KR20180074148A - High strength hot rolled steel sheet having low deviation of mechanical property and excellent surface quality and method for manufacturing the 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 the 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 tensile strength of about 780 MPa.

Description

TECHNICAL FIELD [0001] The present invention relates to a high-strength hot-rolled steel sheet having a low material deviation and an excellent surface quality, and a method of manufacturing the same. BACKGROUND OF THE INVENTION 1. Field of the Invention [0002]

The present invention relates to a high-strength hot-rolled steel sheet having a small material deviation and excellent surface quality, and a manufacturing method thereof.

Recently, steel plates for automobiles are also facing new changes due to the reinforcement of safety standards and environmental regulations and the demand for improved fuel economy. In order to adapt to these changes in the times, major steel makers are making efforts to develop technologies for high strength and light weight with automobile manufacturers. Accordingly, a high strength steel sheet having a strength of 590 MPa or more has been actively developed and applied in order to satisfy both weight reduction and high strength of automobile body. However, as the demand for higher strength and lighter weight gradually increases, development of ultra high strength steel sheet of 780 MPa class or more is required in order to realize ultra high strength and light weight. Further, since most steel sheets for automobiles are formed by press working, it is required to have a low yield strength, high ductility and uniform material properties.

The dual phase steel (DP), which is composed of two phases of ferrite and martensite, is one of the representative steels with low yield strength.

Patent Documents 1 and 2 are related to the manufacturing technology of such a high-strength hot-rolled DP steel, but all of these methods relate to a method of manufacturing in a conventional mill process, in which a material deviation Is difficult to avoid. Further, when DP steel is manufactured from a conventional hot-rolled mill, since the final finish rolling speed is faster than 400 mpm, the target material is to be produced at a low temperature below Ms (martensitic transformation starting temperature) There is a problem in that it is not easy to securely secure it. Further, in order to keep the finishing rolling temperature constant in the conventional hot-rolled mill, there is a problem that material variations in the width and length direction occur largely as the rolling speed is necessarily accelerated in the tail portion.

On the other hand, a fabrication process (mini-mill process) using a so-called thin slab, which is a new steel manufacturing process that has recently been attracting attention, has been widely used in the manufacture of a textured steel having a good material deviation since the width of the strip and the temperature deviation in the longitudinal direction are small It is attracting attention as a process with potential to be able to.

Patent Document 3 relates to a method of manufacturing a hot-rolled DP steel having a tensile strength of 590 MPa by a batch method in a minilmill process, and the final steel sheet thickness is limited to 3.0 mmt. The reason for this is that in the case of the existing mini-mill process, the bar plate is coiled in a coil box, and this process must be performed every time a single steel plate is produced by a disposing arrangement. Therefore, And it is difficult to produce a hot-rolled coil (Coil) having a thickness of 3.0 mm or less because of a high risk of sheet breakage.

Therefore, in order to overcome the above-mentioned problems and to meet increasing demands for higher strength and lighter weight, development of ultra high strength steel sheet having a strength of 780 MPa or more (3.0 mmt or less) and its manufacturing method is required.

US-A-4285741 U.S. Patent Publication No. 4325751 Korean Patent Publication No. 10-2012-0052022

In one aspect of the present invention, there is provided a high strength hot-rolled steel sheet having a surface quality of 780 MPa in which the width and lengthwise material deviations of the steel sheet are remarkably reduced by using continuous continuous rolling mode in the performance- .

On the other hand, the object of the present invention is not limited to the above description. It will be understood by those of ordinary skill in the art that there is no difficulty in understanding the additional problems of the present invention.

An aspect of the present invention is a steel sheet comprising, by weight, 0.03 to 0.06% of C, 1.5 to 2.5% of Mn, 0.1 to 0.5% of Si, 0.01 to 0.05% of P, 0.01 to 0.01% of S, , Al: not more than 0.05%, Ti: 0.01 to 0.05%, N: 0.001 to 0.010%, balance Fe and other unavoidable impurities,

The present invention relates to a high-strength hot-rolled steel sheet having a small area deviation and an excellent surface quality including an area fraction of 50 to 70% of ferrite, 20 to 40% of martensite and 5 to 15% of bainite.

