WO2019009677A1 - Advanced-high strength hot-rolled steel sheet and method for manufacturing same - Google Patents

Abstract

The present invention relates to an advanced-high strength hot-rolled steel sheet, having tensile strength of 980MPa, and a method for manufacturing same, the method enabling enhanced surface quality, processability and weldability as well as significantly reduced deviation of the material in the width and length directions of the steel sheet by means of an endless rolling mode in a continuous casting-direct rolling process. According to a preferable aspect of the present invention, provided are an advanced-high strength hot-rolled steel sheet, having a microstructure, and a method for manufacturing same, the advanced-high strength hot-rolled steel sheet manufactured by means of a continuous casting-direct rolling process and comprising, in weight %, 0.030-0.085% of C, 1.8-3.0% of Mn, 0.03-1.0% of Si, 0.005-0.05% of P, 0.01% or less of S, 0.2-2.0% of Cr, 0.01-0.07% of Al, 0.005-0.10% of Ti, 0.0005-0.0050% of B, 0.001-0.010% of N, the remainder being Fe, and other inevitable impurities and, in an area fraction, 40-60% of ferrite, 30-50% of martensite and 10-20% of bainite.

Description

Ultra-high strength hot-rolled steel sheet and manufacturing method thereof

The present invention relates to a super high strength hot-rolled steel sheet and a method of manufacturing the same, and more particularly, to a super high-strength hot-rolled steel sheet having a high tensile strength of 980 MPa, To a hot-rolled steel sheet and a manufacturing method thereof.

In recent years, as safety regulations for passengers and pedestrians in automobiles have been strengthened, attachment of safety devices has become mandatory, which leads to a problem that the weight of the vehicle body is increased due to the contrary to weight reduction for improving fuel efficiency of automobiles.

In addition, consumers are increasingly interested in hybrid and electric vehicles, which are environmentally friendly and have high fuel efficiency. In order to produce such an environmentally friendly and safe car, the weight of the car body structure and the stability of the car body material must be secured.

However, on the contrary, hybrid cars are being added to existing gasoline engines as well as a number of other devices such as electric engines, electric batteries, and secondary fuel storage tanks. In addition, the weight of the vehicle body is increasing as the driver's convenience facilities are continuously added. It is essential to develop a material that is thin and capable of maintaining high strength and high ductility in order to realize weight saving of the vehicle while compensating for the increase in external weight.

Accordingly, in order to solve such a problem, development of an AHSS (hereinafter referred to as "AHSS") steel sheet capable of securing high strength and high ductility of tensile strength of 980 MPa or more is required.

In addition, welding process is indispensable to manufacture such AHSS steel plate as an end product by applying to an automobile body, and resistance spot welding (RSW) is most widely applied.

Therefore, a steel sheet to be applied to an automobile body is required not only to have high strength and high ductility, but also to have good weldability and welded part mechanical properties.

On the other hand, since most automotive steel sheets are formed by press working, it is necessary to have uniform material characteristics with low yield strength and high ductility.

Among the AHSS steel plates, the so-called DP (dual phase) steel is a steel mainly composed of two phases of ferrite and martensite, and is one of the representative steels having a low yield strength.

The technology related to the production of such AHSS hot-rolled steel is disclosed in Patent Documents 1 and 2. However, all of these methods relate to a method of manufacturing in a conventional hot-melt mill, It 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 stably secure. 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 are largely generated 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 for producing a hot-rolled DP steel having a tensile strength of 590 MPa by batch method in a mini-mill process, and the final steel sheet thickness is about 3.0 mm.

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 coils of hot-rolled steel sheets having a thickness of 3.0 mm or less because of a high risk of plate breakage.

Therefore, it is required to develop a manufacturing process capable of overcoming the above-mentioned problems. In addition to this, it is possible to manufacture AHSS steel sheet with less tensile strength of 980 MPa or more (steel sheet thickness less than 3.0 mm) and excellent weldability due to strong demand for collision stability of passenger and CO ₂ environment around the world. Development is urgently needed.

(Prior art document)

(Patent Document 1) U.S. Patent No. 4285741

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

(Patent Document 3) Korean Published Patent Application No. 10-2012-0052022

A preferred aspect of the present invention is to provide an ultrahigh-strength hot-rolled steel sheet having excellent surface quality and weldability and being significantly reduced in width, lengthwise material deviation, and edge cracks.

Another aspect of the present invention is to provide a method of manufacturing an ultra-high-strength hot-rolled steel sheet excellent in surface quality and weldability, and significantly reduced in width, lengthwise material deviation, and edge cracks, by a 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.

