably 34 nm or less, and most preferably 30 nm or less.

The hot rolled steel sheet provided by the present invention has a yield strength of 1060 to 1400 MPa, a tensile strength of 1470 to 1800 MPa, an elongation of 5% or more, and a Vickers hardness of 420 to 550 Hv (0.5 kgf). Preferably, the variation in tensile strength in the width direction of the strip is 100 MPa or less, and the variation in Vickers hardness in the width direction of the strip is preferably 50 Hv (0.5 kgf) or less. Also, the hot-rolled steel sheet of the present invention may have a thickness of 1.6 mm or less, more preferably 1.4 mm or less, still more preferably 1.3 mm or less, and most preferably 1.2 mm or less. is. The hot-rolled steel sheet of the present invention can effectively replace ultra-high strength cold-rolled steel sheets and hot-formed steels because it has excellent mechanical properties, surface quality, and low material variation as described above.

An embodiment of the hot-rolled steel sheet manufacturing method of the present invention will be described below.
FIG. 1 is a schematic diagram of equipment for a continuous casting-rolling direct connection process applicable to the production of the hot-rolled steel sheet of the present invention. An ultra-high-strength hot-rolled steel sheet with excellent surface quality and little variation in material quality according to one embodiment of the present invention can be produced using continuous casting-rolling direct connection equipment as shown in FIG. The continuous casting-rolling direct connection facility is large and consists of a continuous casting machine 100, a rough rolling mill 400 and a finishing rolling mill 600. The continuous casting-rolling direct connection equipment includes a high-speed continuous casting machine 100 that produces a thin slab (Slab) a having a first thickness, and a bar (Bar) b having a second thickness thinner than the first thickness. It is produced by a roughing mill 400 for rolling, a finishing mill 600 for rolling a second thickness bar with a third thickness strip c, and a winder 900 for winding said strip. Furthermore, there is a roughing mill scale breaker 300 (hereinafter “RSB”) before the roughing mill 400 and a finishing mill scale breaker 500 (hereinafter “FSB”) before the finishing mill 600. can be further included, surface scale can be easily removed, and PO (Pickled & Oiled) steel sheets with excellent surface quality can be produced when hot-rolled steel sheets are pickled in a post-process. In addition, it is possible to perform isothermal and constant velocity rolling in the continuous casting-rolling direct connection process, the temperature variation in the width and length directions of the steel plate is remarkably small, and the run out table (ROT) 700 enables precise cooling control. It is possible to produce ultra-high-strength hot-rolled steel sheets with excellent surface quality and little material variation. The strip thus rolled and cooled is cut by a high speed shearer 800 and wound by a winder 900 to be produced as a product. On the other hand, a bar can be provided in front of the finish rolling scale breaker 500, and a heater 200 for heating can also be provided.

FIG. 2 is another schematic diagram of 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-to-rolling facility disclosed in FIG. 2 has almost the same configuration as the facility disclosed in FIG. , it is easy to secure the slab edge temperature, and it is advantageous to secure the surface quality by lowering the defect occurrence rate of the edge. In addition, a space having a length of at least one slab is secured in front of the roughing mill, and batch rolling is also possible.
The ultra-high-strength hot-rolled steel sheet of the present invention, which has excellent surface quality and little material variation, can be produced by the continuous casting-rolling direct connection equipment shown in FIGS.

An embodiment of the hot-rolled steel sheet manufacturing method of the present invention will be described in detail below.
First, molten steel having the above alloy composition is continuously cast to obtain a thin slab. At this time, the continuous casting is preferably performed at a casting speed of 4 to 8 mpm (m/min) or higher. The reason why the casting speed is set to 4 mpm or more is that the high-speed casting and rolling processes are connected, and a certain or more casting speed is required to secure the target rolling temperature. When the casting speed is slow, segregation may occur in the slab, and if such segregation occurs, it is difficult to ensure strength and workability, and material variations in the width direction or length direction occur. fear grows. If it exceeds 8 mpm, there is a risk that the success rate of operation will decrease due to the instability of the molten steel surface, so the casting speed is preferably in the range of 4 to 8 mpm. The lower limit of the casting speed is more preferably 4.2 mpm, still more preferably 4.4 mpm, and most preferably 4.6 mpm. The upper limit of the casting speed is more preferably 7.5 mpm, still more preferably 7.0 mpm, and most preferably 6.5 mpm.

