WO2018117575A1 - High-strength hot-rolled steel sheet having low deviation of material properties and excellent surface quality, and method for manufacturing same - Google Patents

Abstract

The purpose of the present invention is to provide a high-strength hot-rolled steel sheet and a method for manufacturing same, wherein, in order to overcome the problem of existing mini-mill processes, an endless rolling mode is used in a process that directly combines continuous casting and rolling, thereby providing a hot-rolled steel sheet which has excellent surface quality, has noticeably reduced a deviation of material properties in the transverse and longitudinal directions thereof, and has a tensile strength of about 780MPa.

Description

High strength hot rolled steel sheet with low material deviation and excellent surface quality and manufacturing method

The present invention relates to a high strength hot rolled steel sheet having a low material deviation and excellent surface quality, and a method of manufacturing the same.

Recently, steel sheets for automobiles are also facing new changes due to demands for strengthening safety standards, environmental regulations, and fuel economy. In order to adapt to these changes, major steelmakers are working with automakers to develop technologies for higher strength and lighter weight. Accordingly, in order to satisfy the weight reduction and high strength of the automobile body at the same time, high strength steel sheets of 590 MPa or more have been actively developed and applied. However, as the demand for high strength and light weight is gradually increased, the development of ultra high strength steel sheets of 780 MPa or more is required to realize ultra high strength and ultra light weight. In addition, since most automotive steel sheets are formed by press working, it is required to have uniform material properties along with low yield strength and high ductility.

Among the complex metamorphic steels, the so-called DP (Dual Phase) steel is mainly composed of two phases of ferrite and martensite, and is one of the representative steel grades having low yield strength.

Patent documents related to the manufacturing technology of such high strength hot rolled DP steel are Patent Documents 1 and 2, but these are all related to the method of manufacturing in the existing mill process, the problem that the material deviation in the production in the actual line largely in the width and length direction It is difficult to avoid. In addition, when manufacturing DP steel from conventional hot rolling, the final finish rolling speed is usually 400mpm or more, so the target material must be wound due to the manufacturing characteristics of DP steel, which must be wound at a temperature lower than Ms (martensite transformation start temperature). There is a problem that it is not easy to secure stably. In addition, in order to maintain a constant finish rolling temperature in the existing hot rolling, there is a problem in that a material deviation in the width and length directions is greatly generated as the rolling speed inevitably accelerates the tail portion.

On the other hand, a new steel manufacturing process, which is recently attracting attention, is a process for manufacturing a so-called thin slab (mini-mill process), which has a small temperature deviation in the width and length direction of the strip due to the characteristics of the process. It is attracting attention as a process with potential.

Patent document 3 relates to a method for producing tensile strength 590 MPa class hot rolled DP steel in a batch method in a mini mill process, and limits the final steel sheet thickness to 3.0 mm. The reason for this is that in the case of the existing mini mill process, the bar plate is wound in a coil box and pulled out so that this process is required every time a steel sheet is produced. And it is difficult to produce hot rolled coil (Coil) having a thickness of less than 3.0mm due to the poor flowability and very high risk of sheet breakage.

Therefore, in order to overcome the above-mentioned problems, in order to meet the increasing demand for high strength and light weight, the development of ultra-high strength steel sheet and its manufacturing method of 780 MPa or more (3.0 mm or less in thickness) is required.

(Prior art document)

(Patent Document 1) United States Patent Application Publication No. 4285741

(Patent Document 2) U.S. Patent Publication No.4325751

(Patent Document 3) Korean Unexamined Patent Publication No. 10-2012-0052022

One aspect of the present invention provides a high-strength hot-rolled steel sheet of 780MPa grade and its tensile strength which is excellent in surface quality and at the same time significantly reduced the width, longitudinal material deviation of the steel sheet using the continuous continuous rolling mode in the play-rolling direct connection process To provide.

In addition, the subject of this invention is not limited to the content mentioned above. The problem of the present invention will be understood from the general contents of the present specification, those skilled in the art will have no difficulty understanding the additional problem of the present invention.

One aspect of the present invention is by weight, C: 0.03-0.065%, Mn: 1.5-2.5%, Si: 0.1-0.5%, P: 0.01-0.05%, S: 0.01% or less, Cr: 0.1-0.6% Al: 0.05% or less, Ti: 0.01% to 0.05%, N: 0.001% to 0.010%, remaining Fe and other unavoidable impurities,

The microstructure is a high-strength hot rolled steel sheet having a low surface area deviation and excellent surface quality, including ferrite 50 to 70%, martensite 20 to 40%, and bainite 5 to 15%.

In addition, another aspect of the present invention is by weight, C: 0.03-0.065%, Mn: 1.5-2.5%, Si: 0.1-0.5%, P: 0.01-0.05%, S: 0.01% or less, Cr: 0.1 Continuous casting of molten steel containing -0.6%, Al: 0.05% or less, Ti: 0.01-0.05%, N: 0.001-0.010%, remaining Fe and other unavoidable impurities with a thin slab having a thickness of 60-120 mm;

Spraying coolant at a pressure of 150 bar or more to the thin slab to remove scale;

Roughly rolling the scaled thin slab so that the edge portion temperature of the bar plate is about 850 to 1000 ° C. at the rough rolling side;

Removing the scale by sequentially passing the bar plate through two rows of spraying cooling water at a pressure of 100 to 250 bar and one row of spraying cooling water at a pressure of 50 to 150 bar;

Obtaining a hot rolled steel sheet by finishing rolling the bar plate from which the scale is removed at a temperature range of Ar1 to Ar3; And

After cooling the hot rolled steel sheet for 3 to 8 seconds, cooling at a cooling rate of 200 ° C / s or more and winding up to 250 ° C or less; wherein each step is a material deviation is continuously performed and the surface quality is low It relates to a method for producing an excellent high strength hot rolled steel sheet.