According to a preferred aspect of the present invention, there is provided a method of manufacturing a steel sheet, which is produced by a performance-rolling direct process, comprising 0.030 to 0.085% of C, 1.8 to 3.0% of Mn, 0.03 to 1.0% of Si, 0.005 to 0.05% , S: 0.01% or less, Cr: 0.2 to 2.0%, Al: 0.01 to 0.07%, Ti: 0.005 to 0.10%, B: 0.0005 to 0.0050%, N: 0.001 to 0.010%, and other Fe and other unavoidable impurities And an ultrahigh-strength hot-rolled steel sheet having a microstructure containing 40 to 60% of ferrite, 30 to 50% of martensite and 10 to 20% of bainite in an areal fraction.

Preferably, in the hot-rolled steel sheet, Al and Ti can satisfy the following relational expression (1).

[Relation 1]

1.9Al - 3.4 Ti <0.002

1.9 and 3.4 represent values of Al / N atomic ratio and Ti (atomic weight), respectively. In the formula (1), each symbol represents the weight (%) of each element divided by the atomic weight / N atomic ratio]

In the hot-rolled steel sheet, Ceq expressed by the following formula (2) may be 0.18 to 0.28.

[Relation 2]

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

 [Each symbol of the element in the formula (2) is a value indicating the content of each element in weight%

The hot-rolled steel sheet may contain at least one of Cu, Ni, Mo, Sn, and Pb as a tramp element, and the total content thereof may be 0.2 wt% or less.

The bainite may be formed at the interface between ferrite and martensite.

The mean size of the ferrite crystal grains measured by the circle equivalent diameter may be 5 탆 or less.

The hot-rolled steel sheet may comprise ^25-1000 / ㎛ the Ti (C, N) precipitates, the Ti (C, N) average size of the precipitate can be 50nm or less in circle equivalent diameter.

Preferably, the rolling process includes a rough rolling process for producing a bar plate, and a molar fraction (%) of an AlN precipitate at an edge portion of the roughly rolled bar plate may satisfy the following formula (3) .

[Relation 3]

Molar fraction (%) of AlN precipitate at the edge of the bar plate < 8.3X10 ^-6

The hot-rolled steel sheet may have a tensile strength of 980 MPa or more, an elongation of 10% or more, and a deviation (material deviation) of tensile strength of 30 MPa or less.

The thickness of the hot-rolled steel sheet may be 3.0 mm or less.

According to another aspect of the present invention, there is provided a method of manufacturing a steel sheet, which is manufactured through a performance-rolling direct process, and which comprises 0.030 to 0.085% of C, 1.8 to 3.0% of C, 0.03 to 1.0% of Si, 0.005 0.001 to 0.050% of N, 0.001 to 0.010% of N, 0.001 to 0.010% of Cr, 0.2 to 2.0% of Cr, 0.01 to 0.07% of Al, 0.005 to 0.10% of Ti, And has a microstructure containing an impurity and an area fraction of 40 to 60% of ferrite, 30 to 50% of martensite, and 10 to 20% of bainite, and in the case of resistance spot welding after pickling treatment, The optimum welding range between the lower current satisfying the minimum nugget diameter of 4 t ^1/2 [t = the base metal thickness (mm)] and the upper current limit causing the scattering in steel is 1.5 (kA) or more and the minimum nugget diameter is 5 t ^1/2 The appropriate welding range between the lower current limit and the upper limit current satisfying [t = base metal thickness (mm)] is 1.0 (KA) or more and the tensile shear strength (TS) relative to the cross tensile strength S) having a ductility ratio of 35% or more is provided.

Preferably, in the hot-rolled steel sheet, Al and Ti can satisfy the following relational expression (1).

[Relation 1]

1.9Al - 3.4 Ti <0.002

1.9 and 3.4 represent values of Al / N atomic ratio and Ti (atomic weight), respectively. In the formula (1), each symbol represents the weight (%) of each element divided by the atomic weight / N atomic ratio]

In the hot-rolled steel sheet, Ceq expressed by the following formula (2) may be 0.18 to 0.28.

[Relation 2]

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

 [Each symbol of the element in the formula (2) is a value indicating the content of each element in weight%

The hot-rolled steel sheet may contain at least one of Cu, Ni, Mo, Sn, and Pb as a tramp element, and the total content thereof may be 0.2 wt% or less.

The Vickers hardness of the fused portion corresponding to the nugget may be 350 to 450 Hv, and the difference between the hardness of the base material and the minimum hardness of the softened portion may be 100 Hv or less.

The microstructure of the molten portion corresponding to the nugget may include a martensite structure having an area ratio of 95% or more.

Preferably, the rolling process includes a rough rolling process for producing a bar plate, and the molar fraction (%) of the AlN precipitate at the edge portion of the roughly rolled bar plate can satisfy the following formula (3).