On the other hand, the thickness of the slab is preferably 80-120 mm. When the thickness of the slab exceeds 120 mm, not only is high-speed casting difficult, but also the rolling load during rough rolling increases. , it is difficult to form a uniform structure. In order to solve this problem, additional heating equipment can be installed, but since this is a factor in increasing the production cost, it is preferable to eliminate it as much as possible. Therefore, it is preferable to control the thickness of the slab to 80 to 120 mm. The lower limit of the thickness of the slab is more preferably 82 mm, even more preferably 84 mm, and most preferably 86 mm. The upper limit of the thickness of the slab is more preferably 116 mm, even more preferably 114 mm, and most preferably 110 mm.

Further, the basicity of the mold flux during continuous casting is preferably 0.8 to 1.5. Here, the basicity indicates the CaO (%)/SiO ₂ (%) ratio. In the case of the steel of the present invention, many alloying elements such as C, Mn and B are added to ensure high strength, and the risk of linear crack activation is very high. Therefore, if a mold flux with a basicity of less than 0.8 is used, a large amount of heat is transferred and the surface of the slab is rapidly cooled, which may cause linear cracks. On the other hand, when a mold flux with a basicity exceeding 1.5 is used, the amount of heat transfer is too small, and it may become difficult to obtain sound solidification cells. Therefore, the basicity of the mold flux during continuous casting is preferably 0.8 to 1.5. The lower limit of the basicity of the mold flux is more preferably 0.85, still more preferably 0.90, and most preferably 0.95. The upper limit of the basicity of the mold flux is more preferably 1.45, still more preferably 1.40, and most preferably 1.35.

Further, the secondary cooling specific water amount during the continuous casting is preferably 1.5 to 2.5 L/kg. If the secondary cooling specific water amount during continuous casting exceeds 2.5 L/kg, linear cracks may occur and the quality of the slab may deteriorate, and the edge temperature of the slab or bar may decrease, leading to the risk of edge cracks. become more sexual. On the other hand, if the secondary cooling specific water amount during continuous casting is less than 1.5 L/kg, there is a risk of problems such as outflow of molten steel due to non-solidification of the slab on the continuous casting side. ) Equipment problems may occur due to the risk of roll deterioration. Therefore, it is preferable that the secondary cooling specific water amount during the continuous casting is 1.5 to 2.5 L/kg. The lower limit of the secondary cooling specific water amount during continuous casting is more preferably 1.55 L/kg, still more preferably 1.60 L/kg, and most preferably 1.65 L/kg. The upper limit of the secondary cooling specific water amount during continuous casting is more preferably 2.45 L/kg, still more preferably 2.40 L/kg, and most preferably 2.35 L/kg.

After that, the slab is roughly rolled to obtain a bar. The rough rolling step can be performed by rough rolling the continuously cast slab in a rough rolling mill comprising 2 to 5 stands.
The temperature of the bar edge portion on the delivery side of rough rolling during the rough rolling is preferably 850 to 1000°C. When the temperature of the bar edge portion is lower than 850° C., a large amount of AlN precipitates and the like are generated, and the high-temperature ductility is lowered, which may greatly increase the risk of edge cracks. On the other hand, when the temperature of the bar edge portion exceeds 1000° C., the temperature of not only the edge portion of the bar but also the center portion of the bar rises, and a large amount of scale is generated, resulting in deterioration of the surface quality after pickling. there is a risk of Therefore, it is preferable that the temperature of the bar edge portion on the rough rolling delivery side during the rough rolling is 850 to 1000°C. The lower limit of the temperature of the bar edge portion on the delivery side of rough rolling during the rough rolling is more preferably 860°C, still more preferably 870°C, and most preferably 880°C. The upper limit of the temperature of the bar edge portion on the delivery side of rough rolling during the rough rolling is more preferably 990°C, still more preferably 980°C, and most preferably 970°C.