In addition, the solution of the said subject does not enumerate all the characteristics of this invention. Various features of the present invention and the advantages and effects thereof can be understood in more detail with reference to the following specific embodiments.

According to the present invention, by using the continuous continuous rolling mode in the play-rolling direct connection process, not only the surface quality is excellent, but also the material deviation in the width and length direction of the steel sheet is remarkably reduced, and the tensile error is excellent and the thickness is 3.0 mm or less. Strength 780MPa class high strength hot rolled steel sheet can be manufactured.

In addition, the hot rolled steel sheet produced by the present invention is a thin material (3.0 mm or less in thickness) and the edge portion and the surface scale quality is good, it is possible to manufacture a high-quality PO material in the general hot-rolling pickling process, hot rolled material (more than 3.0 mm thick) It is differentiated from the existing mini mill process that can only produce, and it is excellent in price competition and can greatly improve added value.

In addition, the thin slab playing method can eliminate the reheating process in the existing mill, which can save energy and improve productivity, and can use the steel dissolved in scrap such as scrap in the electric furnace to improve the recycling of resources. Can be.

1 is a photograph taken with a scanning electron microscope (SEM) of the microstructure of Inventive Example 2.

FIG. 2 is a graph showing the ferrite grain size distribution of Inventive Example 2 measured using an EBSD (Electron BackScatter Diffraction). FIG.

3 is a photograph of the surface of the PO material of Inventive Example 2. FIG.

4 is a schematic diagram of a process using a continuous continuous rolling mode in the play-rolling direct connection process.

Hereinafter, preferred embodiments of the present invention will be described. However, embodiments of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below. In addition, the embodiments of the present invention are provided to more completely explain the present invention to those skilled in the art.

In the case of the existing mini mill process, the bar plate is wound on a coil box and pulled out so that this process is required every time a steel sheet is produced in a batch method. And it is not good mailing ability, the risk of plate breakage is very high, it is difficult to produce a hot rolled coil (Coil) of less than 3.0mm thick, and deeply studied to solve this problem.

As a result, by precisely controlling the alloy composition and manufacturing process, the surface quality is excellent by using the continuous rolling mode in the play-rolling direct connection process, and at the same time, the thickness of the material in the width and length direction of the steel sheet is significantly reduced to 3.0. It was confirmed that a high-strength hot-rolled steel sheet having a tensile strength of 780 MPa or less with a tensile strength of mm or less could be produced, and thus, the present invention was completed.

High strength hot rolled steel sheet with low material deviation and excellent surface quality

Hereinafter, a high strength hot rolled steel sheet having a low material deviation and excellent surface quality according to an aspect of the present invention will be described in detail.

High-strength hot-rolled steel sheet having a low material deviation and excellent surface quality according to an aspect of the present invention is a weight%, C: 0.03 to 0.065%, Mn: 1.5 to 2.5%, Si: 0.1 to 0.5%, P: 0.01 to 0.05% , S: 0.01% or less, Cr: 0.1 ~ 0.6%, Al: 0.05% or less, Ti: 0.01 ~ 0.05%, N: 0.001 ~ 0.010%, remaining Fe and other unavoidable impurities, and the microstructure is in the area fraction Ferrites 50-70%, martensite 20-40% and bainite 5-15%.

First, the alloy composition of the present invention will be described in detail. Hereinafter, the unit of each element content is weight%.

C: 0.03-0.065%

Carbon (C) is an important element to increase the strength of the steel sheet and to secure a composite structure composed of ferrite and martensite.

When the C content is less than 0.03%, it may be difficult to secure the strength targeted in the present invention. On the other hand, when the C content is more than 0.065%, molten steel can be leaked because the alloy steel is manufactured by high-speed playing, and a solidification cell having a non-uniform thickness may be formed, which may cause an operation accident. Therefore, it is preferable that C content is 0.03 to 0.065%.

Also, the lower limit of the C content may be 0.035%, and the lower limit may be 0.04%. In addition, the more preferable upper limit of C content may be 0.06%.

Mn: 1.5 ~ 2.5%

Manganese (Mn) is an element that has a very high effect of solid solution strengthening and promotes the formation of a complex structure composed of ferrite and martensite.

If the Mn content is less than 1.5%, it may be difficult to secure the strength targeted in the present invention. On the other hand, when the Mn content is more than 2.5%, weldability, hot rolling property, and the like may be inferior. In addition, excessive addition of Mn content reduces the delta-ferrite region at the temperature near the solidification, so that the apolytic reaction can occur even in the low C region. Molten steel spills can cause operational accidents. Therefore, the Mn content is preferably 1.5 to 2.5%.

In addition, the lower limit of the Mn content may be 1.6%, and the lower limit may be 1.7%. In addition, the more preferable upper limit of Mn content may be 2.4%, and a still more preferable upper limit may be 2.3%.