[Relation 3]

Molar fraction (%) of AlN precipitate at the edge of the bar plate < 8.3X10 ^-6

According to still another aspect of the present invention, there is provided a method of manufacturing a hot-rolled steel sheet by a direct-rolling-performance-rolling process, comprising the steps of: C: 0.030-0.085%; Mn: 1.8-3.0%; Si: 0.001 to 0.05% of P, 0.2 to 2.0% of Cr, 0.01 to 0.07% of Al, 0.005 to 0.10% of Ti, 0.0005 to 0.0050% of B, 0.001 to 0.010% of N, Continuously cast molten steel containing remaining Fe and other unavoidable impurities into a slab having a thickness of 60 to 120 mm;

Spraying the slab with cooling water at a pressure of 50 to 350 bar to remove the surface scale;

Rolling the scaled slab to obtain a bar plate;

Spraying cooling water onto the bar plate at a pressure of 50 to 350 bar to remove surface scale;

Finishing rolling the bar plate from which the scale has been removed in a temperature range of Ar ₃ -30 캜 to Ar ₃ + 60 캜 to obtain a hot-rolled steel sheet;

Cooling the hot-rolled steel sheet in a run-out table for 1 to 8 seconds and cooling to a martensitic transformation completion temperature (Mf) or less at a cooling rate of 150 ° C / sec or more; And

And winding the cooled hot-rolled steel sheet at a martensitic transformation completion temperature (Mf) or lower.

Preferably, in the molten steel, Al and Ti can satisfy the following relational expression (1).

[Relation 1]

1.9Al - 3.4 Ti <0.002

1.9 and 3.4 represent values of Al / N atomic ratio and Ti (atomic weight), respectively. In the formula (1), each symbol represents the weight (%) of each element divided by the atomic weight / N atomic ratio]

In the molten steel, Ceq expressed by the following formula (2) may be 0.18 to 0.28.

[Relation 2]

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

 [Each symbol of the element in the formula (2) is a value indicating the content of each element in weight%

The molten steel may contain at least one of Cu, Ni, Mo, Sn and Pb as a tramp element, and the total content thereof may be 0.2 wt% or less.

The casting speed of the continuous casting may be 4 to 8 mpm.

The slab may be heated before the surface descaling step, and the slab heating temperature may be 900 to 1200 ° C.

The surface temperature of the rough-rolled slab at the time of rough rolling may be 900 to 1200 ° C, and the edge temperature of the rough-rolled bar-plate edge may be 780 to 1100 ° C.

The temperature of the rough rolling out side bar plate edge portion can be controlled so that the AlN mole fraction (%) precipitated at the bar plate edge portion temperature satisfies the following formula (3).

[Relation 3]

Molar fraction (%) of AlN precipitate at the edge of the bar plate < 8.3X10 ^-6

The cumulative rolling reduction during the rough rolling may be 60 to 90%.

During the finish rolling, the sheet passing speed may be 200 to 600 mpm, and the thickness of the hot-rolled steel sheet produced by the finish rolling may be 3.0 mm or less.

The rolling speed difference between the top and the tail of the steel sheet during the finish rolling may be 10% or less.

And picking up the hot-rolled steel sheet to obtain a pickled & &lt; RTI ID = 0.0 &gt; (PO) &lt; / RTI &gt;

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 improved by using the continuous continuous rolling mode in the performance-rolling direct process, but also the material variation in the width and length direction of the steel sheet is remarkably reduced, and the yield rate is excellent. , It is possible to produce an ultra-high strength hot-rolled steel sheet having a thickness of 3.0 mm or less and a tensile strength of 980 MPa. In addition, edge cracks can be significantly reduced.

In addition, the hot-rolled steel sheet produced by the present invention is superior in the quality of the edges and the surface scale, and is capable of producing advanced PO by a general hot-rolling pickling process, It can be differentiated from existing minilmill and hot-milling processes that can only be produced by itself, so it is excellent in terms of price competition and can significantly improve value-added. In addition, it is possible to omit the reheating step in the existing hot melt mill through the thin slab method, thereby saving energy and improving the productivity. In addition, it is possible to use the steel in which the scrap of scrap iron is dissolved in the electric furnace through the thin slab reclamation method, thereby enhancing the recyclability of resources.

BRIEF DESCRIPTION OF THE DRAWINGS Fig.

2 is a scanning electron microscope (SEM) structural photograph of Inventive Example 7. Fig.

Fig. 3 shows the ferrite grain size distribution of Inventive Example 7. Fig.

4 is a transmission electron microscope (TEM) micrograph of the precipitate of Inventive Example 7 taken.

5 shows the distribution of the number of precipitates (탆 ² ) per unit area of Inventive Example 7.

Fig. 6 shows the distribution of the precipitate size (nm) in Inventive Example 7.

7 is a PO material surface photograph of Inventive Example 7. Fig.

8 is a photograph of a slab of Comparative Example 8 in which molten steel outflow occurred.