After the step of obtaining the bar, the step of spraying cooling water on the bar to remove scale may be further included. For example, before finish rolling the bar, cooling water is injected from the nozzle of a finishing mill scale breaker (hereinafter referred to as "FSB") at a pressure of 200 to 300 bar to remove the surface scale to a thickness of 30 μm or less. can do. If the cooling water injection pressure is less than 200 bar, scale removal becomes insufficient, and a large amount of spindle-shaped and scale-shaped scales are formed on the surface of the steel sheet after finish rolling, degrading the surface quality after pickling. On the other hand, when the injection pressure of the cooling water exceeds 300 bar, the delivery side temperature of the finish rolling becomes too low, it becomes difficult to secure an effective austenite fraction, and the target tensile strength is secured. becomes difficult. Therefore, the injection pressure of the cooling water is preferably 200-300 bar. The lower limit of the cooling water injection pressure is more preferably 210 bar, still more preferably 220 bar, and most preferably 230 bar. The upper limit of the injection pressure of the cooling water is more preferably 290 bar, still more preferably 280 bar, and most preferably 270 bar.

Also, when the cooling water is injected, the overlap area ratio of the cooling water is preferably 5 to 25%. If the overlapping area ratio of the cooling water is less than 5%, the overlapping area of the cooling water is too small, and the temperature of the bar may rise locally, resulting in uneven temperature in the width direction. As a result, the scale may not be completely removed, the surface quality may deteriorate, and it may be difficult to control the variation in the tensile strength in the width direction of the finally obtained hot-rolled steel sheet to 100 MPa or less. . Further, if the overlapping area ratio of the cooling water jets exceeds 25%, there is a risk that excessive cooling may occur locally, causing temperature variations in the width direction, resulting in large variations in the quality of the finally obtained hot-rolled steel sheet. Therefore, when the cooling water is injected, the overlap area ratio of the cooling water is preferably 5-25%. The lower limit of the overlapping area ratio of the cooling water is more preferably 6%, still more preferably 7%, and most preferably 8%. The upper limit of the overlapping area ratio of the cooling water is more preferably 24%, still more preferably 23%, and most preferably 22%.

After that, the bar is finish-rolled so that the delivery-side temperature of the finish rolling is Ar ₃ +10° C. to Ar ₃ +60° C. to obtain a hot-rolled steel sheet. The finish rolling step can be performed by finishing rolling the bar produced by the rough rolling mill by a finishing rolling mill comprising 3 to 7 stands. When the delivery-side temperature of the finish rolling is less than Ar ₃ +10° C., the load on the rolls during hot rolling increases greatly, energy consumption increases, the working speed slows down, and the temperature in the width direction increases. Proeutectoid ferrite may occur when the temperature of the hot-rolled steel sheet is locally decreased to Ar ₃ or less when the variation occurs, and a sufficient martensite fraction cannot be obtained after cooling. On the other hand, if the delivery-side temperature of the finish rolling exceeds Ar ₃ +60° C., the crystal grains become coarse and high strength cannot be obtained. In order to obtain a sufficient martensite fraction, there is a problem that the cooling rate must be further increased. Therefore, the delivery side temperature of the finish rolling is preferably Ar ₃ +10°C to Ar ₃ +60°C. The lower limit of the delivery side temperature of the finish rolling is more preferably Ar ₃ +12°C, still more preferably Ar ₃ +14°C, and most preferably Ar ₃ +16°C. The upper limit of the delivery side temperature of the finish rolling is more preferably Ar ₃ +58°C, still more preferably Ar ₃ +56°C, and most preferably Ar ₃ +52°C.

The variation in rolling speed during the finish rolling is preferably 50 mpm or less. Since the ultra-high strength steel targeted by the present invention utilizes the formation of a transformation structure as a strengthening mechanism, it is very likely that the material properties will change according to the deformation rate during finish rolling. That is, if the difference in rolling speed exceeds 50 mpm in the finishing rolling mill consisting of multiple stands, it becomes difficult to ensure a uniform cooling rate and target coiling temperature in the subsequent run out table (ROT), and the strip (Strip) may cause a large material variation in the width or length direction. Therefore, it is preferable that the variation in rolling speed during the finish rolling is 50 mpm or less. The rolling speed variation during the finish rolling is more preferably 48 mpm or less, still more preferably 46 mpm or less, and most preferably 42 mpm or less. On the other hand, in the present invention, the lower the variation in rolling speed during the finish rolling, the more advantageous, so the lower limit is not particularly limited.