In another aspect of the present invention, there is provided a ferritic stainless steel comprising 0.03 to 0.06% of C, 1.5 to 2.5% of Mn, 0.1 to 0.5% of Si, 0.01 to 0.05% of P, 0.01% Continuously casting molten steel containing 0.6% to 0.6% Al, 0.05% or less Al, 0.01-0.05% Ti, 0.001-0.010% N, and the balance Fe and other unavoidable impurities in a thin slab having a thickness of 60-120 mm;

Removing the scale by injecting cooling water into the thin slab at a pressure of 150 bar or more;

Rolling the thin slab from which the scale has been removed from the roughly rolled side so that the edge temperature of the bar plate is 850 to 1000 ° C to obtain a bar plate;

Sequentially passing the bar plate through two rows for spraying cooling water at a pressure of 100 to 250 bar and one column for spraying cooling water at a pressure of 50 to 150 bar;

Finishing the scraped bar plate in a temperature range of Ar1 to Ar3 to obtain a hot rolled steel sheet; And

Cooling the hot-rolled steel sheet for 3 to 8 seconds, cooling at a cooling rate of 200 DEG C / s or more, and winding at 250 DEG C or less, wherein each of the steps has a small material deviation and a surface quality Strength hot-rolled steel sheet.

In addition, the solution of the above-mentioned problems does not list all the features of the present invention. The 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, not only the surface quality is excellent by using the continuous continuous rolling mode in the performance-rolling direct rolling process but also the material deviation in the width and length direction of the steel sheet is remarkably reduced, and the tensile strength, Strength 780 MPa grade high-strength hot-rolled steel sheet can be manufactured.

In addition, the hot-rolled steel sheet produced by the present invention is superior in the quality of the edge portion and surface scale due to the thinness (3.0 mmt or less) and is capable of producing advanced PO by general hot- It is possible to differentiate it from the existing mini-milling process, which is superior in terms of price competition, and can significantly improve the added value.

In addition, it is possible to omit the reheating process in the existing mill through the thin slab-making method, thereby saving energy and improving the productivity. In the electric furnace, it is possible to use steel in which scrap of scrap iron is melted, thereby enhancing the recyclability of resources .

1 is a photograph of a microstructure of Inventive Example 2 taken by a scanning electron microscope (SEM).
2 is a graph showing the ferrite grain size distribution of Inventive Example 2 measured using EBSD (Electron Back Scatter Diffraction).
3 is a photograph of the surface of the PO material of Inventive Example 2. FIG.
4 is a schematic view of a process (CEM) using continuous continuous rolling mode in a performance-rolling direct process.

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

The inventors of the present invention have found that, in the case of a conventional minilmill process, the bar plate is wound on a coil box and wound in a winding manner so as to produce a single steel sheet. Therefore, And it is difficult to produce a hot-rolled coil (Coil) having a thickness of 3.0 mm or less because of a high risk of plate breakage.

As a result, by precisely controlling the alloy composition and the manufacturing process, it is possible to obtain excellent surface quality by using the continuous continuous rolling mode in the performance-to-rolling direct process, and to reduce the material deviation in the width and length direction of the steel sheet by 3.0 mmt or less and a tensile strength of 780 MPa or less. The present invention has been accomplished on the basis of this finding.

High-strength hot-rolled steel sheet with less material variation and superior surface quality

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

According to one aspect of the present invention, there is provided a high-strength hot-rolled steel sheet having a small material variation and excellent surface quality, comprising 0.03 to 0.06% of C, 1.5 to 2.5% of C, 0.1 to 0.5% of Si, 0.01 to 0.05% of P, , S: not more than 0.01%, Cr: 0.1 to 0.6%, Al: not more than 0.05%, Ti: 0.01 to 0.05%, N: 0.001 to 0.010% 50 to 70% of ferrite, 20 to 40% of martensite, and 5 to 15% of bainite.

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

C: 0.03 to 0.06%

Carbon (C) is a very important element for increasing the strength of a steel sheet and securing a composite structure composed of ferrite and martensite.

When the C content is less than 0.03%, it may be difficult to obtain the desired strength in the present invention. On the other hand, when the C content is more than 0.06%, alloy steel produced by high-speed performance is produced, so that molten steel outflow may occur, and solidification cells of non-uniform thickness may be formed, which may lead to operating accidents. Therefore, the C content is preferably 0.03 to 0.06%.

Mn: 1.5 to 2.5%

Manganese (Mn) is an element that has a very strong effect of solid solution strengthening and at the same time promotes the formation of composite structure composed of ferrite and martensite.

When the Mn content is less than 1.5%, it may be difficult to obtain the intended strength in the present invention. On the other hand, if the Mn content exceeds 2.5%, the weldability and hot rolling property may be degraded. In addition, when the Mn content is excessively added, since a delta-ferrite region is reduced at a temperature near the solidification point and apodization reaction occurs even in a low C region, And may cause an accident due to the leakage of molten steel. Therefore, the Mn content is preferably 1.5 to 2.5%.