Fig. 9 shows a state diagram of the inventive example 7. Fig.

10 shows a state diagram of Comparative Example 8. Fig.

11 is a graph showing changes in mole fraction of AlN / TiN according to the temperature of Inventive Example 7.

12 is a graph showing the change in the mole fraction of AlN according to the temperature of Comparative Example 10.

13 is a graph showing the change in the mole fraction of AlN according to the temperature of Conventional Example 1.

14 is a cross-sectional photograph of the nugget according to the welding current for the case 7;

15 shows the distribution of the hardness of the resistance spot welded portion according to Inventive Example 7.

16 shows the ductility ratio variation according to the welding current for the inventive example 7.

17 is a SEM micrograph of the molten portion of the present invention.

Figure 18 shows a lay-out of a preferred example of a performance-rolling direct process that the present invention may be applied to.

Figure 19 shows another preferred lay-out of a performance-rolling direct process in which the present invention may be applied.

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 present inventors conducted research and experiments on an ultra-high-strength hot-rolled steel sheet excellent in surface quality and weldability, and significantly reduced in width, lengthwise material deviation, and edge cracks through a performance-rolling direct process, The present invention has been completed.

The present invention relates to an ultrahigh-strength hot-rolled steel sheet excellent in surface quality and weldability by properly controlling steel composition, microstructure and manufacturing conditions in a performance-rolling direct-cutting process, and significantly reducing width, lengthwise material deviation and edge cracks, And a method for manufacturing the same.

Hereinafter, a super high strength hot rolled steel sheet according to a preferred embodiment of the present invention will be described in detail.

A super high strength hot-rolled steel sheet according to a preferred embodiment of the present invention is manufactured by a performance-rolling direct process, and comprises 0.030 to 0.085% of C, 1.8 to 3.0% of C, 0.03 to 1.0% of Si, 0.005 to 0.05%, S: 0.01% or less, Cr: 0.2 to 2.0%, Al: 0.01 to 0.07%, Ti: 0.005 to 0.10%, B: 0.0005 to 0.0050% Contains inevitable impurities, and has a microstructure including an area fraction of 40 to 60% of ferrite, 30 to 50% of martensite and 10 to 20% 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: 0.030 to 0.085%

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 0.030%, it may be difficult to secure the desired strength in the present invention. On the other hand, when the C content exceeds 0.085%, an apodization reaction (L + Delta Ferrite → Austentite) occurs at the time of solidification of molten steel, and a solidified cell having a non-uniform thickness is formed, resulting in molten steel outflow. Therefore, the C content is preferably 0.03 to 0.085%. , More preferably 0.040 to 0.080%, and even more preferably 0.055 to 0.075%.

Mn: 1.8 to 3.0%

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. If the Mn content is less than 1.8%, it may be difficult to obtain the desired strength in the present invention. On the other hand, when the Mn content exceeds 3.0%, alloy steel cost rise and weldability and hot rolling property may be weakened. Therefore, the Mn content is preferably 1.8 to 3.0%. , More preferably 1.9 to 2.8%, and even more preferably 2.0 to 2.6%.

Si: 0.03 to 1.0%

Silicon (Si) is a useful element that can secure strength without deteriorating the ductility of the 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 0.03%, it is difficult to sufficiently secure the above effect. On the other hand, when the Si content is more than 1.0%, 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 be deteriorated. Therefore, the Si content is preferably 0.03 to 1.0%. , More preferably 0.030 to 0.80%, and even more preferably 0.035 to 0.50%.

P: 0.005 to 0.05%

Phosphorus (P) is an element having an effect of strengthening the steel sheet. When the P content is less than 0.005%, it is difficult to secure the effect. On the other hand, when 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 0.005 to 0.05%. , More preferably 0.006 to 0.040%, and even more preferably 0.010 to 0.025%.

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 preferably controlled to be 0.01% or less.

Cr: 0.2 to 2.0%

Chromium (Cr) is an element that improves hardenability and increases the strength of steel. When the Cr content is less than 0.2%, the above-mentioned effect is insufficient. On the other hand, when the Cr content exceeds 2.0%, there is a problem that the ductility of the steel sheet deteriorates. Therefore, the Cr content is preferably 0.2 to 2.0%. , More preferably 0.3 to 1.8%, and even more preferably 0.5 to 1.4%.

Al: 0.01 to 0.07%

Aluminum (Al) plays a role in suppressing the formation of carbides and increasing the ductility of steel. When the Al content is less than 0.01%, the above-mentioned effect is insufficient. On the other hand, when the Al content is more than 0.07%, a large amount of AlN precipitates are formed to deteriorate the edge quality of the cast steel or bar plate due to deterioration of high temperature ductility, and the steel can be thickened on the surface of the steel plate to deteriorate the plating ability. Therefore, the Al content is preferably 0.01 to 0.07%. , More preferably 0.015 to 0.06%, and even more preferably 0.02 to 0.05%.