Si: 0.1 ~ 0.5%

Silicon (Si) is a useful element that can secure the ductility of the steel sheet. It is also an element that promotes ferrite formation and promotes martensite formation by encouraging C concentration into unmodified austenite.

When the Si content is less than 0.1%, it is difficult to secure the above-described effects sufficiently. On the other hand, when the Si content is more than 0.5%, red scale is generated on the surface of the steel sheet, and traces may remain on the surface of the steel sheet after pickling, thereby degrading the surface quality. Therefore, it is preferable that Si content is 0.1 to 0.5%.

In addition, the lower limit of Si content may be 0.12%, and the upper limit may be 0.4%.

P: 0.01 ~ 0.05%

Phosphorus (P) is an element showing the effect of strengthening the steel sheet.

If the P content is less than 0.01%, it is difficult to secure the effect. On the other hand, when the P content is greater than 0.05%, segregation may occur at grain boundaries and / or interphase boundaries, causing brittleness. Therefore, the content of P is preferably limited to 0.01 to 0.05%.

S: 0.01% or less

Sulfur (S) as impurities may segregate during MnS non-metallic inclusions and performance solidification in steel, causing hot cracks.

Therefore, the content should be controlled as low as possible, preferably at 0.01% or less. The lower limit of the S content does not need to be particularly limited, but excluding the S content to 0% may be excessive, so 0% may be excluded.

Cr: 0.1-0.6%

Chromium (Cr) is an element that improves hardenability and increases the strength of steel.

When the Cr content is less than 0.1%, the above effects are insufficient. On the other hand, when the Cr content is more than 0.6%, there is a problem that the ductility of the steel sheet is lowered. Therefore, it is preferable that Cr content is 0.1 to 0.6%.

Al: 0.05% or less

Aluminum (Al) may be concentrated on the surface of the steel sheet to degrade the plating property, while suppressing carbide formation to increase the ductility of the steel. On the other hand, in the case of thin slabs can be omitted the reheating process in the existing hot-dense mill can improve the energy saving and productivity, but the temperature of the slab surface or the edge portion may drop due to the cold cooling of the slab surface. This can result in excessive precipitation of AlN, resulting in inferior edge quality of the cast and / or bar plates due to high temperature ductility degradation.

Therefore, the content should be controlled as low as possible, preferably at 0.05% or less. The lower limit of the Al content does not need to be particularly limited, but excluding the Al content of 0% may be excessive, so 0% may be excluded.

Ti: 0.01% to 0.05%

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 reduces the amount of AlN precipitates by removing the solid solution N through the formation of TiN near the solidification temperature, thereby preventing the reduction of high-temperature ductility. Therefore, precise control is necessary because Ti is a very useful element to solve the surface and / or edge quality problems and strength obtained in thin slab high-speed performance.

When the Ti content is less than 0.01%, the above effects are insufficient. On the other hand, when the Ti content is more than 0.05%, it is possible to increase the manufacturing cost and lower the ductility of the ferrite. Therefore, the Ti content is preferably 0.01 to 0.05%.

N: 0.001-0.010%

Nitrogen (N) is an austenite stabilizing and nitride forming element.

If the N content is less than 0.001%, the austenite stabilization effect is insufficient. On the other hand, when the N content is more than 0.010%, it increases the precipitation strengthening effect by reacting with the precipitate-forming element, but may cause a sharp drop in ductility. In addition, the surface and edge quality may be inferior due to excessive precipitation such as AlN due to the cooling of the slab in thin slab high speed performance.

Therefore, N content is preferably 0.001 to 0.010%.

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

In this case, in addition to the alloy composition described above, as a tramp element, one or more of Cu, Ni, Mo, Sn, and Pb may be included, and the sum thereof may be 0.2 wt% or less.

The tramp element is an impurity element derived from scrap used as a raw material in the steelmaking process, and when the sum is more than 0.2%, the surface crack of the thin slab and the surface quality of the hot rolled steel sheet may be degraded.

In addition, not only satisfies the above-described alloy composition, Ceq represented by the following formula (1) may be 0.14 ~ 0.24.

Formula (1): Ceq = C + Si / 30 + Mn / 20 + 2P + 3S

(In the formula (1), each element symbol is a value representing each element content in weight%.)

Equation (1) is a component relational formula for securing weldability of the steel sheet. In the present invention, by managing the Ceq value of 0.14 to 0.24, excellent spot weldability can be ensured and excellent mechanical properties can be given to the welded portion. .

If Ceq is less than 0.14, there is a problem that it is difficult to secure the target tensile strength due to the low hardenability. On the other hand, if Ceq is greater than 0.24, the weldability may deteriorate and the physical properties of the weld portion may deteriorate.

Hereinafter, the microstructure of the hot rolled steel sheet according to the present invention will be described in detail.

The microstructure of the hot rolled steel sheet according to the present invention includes 50 to 70% of ferrite, 20 to 40% of martensite, and 5 to 15% of bainite.

When the ferrite fraction of the microstructure of the hot rolled steel sheet according to the present invention is more than 70%, it is difficult to secure the target strength, and if less than 50%, the fraction of the remaining martensite and bainite structure is increased to secure ductility. There is difficulty. In addition, when the martensite fraction is more than 40%, the strength is too high to secure ductility, and less than 20% may be difficult to secure the target strength.

In addition, the reason for including a part of bainite in the microstructure of the hot-rolled steel sheet is as follows.