Si: 0.1 to 0.5%

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

When the Si content is less than 0.1%, it is difficult to sufficiently secure the above-mentioned effect. On the other hand, when the Si content is more than 0.5%, the scale of the steel is generated on the surface of the steel sheet, and traces remain on the surface of the steel sheet after pickling, and the surface quality may deteriorate. Therefore, the Si content is preferably 0.1 to 0.5%.

P: 0.01 to 0.05%

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

When the P content is less than 0.01%, it is difficult to secure the effect. On the other hand, if the P content exceeds 0.05%, the grain boundary and / or the intergranular grain boundary may be segregated to cause brittleness. Therefore, the content of P is preferably limited to 0.01 to 0.05%.

S: not more than 0.01%

Sulfur (S) is an impurity which segregates during MnS nonmetallic inclusions and performance solidification in steel and can cause high temperature cracks.

Therefore, the content thereof should be controlled as low as possible, and it is preferable to control the content to 0.01% or less.

Cr: 0.1 to 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-mentioned effect is insufficient. On the other hand, when the Cr content exceeds 0.6%, there is a problem that the ductility of the steel sheet deteriorates. Therefore, the Cr content is preferably 0.1 to 0.6%.

Al: not more than 0.05%

Aluminum (Al) is concentrated on the surface of the steel sheet to deteriorate the plating ability, while suppressing carbide formation, thereby increasing the ductility of the steel. On the other hand, in the case of thin slabs, it is possible to omit the reheating process in the conventional hot melt mill, thereby saving energy and improving the productivity. However, the temperature may drop on the surface or edge of the slab due to cooling of the surface of the slab. As a result, the AlN is excessively precipitated and the edge quality of the cast steel and / or the bar plate may be degraded due to deterioration of high temperature ductility.

Therefore, the content should be controlled as low as possible, and it is preferable to control the content to 0.05% or less.

Ti: 0.01 to 0.05%

Titanium (Ti) is an element for forming precipitates and nitrides, which increases the strength of steel. 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 deterioration of high temperature ductility and reducing the sensitivity to edge cracking. Therefore, precise control is needed because Ti is a very useful element in solving surface and / or edge quality problems and strengths arising from thin slab high speed performance.

When the Ti content is less than 0.01%, the above-mentioned effect is insufficient. On the other hand, if the Ti content exceeds 0.05%, the manufacturing cost may increase and the ductility of the ferrite may be deteriorated. Therefore, the Ti content is preferably 0.01 to 0.05%.

N: 0.001 to 0.010%

Nitrogen (N) is an austenite stabilizing and nitriding element.

When the N content is less than 0.001%, the effect of stabilizing the austenite is insufficient. On the other hand, when the N content exceeds 0.010%, the precipitation strengthening effect is increased by reacting with the precipitate-forming element, but this may cause a drastic decrease in ductility. In addition, over-precipitation of AlN due to casting cooling in thin slab high-speed performance can result in poor surface and edge quality.

Therefore, the N content is preferably 0.001 to 0.010%.

The remainder of the present invention is iron (Fe). However, in the ordinary manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably incorporated, so that it can not be excluded. These impurities are not specifically mentioned in this specification, as they are known to any person skilled in the art of manufacturing.

At this time, in addition to the above alloy composition, at least one of Cu, Ni, Mo, Sn, and Pb may be included as a tram element, and the total amount may be 0.2 wt% or less.

The tram element is an impurity element derived from scrap used as a raw material in a steelmaking process. If the total amount exceeds 0.2%, surface cracking of the thin slab and surface quality of the hot-rolled steel sheet may be deteriorated.

Further, Ceq not only satisfies the above-described alloy composition but expressed by the following formula (1) may be 0.14 to 0.24.

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

(In the above formula (1), the symbol of each element represents the content of each element in weight%.)

The above formula (1) is a component relational expression for securing the weldability of the steel sheet. In the present invention, by controlling the Ceq value to 0.14 to 0.24, excellent weldability can be ensured and excellent mechanical properties can be imparted to the welded portion .

When Ceq is less than 0.14, the curing ability is low and it is difficult to secure the target tensile strength. On the other hand, when Ceq is more than 0.24, the weldability may deteriorate and the physical properties of the welded 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 in an area fraction.

When the ferrite content of the microstructure of the hot-rolled steel sheet according to the present invention is more than 70%, it is difficult to secure the desired strength. When the ferrite fraction is less than 50%, the remaining martensite and bainite structure fractions are increased to secure ductility There is a difficulty. When the martensite fraction is more than 40%, the strength becomes too high to secure the ductility. When the martensite fraction is less than 20%, it may be difficult to secure the desired strength.

The reason why the bainite structure is partially included in the microstructure of the hot-rolled steel sheet is as follows.