Ti: 0.005 to 0.10%

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 sensitivity of edge cracking by preventing the deterioration of high-temperature ductility by reducing the amount of AlN precipitate by removing solute N through the formation of TiN near the solidification temperature. When the Ti content is less than 0.005%, the above-mentioned effect is insufficient. On the other hand, if the Ti content exceeds 0.10%, the manufacturing cost may increase and the ductility of the ferrite may be deteriorated. Therefore, the Ti content is preferably 0.005 to 0.10%. , More preferably 0.008 to 0.08%, and even more preferably 0.01 to 0.075%.

B: 0.0005 to 0.0050%

Boron (B) is an element that increases the hardenability of steel. When the B content is less than 0.0005%, the above-mentioned effect is insufficient. When the B content exceeds 0.0050%, the austenite recrystallization temperature is increased and the weldability is deteriorated. Further, excessive precipitation of precipitates such as BN may deteriorate the edge quality of the cast steel and / or the bar plate due to deterioration of high-temperature ductility. Therefore, the B content is preferably 0.0005 to 0.0050%. , More preferably 0.001 to 0.0040%, and even more preferably 0.001 to 0.0025%.

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 above-mentioned effect 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 it may cause a drastic decrease in ductility. Therefore, the N content is preferably 0.001 to 0.010%. , More preferably 0.0025 to 0.0095%, and even more preferably 0.0040 to 0.0090%.

Preferably, in the hot-rolled steel sheet, Al and Ti can satisfy the following relational expression (1).

[Relation 1]

1.9Al - 3.4 Ti <0.002

1.9 and 3.4 represent values of Al / N atomic ratio and Ti (atomic weight), respectively. In the formula (1), each symbol represents the weight (%) of each element divided by the atomic weight / N atomic ratio]

The above Ti, Al and N not only satisfy the above-mentioned numerical ranges but can be controlled so as to satisfy the above formula (1) in order to improve the surface quality and significantly reduce the edge crack while securing high strength.

Aluminum (Al) in the steel reacts with nitrogen (N) to form AlN precipitates. The slab / bar plate cracks are induced in the slab cooling conditions under which these precipitates are precipitated during the production of thin slabs to lower the edge quality of the slab or hot- It is possible to appropriately add Ti to remove solute N through the formation of TiN near the solidification temperature of molten steel to reduce the amount of AlN precipitate to control the defect.

When the value of 1.9Al-3.4Ti in the above formula (1) is 0.002 or more, since the amount of Ti to reduce the amount of AlN precipitates is insufficient, the slab / bar plate edge cracks can be caused and the quality of the hot-rolled steel sheet can be reduced.

Meanwhile, in order to produce the final product by applying the steel sheet of the present invention to an automobile body, a welding process is performed, and resistance spot welding (RSW) is applied most. The steel sheet of the present invention has excellent RSW weldability and weld material properties.

The steel sheet of the present invention not only satisfies the alloy composition described above, but also has Ceq of 0.18 to 0.28 expressed by the following formula (2).

[Relation 2]

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

 [Each symbol of the element in the formula (2) is a value indicating the content of each element in weight%

The above formula (2) is a component relational expression for securing the weldability of the steel sheet. In the present invention, by controlling the Ceq value to 0.18 to 0.28, excellent spot weldability can be ensured and excellent mechanical properties can be imparted to the welded portion . The Ceq value is more preferably from 0.18 to 0.27, and still more preferably from 0.18 to 0.26.

When Ceq is less than 0.18, the curing ability is low and it may be difficult to secure the target tensile strength. On the other hand, when Ceq exceeds 0.28, the weldability is lowered and the physical properties of the welded portion may deteriorate. More preferably 0.18 to 0.27, and even more preferably 0.19 to 0.26.

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 alloy composition described above, at least one of Cu, Ni, Mo, Sn, and Pb may be included as a tram element, and the total content thereof 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.

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

In the microstructure of the hot-rolled steel sheet according to the present invention, the fraction of ferrite and martensite combined is 80% or more in area fraction, and the remainder is composed of bainite structure. Preferably, it contains 40 to 60% of ferrite in an area fraction, 30 to 50% of martensite and 10 to 20% of bainite.

When the ferrite fraction exceeds 60%, it is difficult to secure the desired strength. When the ferrite fraction is less than 40%, the fraction of the remaining martensite and bainite structure increases, which is difficult to secure the ductility.

When the martensite fraction exceeds 50%, the strength becomes too high to secure the ductility. When the martensite fraction is less than 30%, it may be difficult to secure the desired strength.

On the other hand, the reason why the bainite structure is partially included in the microstructure of the hot-rolled steel sheet is as follows.