In the case of general DP (Dual Phase) steel composed only of two phases of ferrite and martensite, the martensite fraction has a high martensite tempering in the heat affected zone during welding, causing softening, which causes a decrease in strength. . When the bainite structure instead of martensite is secured to some extent, this problem is not only improved, but the strength and processability of the bainite structure can be secured simultaneously. In addition, in the case of DP steel, there is a problem in that workability is poor because fracture occurs first at an interface due to a difference in interface strength between two phases of soft ferrite and hard martensite. However, bainite is a structure having a medium strength between ferrite and martensite, and when bainite is formed at the interface between these two tissues, the above problems are improved, thereby improving workability.

If the bainite area fraction is less than 5%, the above effects are insufficient. On the other hand, if the content is greater than 15%, the strength may be too high to secure ductility.

On the other hand, in the case of conventional hot rolling, ferrite is inevitably accelerated as the rolling speed inevitably accelerates the rolling speed in the tail part to maintain the finish rolling temperature, and the finish rolling temperature is much higher than that of Ar3. It is difficult to precisely control the austenite fraction. As a result, it is difficult to uniformly obtain the final structures ferrite, martensite and bainite. In addition, the finish rolling rate is too fast, it is difficult to hit the 350 ~ 550 ℃ temperature that occurs bainite transformation due to temperature variation, it is difficult to secure the bainite structure.

In this case, the ferrite grains may have an average size of 5 μm or less as measured by a circle equivalent diameter. In order to secure the strength and processability at the same time through securing the ferrite structure having fine grains, when the size of the ferrite grains exceeds 5㎛ it may be difficult to secure the target strength and processability. More preferably, it may be 3 micrometers or less.

On the other hand, in the case of conventional hot rolling, the grain size exceeds about 5 μm because the finish rolling temperature is finish-rolled at a temperature around 900 ° C., which is much higher than the normal Ar3 temperature (810 to 850 ° C.) to secure the mail flow during finish rolling. Has organization. However, in the play-roll direct connection process according to the present invention, it is possible to secure and control uniform temperature by isothermal and isothermal rolling due to the characteristics of the process, thereby securing the final structure uniformly, and to finish rolling at a lower temperature than Ar3. Fine control

Furthermore, the thickness of the hot rolled steel sheet of the present invention may be 3.0mm or less. This is because, unlike the conventional mini mill process, which can produce only hot rolled material (more than 3.0 mm in thickness), the hot rolled steel sheet can be produced in a thickness of 3.0 mm or less according to the manufacturing method of the present invention. More preferably, the thickness may be 2.0 mm or less.

In addition, the hot rolled steel sheet of the present invention may have a tensile strength of 780 MPa or more, an elongation of 15% or more, and a material deviation of the tensile strength of 15 MPa or less.

Manufacturing method of high strength hot rolled steel sheet with low material deviation and excellent surface quality

Hereinafter, a method of manufacturing a high strength hot rolled steel sheet having a low material deviation and excellent surface quality according to another aspect of the present invention will be described in detail.

According to another aspect of the present invention, there is provided a method of manufacturing a high strength hot rolled steel sheet having a low material deviation and excellent surface quality, comprising: continuously casting molten steel having the above-described alloy composition into a thin slab having a thickness of 60 to 120 mm; Spraying coolant at a pressure of 150 bar or more to the thin slab to remove scale; Roughly rolling the scaled thin slab so that the edge portion temperature of the bar plate is about 850 to 1000 ° C. at the rough rolling side; Removing the scale by sequentially passing the bar plate through two rows of spraying cooling water at a pressure of 100 to 250 bar and one row of spraying cooling water at a pressure of 50 to 150 bar; Obtaining a hot rolled steel sheet by finishing rolling the bar plate from which the scale is removed at a temperature range of Ar1 to Ar3; And air-cooling the hot rolled steel sheet for 3 to 8 seconds, followed by cooling at a cooling rate of 200 ° C./s or more and winding at 250 ° C. or less. Each of the above steps is performed continuously.

When the above steps are performed continuously means that the continuous rolling mode is used in the play-rolling direct connection process.

The new steel manufacturing process, which is attracting attention recently, is a process of using the so-called thin slab (mini mill process), which is a direct rolling-rolling process, and has a good material deviation due to the small temperature deviation in the width and length directions of the strip. It is attracting attention as a process with the potential to manufacture tissue steel.

In such a performance-rolling direct connection process, there is a conventional batch mode and a newly developed continuous continuous rolling mode.

In case of batch mode, in order to compensate for the difference between the playing speed and rolling speed, the coil is rolled up in the coil box before the finishing mill and finish rolling is performed. Problems such as breakage may occur.

In the continuous continuous rolling mode, unlike the batch mode, there is no winding process before finishing rolling, so the problem of the batch mode is solved, but more precise control is required to compensate for the difference between the playing speed and the rolling speed.