In the case of a general DP (dual phase) steel composed of only two phases of ferrite and martensite, the martensite fraction is high, so that martensite is tempered in the heat affected zone during welding, resulting in a softening phenomenon and a decrease in strength . This problem is solved not only by securing a certain amount of bainite structure in place of martensite but also by securing the strength and workability at the same time due to the nature of bainite. Further, in the case of DP steel, there is a problem in that the first-order fracture occurs at the interface due to the difference in the interface strength between the soft ferrite and the hard martensite phase, and the workability is heated. However, bainite is a structure having an intermediate strength between ferrite and martensite, and when the bainite structure is formed at these two texture interfaces, the above problems can be improved and the workability can be improved.

When the bainite area fraction is less than 5%, the above-mentioned effect is insufficient. On the other hand, if it exceeds 15%, the strength becomes too high and it may be difficult to secure ductility.

On the other hand, in the case of conventional hot-rolled mills, in order to keep the finishing rolling temperature at a constant level, the temperature inevitably accelerates the rolling speed of the tail portion, and the finishing rolling temperature is much higher than Ar3, And the austenite fraction are difficult to precisely control. As a result, it is not easy to uniformly obtain ferrite, martensite and bainite as final structures. In addition, it is difficult to secure a bainite structure stably because the finish rolling speed is too fast, and it is difficult to hit 350 to 550 ° C temperature where the bainite transformation occurs due to the temperature deviation.

At this time, the ferrite grains may have an average size of 5 탆 or less as measured by the circle equivalent diameter. In order to simultaneously secure strength and workability through securing a ferrite structure having fine crystal grains, when the size of the ferrite crystal grains exceeds 5 탆, it may be difficult to secure the desired strength and workability. More preferably 3 m or less.

On the other hand, in the case of conventional hot-rolled mills, the finishing rolling temperature is finishing rolled at a temperature of about 900 ° C, which is much higher than the normal Ar3 temperature (810 to 850 ° C) . However, according to the present invention, it is possible to uniformly maintain and control the temperature uniformly by isothermal and constant velocity rolling in the process of casting and rolling, uniformly obtaining the final structure, and finish rolling at a temperature lower than Ar3, Fine control can be performed.

Further, the thickness of the hot-rolled steel sheet of the present invention may be 3.0 mm or less. This is because, unlike the existing mini-mill process, which can produce only the material (more than 3.0 mm) after hot-rolling, the hot-rolled steel sheet can be produced with a thickness of 3.0 mm or less in accordance with the manufacturing method of the present invention. More preferably 2.0 mm or less.

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 tensile strength of 15 MPa or less.

Method of manufacturing high-strength hot-rolled steel sheet with less material deviation and excellent surface quality

Hereinafter, a method for manufacturing a high-strength hot-rolled steel sheet having a small 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 small material deviation and an excellent surface quality, comprising the steps of continuously casting molten steel having the alloy composition described above into a thin slab having a thickness of 60 to 120 mm; Removing the scale by injecting cooling water into the thin slab at a pressure of 150 bar or more; Rolling the thin slab from which the scale has been removed from the roughly rolled side so that the edge temperature of the bar plate is 850 to 1000 ° C to obtain a bar plate; Sequentially passing the bar plate through two rows for spraying cooling water at a pressure of 100 to 250 bar and one column for spraying cooling water at a pressure of 50 to 150 bar; Finishing the scraped bar plate in a temperature range of Ar1 to Ar3 to obtain a hot rolled steel sheet; And cooling the hot-rolled steel sheet for 3 to 8 seconds followed by cooling at a cooling rate of 200 ° C / s or higher and winding at 250 ° C or lower, wherein each of the above steps is performed continuously.

The fact that each of the above steps is carried out continuously means that the continuous rolling mode is used in the performance-rolling direct process.

Since the manufacturing process (mini milling process) using a so-called thin slab, which is a new steel manufacturing process that has recently been attracting attention, has a small temperature deviation in the width direction and the longitudinal direction of the strip due to the process characteristics, Has attracted attention as a process having the potential to manufacture a tissue steel.

There are existing batch type and newly developed Continuous and Endless Mill (CEM) in this performance - rolling direct process.

In the case of the batch mode, in order to compensate for the difference between the performance speed and the rolling speed, the finish rolling is performed in the coil box in front of the finishing mill, and thus the scale peelability, surface quality, And the like.

Unlike the batch mode, Continuous and Endless Mill (CEM) has a problem of the batch mode because there is no winding-up process before finish rolling. However, more precise control is required to compensate the difference between the performance speed and the rolling speed need.