Generally, the DP (dual phase) composed of two phases of ferrite + martensite has a high martensite fraction, so that martensite is tempered in the heat affected zone during welding, resulting in softening phenomenon and a decrease in strength. If the bainite structure is secured to some extent instead of the martensite, such problems can be solved and the strength and workability can be secured simultaneously due to the bainite structure characteristics. Also, in the case of DP steel, there is a problem in that, due to the difference in interfacial strength between the soft ferrite and the hard martensite, fracture occurs at the interface first and the workability is improved. 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 fraction is less than 10%, the above-mentioned effect is insufficient. On the other hand, if it exceeds 20%, the strength becomes too high and it may be difficult to secure the ductility.

The average size of the ferrite grains measured by the circle equivalent diameter may be 5 탆 or less. More preferably not more than 4 mu m, and even more preferably not more than 3 mu m. 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.

On the other hand, the hot-rolled steel sheet of the present invention may contain 5 to 1000 Ti / ² 탆 ² precipitates, more preferably 5 to 500 Ti / ² 탆 ² , and still more preferably 5 To 200 pieces / 탆 ² , and the Ti (C, N) precipitate may have an average size of 50 nm or less in circle equivalent diameter. Here, Ti (C, N) precipitates are meant to include TiN, Ti (C, N), TiC and complex precipitates thereof.

When the size of the precipitate exceeds 50 nm, it may be difficult to effectively secure the strength. When the number of precipitates is less than 5 / 탆 ² , it may be difficult to secure a desired strength. On the other hand, in the case where the number of precipitates is more than 1000 / 탆 ² , the elongation rate is lowered due to the increase of the strength, and processing may become difficult.

In the case where the hot-rolled steel sheet is manufactured by a performance-rolling direct process including a rough rolling process for producing a bar plate, the molar fraction (%) of the AlN precipitate at the edge portion of the rough- (3) can be satisfied.

[Relation 3]

Molar fraction (%) of AlN precipitate at the edge of the bar plate < 8.3X10 ^-6

When the molar fraction (%) of the AlN precipitate at the edge portion of the bar plate in the formula (3) is 8.3X10 ^{-6 or} more, cracking of the bar plate edge may occur, and the quality of the hot rolled steel sheet may be poor.

Further, the thickness of the hot-rolled steel sheet of the present invention may be 3.0 mm or less, preferably 2.0 mm or less, and more preferably 1.5 mm or less.

 The hot-rolled steel sheet of the present invention may have a tensile strength of 980 MPa or more, an elongation of 10% or more, a deviation (material variation) of tensile strength of 30 MPa or less, and preferably 20 MPa or less.

A super high strength hot-rolled steel sheet according to another preferred embodiment of the present invention is manufactured by a performance-rolling direct process, and includes 0.030 to 0.085% of C, 1.8 to 3.0% of C, 0.03 to 1.0% of Si, 0.03 to 1.0% of C, : 0.005 to 0.05%, S: 0.01% or less, Cr: 0.2 to 2.0%, Al: 0.01 to 0.07%, Ti: 0.005 to 0.10%, B: 0.0005 to 0.0050% And other inevitable impurities and has a microstructure containing 40 to 60% of ferrite in an area fraction, 30 to 50% of martensite and 10 to 20% of bainite, and in resistance point welding after pickling treatment, The optimum welding range between the lower current satisfying the minimum nugget diameter of 4 t ^1/2 [t = the base material thickness (mm)] and the upper current causing the scattering is 1.5 (kA) or more and the minimum nugget diameter is 5 t ^{1 / 2} [t = thickness of base material (mm)] appropriate range of the welding current and the upper current to meet the lower limit is not less than 1.0 (KA), the cross tensile strength (cross tensile strength, CTS) compared to the tensile shear strength (Tensi le shear strength, TSS) is 35% or more.

The appropriate welding range in resistance spot welding (RSW) is a key index for evaluating the weldability and can be defined as the range between the upper limit current and the lower limit current. The optimum welding range between the lower current satisfying the minimum nugget diameter of 4t ^1/2 (t is the base material thickness (mm)) and the upper current causing the expansion is more than 1.5 (kA), and the minimum nugget diameter is 5t ^{1 / 2} , the appropriate welding range between the lower current limit and the upper current limit may be 1.0 (KA) or more.

If the appropriate welding range is less than the above range, the range in which the welding is possible is too narrow, which is not only difficult to weld in the case of the body part welding but also may cause excessive welding defective products. Here, the minimum nugget diameter 4t ^1/2 means a minimum nugget diameter required to switch to the pull-out fracture (Pull Out Fracture, POF) at the interface fracture (Fracture Interfacial, IF). In AHSS with a tensile strength of 980 MPa or more, martensite structure is formed in the molten portion due to a high carbon equivalent (Ceq) and a rapid cooling rate peculiar to the RSW, so that the sensitivity of the interfacial determination at the interface is increased, 4t ^1/2 can not be matched and the lower current management satisfying the minimum nugget diameter 5t ^1/2 may be important. The upper limit current is defined as the value obtained by subtracting 0.2 (kA) from the current generated by scattering.