Figure 4 shows an example of a process using the continuous continuous rolling mode in the play-rolling direct connection process. In the continuous casting machine 100, a thin slab (a) having a thickness of 50 to 150 mm is manufactured, and there is no coil box between the roughing mill 400 and the finishing mill 600, so that the steel sheet can be continuously rolled to obtain a good flowability. The risk of plate breakage is very low, making it possible to produce thin films with a thickness of less than 3.0mm. Rough mill scale breaker (RSB) 300 in front of rough mill 400 and finishing mill scale breaker 500 in front of finishing mill 600 (FSB) for easy surface scale removal. When pickling hot rolled steel plate in post process, it is possible to produce PO (Pickled & Oiled) material with excellent surface quality. In addition, in the finish rolling step, isothermal isothermal rolling is possible with a difference in rolling speed of 5% or less in one steel sheet, so that the steel plate width and longitudinal temperature variation are remarkably low and precisely cooled in the run out table (ROT). The controllable steel sheet can be manufactured with excellent material variation.

Hereinafter, each step will be described in detail.

Continuous casting step

The molten steel having the alloy composition described above is continuously cast into a thin slab having a thickness of 60 to 120 mm.

When the thickness of the thin slab exceeds 120mm, not only the high speed casting is difficult, but also the rolling load increases during rough rolling, and when the thickness of the slab is less than 60mm, a temperature drop of the cast slab rapidly occurs to form a uniform structure. In order to solve this problem, it is possible to additionally install a heating device, but this is a factor to improve the production cost, it is desirable to exclude as possible. Therefore, the thickness of the thin slab is limited to 60 ~ 120mm.

At this time, the casting speed of the continuous casting may be 4 ~ 8mpm.

The reason why the casting speed is more than 4mpm is because the high-speed casting and the rolling process are connected, and a certain casting speed is required to secure a target rolling temperature. In addition, when the casting speed is low, there is a risk of segregation from the cast, when this segregation is not only difficult to secure strength and workability, but also increases the risk of material deviation in the width direction or longitudinal direction. However, if the casting speed exceeds 8mpm, the success rate of operation may be reduced by the instability of molten steel. Therefore, the casting speed of the continuous casting is preferably 4 ~ 8mpm.

Thin slab descaling step

Cooling water is sprayed to the heated thin slab at a pressure of 150 bar or more to remove scale. For example, the surface scale thickness may be removed to 300 μm or less by spraying a cooling water of 40 ° C. or less at a pressure of 150 bar or more from a rough mill scale breaker (RSB) nozzle.

When the pressure is less than 150bar, a large number of arithmetic scales and the like remain on the surface of the thin slab, and the surface quality may be inferior after pickling.

Rough rolling stage

The thin slab from which the scale is removed is roughly rolled on the rough rolling side so that the edge temperature of the bar plate is 850 to 1000 ° C. to obtain a bar plate. For example, it can be rough rolling in a roughing mill consisting of 2 to 5 stands.

When the edge portion temperature is less than 850 ° C., a large amount of AlN precipitates are generated, and thus there is a problem in that edge crack generation sensitivity is very high due to the decrease in high temperature ductility. On the other hand, if the edge temperature is more than 1000 ℃ the temperature of the thin slab core is too high, there is a possibility that a large number of arithmetic scale occurs, the surface quality is inferior after pickling.

At this time, the rough rolling may be performed so that the surface temperature of the thin slab is 1000 ~ 1200 ℃ at the entrance side of the rough rolling.

If the surface temperature of the thin slab at the inlet side of the roughing mill is less than 1000 ° C., there is a possibility that cracks may occur at the edge of the bar plate during the rough rolling load increase and the rough rolling process, and this may cause an edge defect of the hot rolled steel sheet. If the slab surface temperature exceeds 1200 ° C., problems such as deterioration of the hot rolled surface due to the remaining hot rolled scale may occur.

On the other hand, the rough rolling can be performed so that the cumulative reduction rate is 60 ~ 90%. The higher the rolling reduction rate during rough rolling, the more uniform the microscopic distribution of Mn, Si, Cr, etc., which are important elements for the production of high strength steel, and the smaller the temperature gradient in the width and thickness direction of the strip. Do. However, when the cumulative reduction ratio is less than 60%, the above-described effects are insufficient, and when the cumulative reduction ratio is greater than 90%, the rolling deformation resistance is greatly increased, thereby increasing the manufacturing cost.

Bar Plate Descaling Step

The bar plate is sequentially removed through a first row of spraying the coolant at a pressure of 50 to 150 bar and two rows of spraying the coolant at a pressure of 100 to 250 bar to remove scale. For example, the surface scale thickness can be removed up to 50 μm using a single row and two row nozzles of a Finishing Mill Scale Breaker (FSB) prior to finishing rolling the bar plate.

When the pressure of the first row and the second row nozzles is less than 50 bar and 100 bar, respectively, the scale is insufficient to be removed, and a large amount of fusiform and scale scales are generated on the surface of the steel sheet after finishing rolling, resulting in inferior surface quality after pickling. On the other hand, when the pressure of the first row nozzle is more than 150bar or the pressure of the second row nozzle is more than 250bar, the finish rolling temperature is too low to obtain an effective austenite fraction, it is difficult to secure the target tensile strength.

In addition, since it is difficult to remove the scale sufficiently with only one row of nozzles, and a fusiform scale, which is a fatal defect in the product, may occur during finishing rolling, it is preferable to remove the scale by using both the first row and the second row nozzles as described above.

Finishing rolling steps

The bar plate from which the scale is removed is finish rolled at a temperature range of Ar1 to Ar3 to obtain a hot rolled steel sheet. For example, it can finish-roll in the finishing mill which consists of 3-6 stands.