FIG. 4 shows an example of a process (CEM) using a continuous continuous rolling mode in a performance-to-rolling direct process. The thin slabs (a) having a thickness of 50 to 150 mm are manufactured in the continuous casting machine 100 and the steel plates can be continuously rolled because there is no coil box between the roughing mills 400 and the finishing mills 600, Since the risk of plate breakage is very low, it is possible to produce products with a diameter of 3.0mm or less. Surface scaling is easy due to the finishing mill scale breaker (FSB) 500 in front of the roughing mill scale breaker (RSB) and the finish rolling mill 600 before the roughing mill 400 It is possible to produce Pickled & Oiled (PO) with superior surface quality when picking hot-rolled steel sheet in post-process. In addition, it is possible to perform isothermal constant speed rolling with a difference in rolling speed within one steel sheet in the finish rolling step so that the steel sheet width and longitudinal temperature deviation are remarkably low, It is possible to manufacture a steel sheet having excellent material deviation.

Hereinafter, each step will be described in detail.

Continuous casting step

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

When the thickness of the thin slab is more than 120 mm, high-speed casting is difficult, and the rolling load during rough rolling is increased. When the thickness is less than 60 mm, the temperature of the cast steel is rapidly decreased and uniform texture is hardly formed. In order to solve this problem, it is possible to additionally provide a heating apparatus, but this is a factor for improving the production cost, so it is preferable to exclude it. Therefore, the thickness of the thin slab is limited to 60 to 120 mm.

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

The reason why the casting speed is set to 4 mpm or more is that a high speed casting and rolling process are connected to each other and a casting speed higher than a certain level is required to secure the target rolling temperature. Further, if the casting speed is slow, there is a risk of segregation from the cast steel. If such a segregation occurs, it is difficult to secure strength and workability, and the risk of material variation in the width direction or the longitudinal direction is increased. However, when the casting speed exceeds 8 mpm, the operation success rate may be reduced due to instability of the molten steel bath surface. Therefore, the casting speed of the continuous casting is preferably 4 to 8 mpm.

Thin slab scale removal step

The scale is removed by injecting cooling water into the heated thin slab at a pressure of 150 bar or more. For example, in a roughing scale breaker (hereinafter, referred to as 'RSB') nozzle, the cooling water of 40 ° C or less can be sprayed at a pressure of 150 bar or more to remove the surface scale thickness to 300 μm or less.

When the pressure is less than 150 bar, a large amount of arithmetic scale scale remains on the surface of the thin slab, and the surface quality may become dull after pickling.

Rough rolling  step

The thin slab from which the scale has been removed is rough-rolled from the rough-rolled 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-rolled in a roughing mill consisting of 2 to 5 stands.

When the edge temperature is lower than 850 ° C, a large amount of AlN precipitates or the like is generated, and the susceptibility to edge cracking becomes very high due to deterioration of high temperature ductility. On the other hand, when the edge portion temperature is higher than 1000 ° C, the temperature of the central portion of the thin slab becomes too high, so that a large number of arithmetic scale may occur, which may result in poor surface quality after pickling.

At this time, the rough rolling can be carried out such that the surface temperature of the thin slab is 1000 to 1200 DEG C at the rough rolling inlet side.

If the surface temperature of the thin slab is less than 1000 캜 at the entrance side of the rough rolling mill, there is a possibility that cracks are generated in the edge of the bar plate during the rough rolling load increase and the rough rolling process. In this case, the edge of the hot rolled steel sheet may be defective. If the slab surface temperature exceeds 1200 ° C, problems such as deterioration of hot rolling surface quality due to the remnant of a hot rolling scale may occur.

On the other hand, the rough rolling can be performed such that the cumulative rolling reduction is 60 to 90%. The higher the reduction rate in rough rolling, the more uniform the microscopic distribution of Mn, Si, and Cr, which are important elements in the manufacture of high strength steel, and the smaller the temperature gradient in the width and thickness direction of the strip, Do. However, when the cumulative rolling reduction is less than 60%, the above-mentioned effect is insufficient, and when the cumulative rolling reduction is more than 90%, the rolling deformation resistance is greatly increased and the manufacturing cost may increase.

Bar plate scale removal step

The bar plate is sequentially passed through one row for spraying cooling water at a pressure of 50 to 150 bar and two rows for spraying cooling water at a pressure of 100 to 250 bar to remove the scale. For example, the surface scale thickness can be reduced to 50um or less by using a single row nozzle and a two row nozzle of Finishing Mill Scale Breaker (FSB) before finishing the bar plate.