The temperature variation in the width direction of the hot-rolled steel sheet during the finish rolling is preferably 50°C or less. If the temperature variation in the width direction of the hot-rolled steel sheet at the time of finish rolling exceeds 50° C., differences in the austenite fraction and grain size may occur locally, resulting in severe material variation. Therefore, the temperature variation in the width direction of the hot-rolled steel sheet during the finish rolling is preferably 50°C or less. The temperature variation in the width direction of the hot-rolled steel sheet during finish rolling is more preferably 48° C. or less, still more preferably 46° C. or less, and most preferably 42° C. or less. On the other hand, in the present invention, the lower the temperature variation in the width direction of the hot-rolled steel sheet during the finish rolling, the more advantageous it is, so the lower limit is not particularly limited.

The rolling speed during the finish rolling is preferably 200 to 600 mpm. If the rolling speed during the finish rolling exceeds 600 mpm, there is a possibility that an operation accident such as plate breakage may occur, it is difficult to perform isothermal and constant velocity rolling, a uniform temperature cannot be ensured, and there is a risk of material variation. be. On the other hand, if the rolling speed at the time of finish rolling is less than 200 mpm, the finish rolling speed is too slow, and it may become difficult to secure the target finish rolling temperature of the present invention. Therefore, the rolling speed during the finish rolling is preferably 200 to 600 mpm. The lower limit of the rolling speed during finish rolling is more preferably 220 mpm, still more preferably 250 mpm, and most preferably 280 mpm. The upper limit of the rolling speed during finish rolling is more preferably 580 mpm, still more preferably 550 mpm, and most preferably 500 mpm.

After that, the hot-rolled steel sheet is cooled at 200° C./sec or more directly above Ar ₃ and coiled at Mf(90)−50° C. or less. If the cooling rate is less than 200° C./sec, ferrite and bainite may be formed, making it difficult to secure a sufficient martensite structure. Therefore, the cooling rate is preferably 200° C./sec or more. The cooling rate is more preferably 220° C./sec or higher, still more preferably 240° C./sec or higher, most preferably 260° C./sec or higher. Further, when the winding temperature exceeds Mf(90)-50°C, not only is it difficult to obtain a martensitic structure, but the martensitic structure obtained by cooling is excessively auto-tempered. It may become difficult to obtain the tensile strength targeted by the present invention. Therefore, the winding temperature is preferably Mf-50° C. or lower. The winding temperature is more preferably Mf-60° C. or lower, further preferably Mf-70° C. or lower, and most preferably Mf-80° C. or lower. Meanwhile, Mf means the temperature at which the transformation of the austenite structure to martensite is 100% completed.

On the other hand, the interval between cooling nozzles during cooling is preferably 150 to 400 mm. If the distance between the cooling nozzles exceeds 400 mm, the temperature of the hot-rolled steel sheet may rise locally, resulting in severe material variation. It may become low locally and the material variation may become severe. Therefore, the interval between the cooling nozzles during cooling is preferably 150 to 400 mm. The lower limit of the distance between the cooling nozzles during cooling is more preferably 160 mm, still more preferably 170 mm, and most preferably 180 mm. The upper limit of the distance between the cooling nozzles during cooling is more preferably 380 mm, still more preferably 360 mm, and most preferably 340 mm.

After the coiling step, the coiled hot-rolled steel sheet may be further pickled, and a PO (Pickled & Oiled) material may be obtained by the pickling treatment. In the present invention, the scale can be sufficiently removed in the step of removing slab and bar scale, so that a PO material with excellent surface quality can be obtained even with a general pickling treatment. Therefore, in the present invention, any method generally used in the hot-rolling pickling process can be applied, and the pickling treatment method is not particularly limited.

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, it should be noted that the following examples are merely to illustrate and explain the present invention in more detail, and are not intended to limit the scope of rights of the present invention. The scope of rights of the present invention is determined by matters described in the claims and matters reasonably inferred therefrom.