In the present invention, a high fraction of precipitates is formed in the finish rolling process, thereby reducing the grain size by reducing the grain strengthening effect by reducing the fraction of precipitates to be precipitated finely at a low temperature in the temperature range of Ar1 to Ar3. To compensate with an increase in strengthening effect.

In the case of metamorphic steel, it is important to concentrate the austenite stabilizing elements such as C and Mn on the unmodified austenite in order to improve the strength and ductility at the same time. As a result, the martensite is more stabilized and the ferrite is cleansed, even in the same component.

On the other hand, when the finishing rolling temperature is lower than Ar3 in the conventional blast furnace mill and mini-mill process, there is a problem in the rolling sheetability, but in the performance-rolling direct connection process according to the present invention, because it is rolled at isothermal and constant velocity in the characteristics of the process, No problem, low temperature rolling is possible at the temperature of Ar1 ~ Ar3.

At this time, the finish rolling may be performed so that the sheet speed is 200 ~ 600mpm, the thickness of the hot rolled steel sheet is 3.0mm or less.

When the finishing rolling speed is more than 600mpm, an operation accident such as a panda stage may occur, and isothermal and constant isothermal rolling may be difficult, so that a uniform temperature may not be secured and material deviation may occur. On the other hand, when less than 200mpm it may be difficult to secure the finish rolling temperature because the finish rolling speed is too slow.

In addition, the finish rolling may be performed such that 20-40% of ferrite and 60-80% of austenite are formed in an area fraction.

As a result of the repetitive experiment, when the ferrite fraction exceeds 40%, the austenite fraction is relatively low, so that the martensite and bainite fractions are lowered after cooling, thereby making it difficult to secure the target tensile strength. On the other hand, when the ferrite fraction is less than 20%, the austenite fraction is too large, so that the martensite and bainite fraction after phase transformation may be difficult to secure the elongation.

At this time, the finish rolling can be performed so that the rolling speed difference in one bar plate is 10% or less.

High-strength steel of 780MPa class for the purpose of the present invention is very likely to change the material properties according to the deformation rate during the finish rolling because the formation of the transformation structure as a reinforcing mechanism. In other words, if the difference in rolling speed exceeds 10% in a finishing mill consisting of a plurality of stands, it is difficult to obtain a uniform cooling rate and a target winding temperature in a subsequent run out table, and thus the width or length of the strip. This may cause a large material deviation in the direction.

Cooling and winding stage

After cooling the hot rolled steel sheet for 3 to 8 seconds, it is cooled at a cooling rate of 200 ° C / s or more and wound up to 250 ° C or less.

If the air cooling time is less than 3 seconds, the concentration of C to the residual austenite is insufficient and the time for ferrite transformation is insufficient, which increases the risk of deterioration of elongation. Not only is there difficulty in securing strength, but also the length of the installation may be long or productivity may be reduced.

If the cooling rate is slower than 200 ° C / sec is promoted ferrite transformation and cementite is difficult to obtain the desired material. In addition, when the coiling temperature is 250 ° C or higher, it is difficult to obtain martensite structure, and martensite obtained by cooling may be auto tempered, thereby making it difficult to obtain a target tensile strength.

At this time, the pickled hot rolled steel sheet may further comprise the step of obtaining a PO material.

In the present invention, since the scale is sufficiently removed in the thin slab and bar plate descaling step, it is possible to obtain a PO material having excellent surface quality even by general pickling treatment. Therefore, the pickling treatment which can be used in the present invention is not particularly limited since it can be applied as long as it is a treatment method generally used in the hot acid pickling process.

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

The molten steel having the component composition shown in Table 1 below was prepared.

Inventive Examples 1 and 2 and Comparative Examples 1 to 8 were continuously cast 96 mm thick thin slabs, and then subjected to the manufacturing conditions shown in Table 2, followed by 2.0 mm thick hot rolled steel sheet in continuous continuous rolling mode in a performance-rolling direct connection process. Was prepared. In the case of the prior art example 1, 3.2 mm thick hot rolled steel sheet was manufactured in a batch mode by applying the manufacturing conditions shown in Table 2.

After pickling the prepared hot-rolled steel sheet to obtain PO material, the microstructure, yield strength (YS), tensile strength (TS), elongation (EL), material deviation of tensile strength (△ TS), edge crack occurrence and surface The quality was measured and listed in Table 3 below.

In addition, Ar1 and Ar3 temperature in Table 2 is the value computed using the commercial thermodynamic software JmatPro V-8.

The area fractions of ferrite (F), martensite (M) and bainite (B) were measured using a scanning electron microscope (SEM).

Ferrite Grain Size (FGS) was measured at random diameters of 10 spots at 3,000x magnification by using EBSD (Electron BackScatter Diffraction), and then measured by the equivalent diameter using Image-Plus Pro software. One average value is listed.

Tensile strength is the value obtained by taking JIS 5 specimens at the width W / 4 point in the rolling direction. Material deviation is the maximum value minus the minimum value of the tensile strength measured in the width and length direction of the coil. will be.

Edge cracks were visually checked first at the bar plate and the coil, and secondly, using the SDD (Surface Defect Detector) device, which is a surface defect detector.

Evaluation criteria for the surface quality of the PO material is as follows.