When the pressures of the first and second nozzles are less than 50 bar and 100 bar respectively, scale removal is insufficient, so that a large amount of spindle-shaped scale scale is produced on the surface of the steel sheet after finishing rolling, and surface quality after pickling becomes poor. On the other hand, when the pressure of the one-row nozzle is more than 150 bar or the pressure of the two-row nozzle is more than 250 bar, the finish rolling temperature becomes too low to obtain an effective austenite fraction and it is difficult to secure the target tensile strength.

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

Finishing rolling step

The bar plate from which the scale has been removed is subjected to finish rolling in the temperature range of Ar1 to Ar3 to obtain a hot-rolled steel sheet. For example, finishing rolling can be carried out in a finishing mill composed of 3 to 6 stands.

In the present invention, a high fraction of precipitates are formed in the finish rolling process, and the reduction of the precipitation strengthening effect due to the reduction of the fraction of the precipitates to be finely precipitated at low temperatures is low-temperature rolled in the range of Ar1 to Ar3 to reduce the grain size, This is to compensate with an increase in strengthening effect.

In the case of transformed textured steel, it is important to enhance the austenite stabilizing element such as C and Mn to the untreated austenite in order to improve the strength and ductility at the same time. In the case of finishing rolling, The martensite becomes more stable and the ferrite becomes clean, and the strength and ductility are improved at the same time.

On the other hand, when the finishing rolling temperature is lower than Ar3 in the conventional blast furnace and mini-milling process, there is a problem in the rolling ductability. However, in the process of direct rolling of the steel to rolling according to the present invention, rolling is performed at isothermal and constant speed, There is no problem and low-temperature rolling is possible at Ar1-Ar3 temperature.

At this time, the finish rolling can be carried out so that the passing speed is 200 to 600 mpm, and the thickness of the hot-rolled steel sheet is 3.0 mm or less.

If the finish rolling speed is higher than 600 mPm, it is possible to cause an accident such as a pandanade, and it is difficult to perform isothermal and constant speed rolling, so that a uniform temperature can not be ensured and a material deviation may occur. On the other hand, in the case of less than 200 mpm, the finishing rolling speed is too slow to secure the finishing rolling temperature.

Further, the finish rolling may be performed so that the area fraction is 20 to 40% of ferrite and 60 to 80% of austenite.

As a result of repeated experiments, when the ferrite fraction exceeds 40%, the austenite fraction is relatively low, and after cooling, the martensite and bainite fractions are low, making it difficult to secure the desired tensile strength. On the other hand, when the ferrite fraction is less than 20%, the austenite fraction becomes too large and the martensite and bainite fractions after the phase transformation are high, so that it 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 becomes 10% or less.

Since the high strength steel of 780 MPa class, which is the object of the present invention, utilizes the formation of the transformed structure as the strengthening mechanism, there is a high possibility that the material characteristics change according to the deformation rate during finish rolling. That is, if the difference in rolling speed exceeds 10% in a finishing mill composed of a plurality of stands, it is difficult to obtain a uniform cooling rate and a target coiling temperature in a subsequent run out table, It is possible to cause a large variation in material in the direction.

Cooling and Winding step

The hot-rolled steel sheet is air-cooled for 3 to 8 seconds, cooled at a cooling rate of 200 DEG C / s or more, and wound at 250 DEG C or less.

When the air cooling time is less than 3 seconds, the C concentration in the retained austenite is insufficient and the time for the ferrite transformation is insufficient, so that the risk of the elongation decreases. In the case of exceeding 8 seconds, Not only the difficulty in securing the strength but also the length of the equipment or the productivity may be decreased.

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

At this time, a step of pickling the rolled hot-rolled steel sheet to obtain a PO material may be further included.

In the present invention, scales are sufficiently removed in the thin slab and bar plate scale removal step, so that a PO material having excellent surface quality can be obtained even by a general pickling treatment. Therefore, the pickling treatment that can be used in the present invention is not particularly limited as long as it is generally applicable to a treatment method used in a hot-rolling pickling process.

Hereinafter, the present invention will be described more specifically by way of examples. It should be noted, however, that the following examples are intended to illustrate the invention in more detail and not to limit the scope of the invention. The scope of the present invention is determined by the matters set forth in the claims and the matters reasonably inferred therefrom.

( Example )

Molten steel having the composition shown in Table 1 below was prepared.

In the case of Examples 1 and 2 and Comparative Examples 1 to 8, a thin slab having a thickness of 96 mm was continuously cast and then subjected to a continuous rolling process in the continuous rolling mode in the performance-rolling direct process by applying the manufacturing conditions described in Table 2, . In the case of Conventional Example 1, a hot-rolled steel sheet having a thickness of 3.2 mm was produced in a batch mode in an existing mini-mill process by applying the manufacturing conditions described in Table 2. [

The produced hot-rolled steel sheet was subjected to pickling treatment to obtain a PO material, and then the obtained material was subjected to a heat treatment to obtain a PO material, and then a microstructure, a yield strength (YS), a tensile strength (TS), an elongation (EL) The quality was measured and reported in Table 3 below.