In RSW, the ductility ratio (CTS / TSS) is defined as the ratio of Tensile Shear Strength (TSS) to Cross Tensile Strength (CTS), and is a comprehensive index that determines the mechanical properties of the weld in the AHSS RSW . The ductility ratio of the developed steel may be at least 35% over a given appropriate welding current range. If it is less than 35%, the passenger may not be able to stably protect the passenger in the event of a body collision in order to open the weld strength characteristic and the collision stability.

Furthermore, it is preferable that the Vickers hardness of the fusion zone, which is the RSW nugget of the steel sheet of the present invention, is 350 to 450 Hv. If it is lower than 350Hv, sufficient hardness of the molten part can not be secured and the strength of the welded part may be low. However, when it exceeds 450 Hv, the strength of the welded portion and especially the impact absorption energy may be low because the hardness of the molten portion is too high, the sensitivity of cracking is high, and the interfacial fracture where fracture occurs at the nugget interface becomes high. The Vickers hardness of the fusion zone is more preferably 360 to 440 Hv, and even more preferably 370 to 430 Hv.

On the other hand, the softened part is a typical phenomenon observed in the AHSS welded part having a high martensite fraction, and when the softened part hardness is too low, the softened part may be judged and the strength of the welded part may be low. Accordingly, in the present invention, the difference between the hardness of the base metal and the minimum hardness of the softening zone may be 100 Hv or less. More preferably 90 Hv or less, and even more preferably 80 Hv or less. If it is more than this value, breakage occurs in the softened portion, and it may be difficult to secure sufficient strength of the welded portion.

The microstructure of the nugget (melting portion) of the RSW of the inventive steel sheet preferably has a martensite structure in the nugget of 95% or more.

Hereinafter, a method of manufacturing an ultra-high strength hot-rolled steel sheet according to another preferred embodiment of the present invention will be described in detail.

According to still another aspect of the present invention, there is provided a method of manufacturing a hot-rolled steel sheet by a direct-rolling process, which comprises continuously casting molten steel having the alloy composition described above into a slab having a thickness of 60 to 120 mm ;

Spraying the slab with cooling water at a pressure of 50 to 350 bar to remove the surface scale;

Rolling the scaled slab to obtain a bar plate;

Spraying cooling water onto the bar plate at a pressure of 50 to 350 bar to remove surface scale;

Finishing rolling the bar plate from which the scale has been removed in a temperature range of Ar ₃ -30 캜 to Ar ₃ + 60 캜 to obtain a hot-rolled steel sheet;

Cooling the hot-rolled steel sheet in a run-out table for 1 to 8 seconds and then cooling it to a martensitic transformation completion temperature (Mf) of 150 ° C / sec or more; And

And winding the cooled hot-rolled steel sheet at a martensitic transformation completion temperature (Mf) or lower.

Hereinafter, each step will be described in detail.

Continuous casting step

The molten steel having the above-mentioned alloy composition is continuously cast to prepare a slab having a thickness of 60 to 120 mm. At this time, the casting speed is preferably set to, for example, 4 to 8 mpm. The reason why the casting speed is preferably 4 mpm or more is because a high speed casting and a rolling process are connected to each other and a casting speed higher than a certain level is required to secure the target rolling temperature. In addition, if the casting speed is slow, there is a risk of segregation from the cast steel. If such 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. When the casting speed exceeds 8 mpm, the operation success rate may be reduced due to instability of the molten steel bath surface, so that the casting speed is preferably set to 4 to 8 mpm. More preferably 4.2 to 7.2 mpm, and even more preferably 4.5 to 6.5 mpm.

On the other hand, when the thickness of the slab is more than 120 mm, high-speed casting is difficult, and the rolling load during rough rolling is increased. When the slab thickness is less than 60 mm, the temperature of the cast steel is rapidly decreased. In order to solve this problem, it is possible to additionally provide a heating device, but this increases the production cost, so it is preferable to exclude it. Therefore, the thickness of the slab is limited to 60 to 120 mm. More preferably 70 to 110 mm, and even more preferably 80 to 100 mm.

The slab may be heated before the surface descaling step, and the slab heating temperature may be 900 to 1200 ° C. When the surface temperature of the slab is less than 900 캜, there is a possibility that cracks are generated in the edge portion of the bar plate in the course of an increase in the rough rolling load and in the rough rolling. 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. Therefore, the heating temperature of the slab is preferably in the range of 900 to 1200 ° C. More preferably 950 to 1150 占 폚, and still more preferably 1000 to 1100 占 폚.