(Example 1)
After preparing molten steel having the alloy composition shown in Table 1 below, a hot-rolled steel sheet having a thickness of 1.2 mm was produced under the production conditions shown in Tables 2 and 3 below by applying a direct casting-rolling process. After pickling the hot-rolled steel sheet, the presence or absence of nozzle clogging was observed, and the microstructure and precipitates were measured. The results are shown in Table 4 below. ), elongation (El), Vickers hardness (Hv (0.5 kgf)), tensile strength variation (ΔTS), Vickers hardness variation (ΔHv (0.5 kgf)), and the presence or absence of cracks were measured. The results are shown in Table 5 below. Meanwhile, the Ar ₃ and Mf temperatures in Table 3 are values calculated using JmatPro V-8, a commonly used thermodynamic software.
Microstructures and precipitates were observed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM).

Yield strength, tensile strength, and elongation were obtained by measuring JIS No. 5 test specimens sampled in the rolling direction over the entire width of the strip [constant intervals (7 locations)], and then the average values were recorded.
The hardness was measured 10 times under a load of 0.5 kgf using a Vickers hardness machine, and the average value was recorded.
Tensile strength variation (ΔTS) and Vickers hardness variation (ΔHv (0.5 kgf)) indicate the difference between the maximum and minimum values measured across the width.
The presence or absence of cracks was first checked with the naked eye from the slab, bar, and strip, and secondarily checked using an SDD (Surface Defect Detector) device of a surface defect detector.

As shown in Tables 1 to 5 above, in the case of invention examples 1 to 15 that satisfy all of the alloy composition, relational expressions 1 to 3, and manufacturing conditions proposed by the present invention, the microstructure and precipitate conditions of the present invention are found to be fulfilled. In addition, it can be confirmed that no linear cracks and edge cracks are generated and good surface quality is ensured. Furthermore, it can be confirmed that the target yield strength, tensile strength, elongation ratio, Vickers hardness, variation in tensile strength in the width direction of the strip, and variation in Vickers hardness in the width direction of the strip are achieved.
However, in the case of Comparative Examples 1 to 12, which do not satisfy one or more of the alloy composition proposed by the present invention, the relational expressions 1 to 3, and the manufacturing conditions (temperature at the delivery side of finish rolling), edge cracks occur, It can be confirmed that the conditions for mechanical properties and material variation targeted by the present invention are not ensured.

Comparative Example 13 is a case in which the Ca content of the alloy composition proposed by the present invention does not satisfy the range, and it can be confirmed that casting was interrupted due to clogging of the nozzle.
Comparative Examples 14 and 15 are cases where the alloy composition proposed by the present invention and the relational expressions 1 to 3 are satisfied, but the manufacturing conditions (delivery side temperature of finish rolling) are not satisfied, and the microstructure proposed by the present invention could not be ensured, it can be confirmed that the conditions for the mechanical properties and material variation targeted by the present invention are not ensured.

FIG. 3 is a graph showing values of relational expressions 1 and 2 for Inventive Examples 1-15 and Comparative Examples 1-13. In the case of the invention area, the range satisfies the relational expression 3 of the present invention, and in the case of invention examples 1 to 15, it can be confirmed that it is included in the above invention area, whereas in the case of comparative examples 1 to 12, It can be confirmed that it is out of the above invention area. Comparative Example 13 is included in the above invention area, but does not satisfy the Ca content range of the present invention.

FIG. 4 is a microstructure photograph of Invention Example 1 observed with a scanning electron microscope (SEM). As shown in FIG. 4 , it can be confirmed that in Invention Example 1, the main structures are martensite and autotempered martensite, and ferrite is partially formed.

FIG. 5 is a transmission electron microscope (TEM) microstructure photograph of Invention Example 1 according to an embodiment of the present invention, where (a) shows lath and (b) shows carbide. As shown in FIGS. 5(a) and 5(b), in Example 1, martensite laths are finely and well developed, and fine carbides are present in the martensite laths. It can be confirmed that the tempered martensitic structure exists together.

FIG. 6 is a graph showing the distribution of martensite and autotempered martensite of Inventive Example 1 with respect to lath width. As shown in FIG. 6, in the case of Invention Example 1, the laths of martensite and autotempered martensite exist in the range of 0.05 to 1.0 μm, and the width of martensite and autotempered martensite is 0.3 μm. It can be confirmed that there are many site laths.