On the other hand, Ar1 and Ar3 temperatures in Table 2 are values calculated using a commercial thermodynamic software JmatPro V-8.

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

The ferrite grain size (FGS, Ferrite Grain Size) was measured randomly at 10 locations at 3,000 times magnification using Electron Back Scatter Diffraction (EBSD), and then measured with a circle-equivalent diameter using Image-Plus Pro software An average value is described

The tensile strength is measured by taking JIS No. 5 specimen in a direction perpendicular to the rolling direction at a width of W / 4, and the material deviation is obtained by subtracting the minimum value from the maximum value of the tensile strength measured in the width and length direction of the coil will be.

The presence or absence of edge cracks was visually confirmed in the bar plate and coil by the naked eye and secondly confirmed by SDD (Surface Defect Detector), which is a surface defect detector.

The evaluation criteria of the PO re-surface quality are as follows.

○: gloss average deviation in the width direction is not more than 20%

X: gloss average deviation in width direction is more than 20%

division Steel grade Alloy element (% by weight) Equation (1) C Mn Si P S Ti Al Cr N Invention river A 0.050 1.91 0.18 0.018 0.0006 0.027 0.018 0.40 0.0064 0.19 Invention river 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, the formula (1) is Ceq = C + Si / 30 + Mn / 20 + 2P + 3S where each element symbol represents the content of each element in weight%.

division Steel grade RSB
(Bar) FSB (Bar) Rough rolling temperature (캜) Wrap-up
Rolling
Temperature
(° C) Ar3
(° C) Ar1
(° C) Air cooling
time
(sec) Cooling
speed
(° C / sec) Coiling
Temperature
(° C) Column 1 Column 2 Inside
surface Exit
edge Inventory 1 A 191 102 203 1052 890 785 815 640 5.0 335 145 Inventory 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 cooling water injection pressure before rough rolling, and FSB (Finishing Mill Scale Breaker, finish rolling scale brake) is cooling water injection pressure after rough rolling.

division Steel grade Microstructure (area%) FGS
(탆) YS
(MPa) TS
(MPa) △ TS
(MPa) EL
(%) Edge crack
Occurrence PO ash
Surface quality F M B Inventory 1 A 63 28 9 2.01 567 818 11 17 X ○ Inventory 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 - -

Examples 1 and 2 satisfying the conditions of the present invention satisfy the target tensile strength (780 MPa or more) and elongation (15% or more), and both the edge surface quality and the surface quality of the PO material are excellent have.

1 is a photograph of a microstructure of Inventive Example 2 taken by a scanning electron microscope (SEM). It can be confirmed that ferrite (F) and martensite (M) are composed of main phases and that some bainite (B) exists.

2 is a graph showing the ferrite grain size distribution of Inventive Example 2 measured using EBSD. From these results, it is confirmed that crystal grains of 1 to 3 mu m or less are finely distributed and the average crystal grain size is 1.95 mu m.

Fig. 3 shows a surface photograph of a PO material obtained by pickling the inventive hot-rolled steel sheet, and it can be confirmed that the surface quality is excellent.

Comparative Examples 1 to 5 satisfied the composition of the alloys proposed in the present invention, but did not satisfy the manufacturing conditions and failed to secure the desired materials.

In Comparative Examples 1 and 2, the cooling conditions were not satisfied and the strength was inferior. In Comparative Example 3, the edge quality was poor when the edge temperature of the rough rolling out side was insufficient. Comparative Examples 4 and 5 did not satisfy the RSB or FSB pressure suggested in the present invention, and the surface quality was poor.

In Comparative Example 6, the operating conditions were all the same as those in Inventive Example 2, but the Si content exceeded the target tensile strength, and the surface quality of the PO material was also poor.

In Comparative Example 7, the target tensile strength was not satisfied when the Cr content was low. In Comparative Example 8, the elongation was poor when the Si content was low.