○: glossiness width direction average deviation is 20% or less

X: glossiness width direction average deviation exceeds 20%

division	Steel grade	Alloy element (wt%)									Formula (1)
		C	Mn	Si	P	S	Ti	Al	Cr	N
Invention steel	A	0.050	1.91	0.18	0.018	0.0006	0.027	0.018	0.40	0.0064	0.19
Invention steel	B	0.050	1.91	0.19	0.015	0.0009	0.020	0.020	0.42	0.0068	0.18
Comparative steel	C	0.051	1.90	0.60	0.016	0.0008	0.021	0.019	0.41	0.0068	0.19
Comparative steel	D	0.050	1.86	0.18	0.020	0.0011	0.020	0.021	0.08	0.0053	0.17
Comparative steel	E	0.054	1.85	0.08	0.024	0.0008	0.020	0.022	0.42	0.0064	0.20
Conventional Steel	F	0.070	1.40	0.70	0.030	0.0030	-	0.040	-	0.0070	0.23

In Table 1, Equation (1) is Ceq = C + Si / 30 + Mn / 20 + 2P + 3S, each element symbol in the relation is a value representing each element content in weight%.

division	Steel grade	RSB (Bar)	FSB (Bar)		Rough rolling temperature (℃)		Finish rolling temperature (℃)	Ar3 (℃)	Ar1 (℃)	Air Cooling Time (sec)	Cooling rate (℃ / sec)	Winding temperature (℃)
			1 row	2 rows	Entrance surface	Exit edge
Inventive Example 1	A	191	102	203	1052	890	785	815	640	5.0	335	145
Inventive Example 2	B	180	106	202	1054	896	786	816	640	5.1	326	151
Comparative Example 1		180	99	202	1054	896	786			10.5	331	161
Comparative Example 2		180	106	202	1054	879	786			5.1	80	321
Comparative Example 3		180	110	202	1054	810	786			5.1	323	169
Comparative Example 4		10	109	210	1049	889	780			5.2	310	171
Comparative Example 5		191	15	19	1041	892	791			5.0	326	169
Comparative Example 6	C	189	105	200	1049	889	786	830	650	5.3	321	171
Comparative Example 7	D	209	101	200	1038	891	787	815	640	4.8	350	142
Comparative Example 8	E	212	99	209	1031	893	788	815	640	3.0	350	132
Conventional Example 1	F	-	-	-	1080	-	780	-	-	-	70	200

In Table 2, RSB (Roughing Mill Scale Breaker, rough rolling scale brake) is the coolant injection pressure before the rough rolling, FSB (Finishing Mill Scale Breaker, finish rolling scale brake) is the cooling water injection pressure after rough rolling.

division	Steel grade	Microstructure (area%)			FGS (μm)	YS (MPa)	TS (MPa)	△ TS (MPa)	EL (%)	Edge cracks occur	PO material surface quality
		F	M	B
Inventive Example 1	A	63	28	9	2.01	567	818	11	17	X	○
Inventive Example 2	B	62	29	9	1.95	572	821	12	16	X	○
Comparative Example 1		73	20	7	2.87	495	721	16	19	X	○
Comparative Example 2		80	16	4	2.58	425	610	18	25	X	○
Comparative Example 3		61	29	10	2.02	561	819	14	16	○	○
Comparative Example 4		63	29	8	1.98	565	815	15	17	X	X
Comparative Example 5		61	28	11	2.01	555	810	13	17	X	X
Comparative Example 6	C	71	21	8	2.56	521	776	15	21	X	X
Comparative Example 7	D	73	24	3	2.10	485	720	16	22	X	○
Comparative Example 8	E	44	50	6	1.96	717	939	23	10	X	○
Conventional Example 1	F	-				-	608	20	30	-	-

Inventive examples 1 and 2 satisfying all the conditions presented in the present invention satisfy the target tensile strength (780 MPa or more) and elongation (15% or more), and it can be seen that both the edge surface quality and the surface quality of the PO material are excellent. have.

1 is a photograph taken with a scanning electron microscope (SEM) of the microstructure of Inventive Example 2. It is confirmed that ferrite (F) and martensite (M) are composed of a main phase, and some bainite (B) is present.

FIG. 2 is a graph showing the ferrite grain size distribution of Inventive Example 2 measured using EBSD. FIG. From this result, it can be confirmed that the crystal grains of 1-3 micrometers or less are minutely distributed, and the average grain size is 1.95 micrometers.

Figure 3 shows the surface photograph of the PO material obtained by pickling the hot rolled steel sheet of Inventive Example 2, it can be confirmed that the surface quality is excellent.

Comparative Examples 1 to 5 were satisfied with the alloy composition proposed in the present invention, but did not satisfy the manufacturing conditions did not secure the target material.

Comparative Examples 1 and 2 did not satisfy the cooling conditions, the strength was inferior. In Comparative Example 3, the edge quality was inferior when the rough rolling side edge temperature was less than that. Comparative Examples 4 and 5 did not satisfy the RSB or FSB pressure presented in the present invention, the surface quality was inferior.

In Comparative Example 6, all of the operating conditions were the same as in Inventive Example 2, but the Si content was exceeded, and thus, the target tensile strength was not satisfied, and the PO material surface quality was also inferior.

Comparative Example 7 did not satisfy the target tensile strength when the Cr content was low. In Comparative Example 8, the elongation was inferior when the Si content was low.

Although described with reference to the embodiments above, those skilled in the art will understand that the present invention can be variously modified and changed without departing from the spirit and scope of the invention as set forth in the claims below. Could be.