7 and 8 are photographs of the precipitates of Inventive Example 1 and Comparative Example 8, respectively, observed with a transmission electron microscope (TEM). At this time, the TEM test piece was prepared by a carbon replica method. As shown in FIGS. 7 and 8, in the case of Invention Example 1, fine composite precipitates of 40 nm or less [M (X), where M is Ti, Nb, Si, Al, B, Mg, Cr, Ca , P, and X is C or N] are distributed. can be confirmed.

(Example 2)
After preparing a molten steel having the alloy composition of Inventive Steel 1, a 1.2 mm thick hot-rolled steel sheet was manufactured under the manufacturing conditions shown in Table 6 below by applying the continuous casting-rolling direct connection process. In addition to the production conditions shown in Table 6 below, the conditions for scale removal, finish rolling and cooling were the same as those for Invention Example 1 in Table 2 above. After pickling the hot-rolled steel sheets manufactured above, the degree of occurrence of linear cracks and edge cracks was measured, and the results are shown in Table 6 below.

As shown in Table 6 above, it can be confirmed that invention examples 16 to 18 satisfying the alloy composition and manufacturing conditions proposed by the present invention do not generate linear cracks and edge cracks.
On the other hand, Comparative Examples 16 to 19 satisfy the alloy composition proposed by the present invention, but one of the manufacturing conditions of the basicity of the mold flux, the secondary cooling specific water amount, and the temperature of the bar edge on the delivery side of rough rolling It can be confirmed that linear cracks and edge cracks occurred.

(Example 3)
After preparing molten steel having the alloy composition of Inventive Steel 5, hot-rolled steel sheets with a thickness of 1.2 mm were manufactured under the manufacturing conditions shown in Tables 7 and 8 below by applying a continuous casting-rolling direct connection process. In addition to the manufacturing conditions described in Tables 7 and 8 below, the conditions for continuous casting and rough rolling were the same as the conditions for Invention Example 5 in Table 2 above. After pickling the hot-rolled steel sheet produced above, yield strength (YS), tensile strength (TS), elongation (El), Vickers hardness (Hv (0.5 kgf)), tensile strength variation (ΔTS) ) and Vickers hardness variation (ΔHv (0.5 kgf)) were measured, and the results are shown in Table 9 below. The Ar ₃ and Mf temperatures in Table 7 below are values calculated using JmatPro V-8, a commonly used thermodynamic software.

As shown in Tables 7 to 9 above, Invention Examples 19 to 21, which satisfy the alloy composition and production conditions proposed by the present invention, are the yield strength, tensile strength, elongation ratio, Vickers hardness, and strip width targeted by the present invention. It can be confirmed that the tensile strength variation in the direction and the Vickers hardness variation in the width direction of the strip are ensured.
On the other hand, Comparative Examples 20 to 24 satisfy the alloy composition proposed by the present invention, but among the manufacturing conditions, the overlapping area ratio of the cooling water injection, the rolling speed variation during finish rolling, and the width direction of the hot rolled steel sheet during finish rolling It can be confirmed that linear cracks and edge cracks occurred because one of the temperature variation and the distance between the cooling nozzles during cooling was not satisfied.

100 Continuous Caster 200, 200' Heater 300 Roughing Mill Scale Breaker (RSB)
400 Rough rolling mill 500 Finish rolling scale breaker (Fishing Mill Scale Breaker: FSB)
600 finishing mill 700 runout table 800 high speed shear 900 winder a slab b bar c strip

Claims (17)