It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined in the appended claims. It will be possible.

a: Slab b: Coil
100: continuous casting machine 200: heater
300: RSB (Roughing Mill Scale Breaker, rough rolling scale brake)
400: rough rolling mill
500: FSB (finishing mill scale breaker, finishing rolling scale brake)
600: finishing mill 700: run-out table
800: High speed shear machine 900: Winder

Claims (16)

0.1 to 0.5% of Si, 0.01 to 0.05% of P, 0.01 to 0.01% of S, 0.1 to 0.6% of Cr, and 0.05% or less of Al in terms of% by weight of C, 0.03 to 0.06%, 1.5 to 2.5% , Ti: 0.01 to 0.05%, N: 0.001 to 0.010%, balance Fe and other unavoidable impurities,
The microstructure is a high-strength hot-rolled steel sheet with an area fraction of 50 to 70% of ferrite, 20 to 40% of martensite, and 5 to 15% of bainite,
The method according to claim 1,
Wherein the hot-rolled steel sheet further contains at least one of Cu, Ni, Mo, Sn and Pb as a tramp element and has a total amount of not more than 0.2% by weight.
The method according to claim 1,
Wherein the hot-rolled steel sheet has a material deviation of 0.10 to 0.24 and a surface roughness of Ceq of 0.10 to 0.24.
(1): Ceq = C + Si / 30 + Mn / 20 + 2P + 3S
(In the above relational expression, each element symbol represents the content of each element in weight%.)
The method according to claim 1,
The high-strength hot-rolled steel sheet of the present invention is characterized in that the grain size of the ferrite is less than 5 占 퐉 and the average size is not more than 5 占 퐉.
The method according to claim 1,
Wherein the thickness of the hot-rolled steel sheet is 3.0 mm or less, and the surface quality is excellent.
The method according to claim 1,
The hot-rolled steel sheet has a tensile strength of 780 MPa or more, an elongation of 15% or more, and a material deviation of 15 MPa or less in tensile strength.
0.1 to 0.5% of Si, 0.01 to 0.05% of P, 0.01 to 0.01% of S, 0.1 to 0.6% of Cr, and 0.05% or less of Al in terms of% by weight of C, 0.03 to 0.06%, 1.5 to 2.5% 0.01 to 0.05% of Ti, 0.001 to 0.010% of N, and the remaining Fe and other unavoidable impurities in a thin slab having a thickness of 60 to 120 mm;
Removing the scale by injecting cooling water into the thin slab at a pressure of 150 bar or more;
Rolling the thin slab from which the scale has been removed from the roughly rolled side so that the edge temperature of the bar plate is 850 to 1000 ° C to obtain a bar plate;
Sequentially passing the bar plate through two rows for spraying cooling water at a pressure of 100 to 250 bar and one column for spraying cooling water at a pressure of 50 to 150 bar;
Finishing the scraped bar plate in a temperature range of Ar1 to Ar3 to obtain a hot rolled steel sheet; And
Cooling the hot-rolled steel sheet for 3 to 8 seconds, cooling at a cooling rate of 200 DEG C / s or more, and winding at 250 DEG C or less, wherein each of the steps has a small material deviation and a surface quality A method for manufacturing a high - strength hot - rolled steel sheet.
8. The method of claim 7,
Wherein the molten steel contains at least one of Cu, Ni, Mo, Sn and Pb as a tramp element, and the total amount thereof is 0.2 wt% or less, and the surface quality is excellent.
8. The method of claim 7,
Wherein said molten steel has a small material deviation of 0.14 to 0.24 in Ceq and has excellent surface quality.
Ceq = C + Si / 30 + Mn / 20 + 2P + 3S
(In the above relational expression, each element symbol represents the content of each element in weight%.)
8. The method of claim 7,
Wherein the casting speed of the continuous casting is 4 to 8 mpm and the material quality is small and the surface quality is excellent.
8. The method of claim 7,
Wherein the rough rolling is performed such that the surface temperature of the thin slab is roughly 1000 to 1200 DEG C at the rough rolling inlet side, and the surface quality is excellent.
8. The method of claim 7,
A method for manufacturing a high-strength hot-rolled steel sheet, wherein the rough rolling has a small material deviation so that the cumulative rolling reduction becomes 60 to 90% and the surface quality is excellent.
8. The method of claim 7,
Wherein said finish rolling is a process for producing a high-strength hot-rolled steel sheet having a sheet speed of 200 to 600 mpm and a material deviation less than 3.0 mmt and having excellent surface quality.
8. The method of claim 7,
The method of manufacturing a high-strength hot-rolled steel sheet according to claim 1, wherein the finishing rolling is performed such that ferrite and an austenite are formed in an area fraction of 20 to 40% and 60 to 80%, respectively.
8. The method of claim 7,
The method of manufacturing a high-strength hot-rolled steel sheet according to any one of claims 1 to 3, wherein the finishing rolling is performed such that a rolling speed difference in one bar plate is 10% or less.
8. The method of claim 7,
And picking up the wound hot-rolled steel sheet to obtain a pickled &< RTI ID = 0.0 > (PO) < / RTI > material.

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