(Explanation of the sign)

a: slab b: coil

100: continuous casting machine 200: heater

300: Roughing Mill Scale Breaker (RSB)

400: roughing mill

500: Finishing Mill Scale Breaker (FSB)

600: finish rolling mill 700: runout table

800: high speed shear 900: winder

Claims (16)

1. By weight%, C: 0.03-0.065%, Mn: 1.5-2.5%, Si: 0.1-0.5%, P: 0.01-0.05%, S: 0.01% or less, Cr: 0.1-0.6%, Al: 0.05% or less , Ti: 0.01% to 0.05%, N: 0.001% to 0.010%, remaining Fe and other unavoidable impurities,

   High-strength hot-rolled steel sheet with excellent surface quality and low material deviation, including microstructure, ferrite 50-70%, martensite 20-40%, and bainite 5-15%.
2. The method of claim 1,

   The hot rolled steel sheet further comprises at least one of Cu, Ni, Mo, Sn, and Pb as a tramp element, and has a material deviation of less than 0.2% by weight in total and excellent surface quality.
3. The method of claim 1,

   The hot rolled steel sheet is a high strength hot rolled steel sheet having a low Ceq of 0.10 to 0.24 and a good surface quality.

   Formula (1): Ceq = C + Si / 30 + Mn / 20 + 2P + 3S

   (In the above relations, each element symbol represents a value expressed in weight percent of each element.)
4. The method of claim 1,

   The grain of the ferrite is a high-strength hot-rolled steel sheet having a low material deviation having an average size of 5 μm or less and excellent surface quality measured by a circular equivalent diameter.
5. The method of claim 1,

   The hot rolled steel sheet is a high strength hot rolled steel sheet with less material deviation less than 3.0mm and excellent surface quality.
6. The method of claim 1,

   The hot rolled steel sheet has a tensile strength of 780MPa or more, an elongation of 15% or more, and a material deviation of the tensile strength of 15MPa or less.
7. By weight%, C: 0.03-0.065%, Mn: 1.5-2.5%, Si: 0.1-0.5%, P: 0.01-0.05%, S: 0.01% or less, Cr: 0.1-0.6%, Al: 0.05% or less Continuous casting of molten steel containing Ti: 0.01 to 0.05%, N: 0.001 to 0.010%, remaining Fe and other unavoidable impurities with a thin slab of 60 to 120 mm in thickness;

   Spraying coolant at a pressure of 150 bar or more to the thin slab to remove scale;

   Roughly rolling the scaled thin slab so that the edge portion temperature of the bar plate is about 850 to 1000 ° C. at the rough rolling side;

   Removing the scale by sequentially passing the bar plate through two rows of spraying cooling water at a pressure of 100 to 250 bar and one row of spraying cooling water at a pressure of 50 to 150 bar;

   Obtaining a hot rolled steel sheet by finishing rolling the bar plate from which the scale is removed at a temperature range of Ar1 to Ar3; And

   After cooling the hot rolled steel sheet for 3 to 8 seconds, cooling at a cooling rate of 200 ° C / s or more and winding up to 250 ° C or less; wherein each step is a material deviation is continuously performed and the surface quality is low Excellent high strength hot rolled steel sheet production method.
8. The method of claim 7, wherein

   The molten steel includes at least one of Cu, Ni, Mo, Sn, and Pb as a tramp element, and has a material deviation of 0.2 wt% or less in total and has a high surface quality, and has a high surface quality.
9. The method of claim 7, wherein

   The molten steel is a method of producing a high-strength hot rolled steel sheet having a low Ceq is 0.14 ~ 0.24 material deviation and excellent surface quality.

   Ceq = C + Si / 30 + Mn / 20 + 2P + 3S

   (In the above relations, each element symbol represents a value expressed in weight percent of each element.)
10. The method of claim 7, wherein

    The casting speed of the continuous casting is a method of producing a high strength hot rolled steel sheet with a low material deviation of 4 ~ 8mpm and excellent surface quality.
11. The method of claim 7, wherein

    The rough rolling is a method of producing a high-strength hot rolled steel sheet having a low material deviation and excellent surface quality to be performed so that the surface temperature of the thin slab at the rough rolling entrance side is 1000 ~ 1200 ℃.
12. The method of claim 7, wherein

    The rough rolling is a method of producing a high strength hot rolled steel sheet with a low material deviation and excellent surface quality to be carried out so that the cumulative reduction rate is 60 ~ 90%.
13. The method of claim 7, wherein

    The finish rolling is a method of producing a high-strength hot rolled steel sheet having a sheet thickness of 200 ~ 600mpm, the thickness of the hot rolled steel sheet is less than 3.0mm and the material deviation is small and excellent surface quality.
14. The method of claim 7, wherein

    The finish rolling is a method of producing a high strength hot rolled steel sheet having a low surface area and excellent surface quality of the ferrite 20 to 40%, austenitic 60 to 80% by area fraction.
15. The method of claim 7, wherein

    The finish rolling is a method of producing a high strength hot rolled steel sheet with a low material deviation and excellent surface quality, so that the rolling speed difference in one bar plate is 10% or less.
16. The method of claim 7, wherein

    The method of manufacturing a high strength hot rolled steel sheet having a low material deviation and excellent surface quality further comprising the step of pickling the wound hot rolled steel sheet to obtain a Pickled & Oiled (PO) material.

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