% by weight, C: 0.16-0.27%, Mn: 0.8-2.6%, Si: 0.05-0.3%, Al: 0.05% or less, Ti: 0.01 ~0.08%, B: 0.001 to 0.005%, Ca: 0.001 to 0.005%, N: 0.001 to 0.010%, the balance consisting of Fe and other inevitable impurities,
satisfying the following relational expressions 1 to 3,
a microstructure in which the total area fraction of martensite and autotempered martensite is 95% or more and the ferrite is 5% or less (including 0%);
Characterized by containing composite precipitates of M(X) (M = Ti, Nb, Si, Al, B, Mg, Cr, Ca, P, X = C, N) having an average size of 40 nm or less Hot-rolled steel plate.
[Relationship 1]
16≤100(C+Mn/100+B/10)≤28
[Relational expression 2]
1≦[(Al/27)×(N/14)]/[(Ti/48)×(B/11)]≦14
[Relational expression 3]
0.05≦[(Al/27)×(N/14)]/[(Ti/48)×(B/11)]/100(C+Mn/100+B/10)≦0.66
(However, the contents of the alloy components described in the above relational expressions 1 to 3 are weight percent.) The hot-rolled steel sheet contains, as a tramp element, one or more selected from the group consisting of Nb, V , Mo , Cu, Cr, Ni, Zn, Se, Sb, Zr, W, Ga, Ge and Mg. 2. The hot -rolled steel sheet according to claim 1, wherein the content is 0.1% by weight or less. 2. The hot -rolled steel sheet according to claim 1, wherein the martensite lath has an average width of 1 [mu]m or less. The hot rolled steel sheet has a yield strength of 1060 to 1400 MPa, a tensile strength of 1470 to 1800 MPa, an elongation of 5% or more, a Vickers hardness of 420 to 550 Hv (0.5 kgf), and a strip width of 2. The hot -rolled steel sheet according to claim 1, wherein the variation in tensile strength in the direction is 100 MPa or less, and the variation in Vickers hardness in the width direction of the strip is 50 Hv (0.5 kgf) or less. The hot-rolled steel sheet according to claim 1, wherein the hot -rolled steel sheet has a thickness of 1.6 mm or less. A method for manufacturing a hot-rolled steel sheet according to claim 1,
% by weight, C: 0.16-0.27%, Mn: 0.8-2.6%, Si: 0.05-0.3%, Al: 0.05% or less, Ti: 0.01 ~0.08%, B: 0.001 to 0.005%, Ca: 0.001 to 0.005%, N: 0.001 to 0.010%, the balance consisting of Fe and other inevitable impurities, Obtaining a thin slab having a thickness of 80 to 120 mm by continuously casting molten steel that satisfies the following equations 1 to 3;
rough rolling the thin slab to obtain a bar;
obtaining a _hot -rolled steel sheet by finish-rolling the bar so that the delivery side temperature of the finish rolling is Ar ₃ +10 ° C. to Ar ₃ +60 ° C.; Cooling and winding at Mf -50 ° C or less,
A method of manufacturing a hot-rolled steel sheet, wherein the steps are performed continuously.
[Relationship 1]
16≤100(C+Mn/100+B/10)≤28
[Relational expression 2]
1≦[(Al/27)×(N/14)]/[(Ti/48)×(B/11)]≦14
[Relational expression 3]
0.05≦[(Al/27)×(N/14)]/[(Ti/48)×(B/11)]/100(C+Mn/100+B/10)≦0.66
(However, the contents of the alloy components described in the above relational expressions 1 to 3 are weight percent.) 7. The method for producing a hot- rolled steel sheet according to claim 6, wherein the casting speed during the continuous casting is 4 to 8 mpm (m/min). 7. The method for producing a hot- rolled steel sheet according to claim 6, wherein the basicity of the mold flux during the continuous casting is 0.8 to 1.5. 7. The method for producing a hot-rolled steel sheet according to claim 6, wherein the secondary cooling specific water amount during the continuous casting is 1.5 to 2.5 L/kg. 7. The method for producing a hot- rolled steel sheet according to claim 6, wherein the temperature of the bar edge portion on the delivery side of the rough rolling during the rough rolling is 850 to 1000.degree. 7. The method of manufacturing a hot- rolled steel sheet according to claim 6, further comprising spraying cooling water onto the bar at a pressure of 200-300 bar after obtaining the bar. The method for producing a hot- rolled steel sheet according to claim 11 , wherein the cooling water has an overlap area ratio of 5 to 25% when the cooling water is injected. 7. The method for producing a hot- rolled steel sheet according to claim 6, wherein variation in rolling speed during the finish rolling is 50 mpm or less. The method for producing a hot- rolled steel sheet according to claim 6, wherein the temperature variation in the width direction of the hot-rolled steel sheet during the finish rolling is 50°C or less. 7. The method for producing a hot- rolled steel sheet according to claim 6, wherein the rolling speed during the finish rolling is 200-600 mpm. 7. The method for producing a hot- rolled steel sheet according to claim 6, wherein the interval between the cooling nozzles during cooling is 150 to 400 mm. 7. The method of manufacturing a hot- rolled steel sheet according to claim 6, further comprising pickling the wound hot-rolled steel sheet after the winding.

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