Slab scale removal step

The scale is removed by injecting cooling water at a pressure of 50 to 350 bar into the cast slab or the heated slab after casting. For example, cooling water of 50 ° C or less is sprayed at a pressure of 50 to 350 bar in a roughing scale breaker (hereinafter, referred to as 'RSB') to obtain a surface scale thickness of, for example, 300 μm or less The scale can be removed. If the pressure is less than 50 bar, a large amount of arithmetic scale scale or the like may remain on the surface of the slab and the surface quality may become dull after pickling. On the other hand, if the temperature exceeds 350 bar, the edge temperature of the bar plate may drop rapidly, and an edge crack may occur. The cooling water injection pressure may be more preferably 100 to 300 bar, and still more preferably 150 to 250 bar.

Rough rolling step

The scale-removed slab is rough-rolled, for example, in a roughing mill composed of 2 to 5 stands to obtain a bar plate. The surface temperature of the slab at the inlet side of the roughing mill is preferably in the range of 900 to 1200 ° C. When the surface temperature of the slab is less than 900 캜, there is a possibility that cracks are generated in the edge portion of the bar plate in the course of an increase in the rough rolling load and in the rough rolling. 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. Therefore, the surface temperature of the slab preferably ranges from 900 to 1200 ° C. More preferably 950 to 1150 占 폚, and still more preferably 1000 to 1100 占 폚.

It is preferable that the edge temperature of the rough-rolled bar plate at the time of rough rolling has a range of 780 to 1100 ° C. If the edge portion temperature is less than 780 ° C, the AlN precipitates precipitate and the susceptibility to edge cracking may become very high as the high temperature ductility deteriorates. On the other hand, when the edge temperature exceeds 1100 ° C, the temperature of the center of the thin slab becomes too high, so that a large number of arithmetic scale may occur and the surface quality may become poor after pickling. The temperature at the edge of the bar plate at the time of rough rolling during the rough rolling may be more preferably 800 to 1080 ° C, and still more preferably 820 to 1060 ° C.

At this time, it is preferable that the edge temperature of the bar-rolling-out side bar plate not only satisfies the above-described respective numerical ranges but also satisfies the following formula (3) in order to improve surface and edge quality while ensuring high strength.

[Relation 3]

Molar fraction (%) of AlN precipitate at the edge of the bar plate < 8.3X10 ^-6

(3), that is, when the AlN mole fraction (%) precipitated at the bar plate edge portion temperature in the equation (3) is 8.3 If X10 ^{-6 or} higher, cracking of the bar plate edge may occur and the quality of the hot-rolled steel sheet may be poor.

On the other hand, cumulative rolling reduction during rough rolling plays an important role in obtaining a uniform target material in the present invention. That is, as the reduction rate in rough rolling increases, microstructures such as Mn, Si, and Cr, which are important elements in the manufacture of high strength steel, become uniform and the temperature gradient in the width and thickness direction of the strip becomes smaller. Valid. However, if the cumulative rolling reduction is less than 60%, the above effects can not be sufficiently exhibited. If the cumulative rolling reduction exceeds 90%, the rolling resistance increases greatly and the manufacturing cost rises. It is preferable to perform rolling. , More preferably 65 to 85%, and still more preferably 70 to 80%.

Bar plate scale removal step

The scale is removed by injecting cooling water into the bar plate at a pressure of 50 to 350 bar. For example, before the finish rolling of the bar plate, cooling water of 50 ° C or less is sprayed at a pressure of 50 to 350 bar in a Finishing Mill Scale Breaker (FSB) nozzle, so that the surface scale thickness is, for example, The scale can be removed so as to be not more than 30 mu m. The cooling water injection pressure is more preferably 100 to 300 bar, and even more preferably 150 to 250 bar.

If the pressure is less than 50 bar, scale removal is insufficient, and 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 exceeds 350 bar, the finish rolling temperature becomes too low to obtain an effective austenite fraction and it is difficult to secure a target tensile strength.

Finishing rolling step

The bar plate on which the scale has been removed is finish-rolled in a finishing mill composed of a plurality of stands, for example, three to six stands.

Since the ultrahigh strength steel of 980 MPa class, which is the object of the present invention, utilizes the formation of the transformed structure as an strengthening mechanism, there is a high possibility that the material properties change according to the strain rate during finish rolling. That is, when the rolling speed difference between the top and the tail of the steel sheet exceeds 10% during finishing rolling in a finishing mill composed of a plurality of stands, a uniform cooling rate and a uniform cooling rate in a subsequent run- It is difficult to obtain the target coiling temperature, which results in causing a material variation in the width or length direction of the strip. Therefore, it is preferable to control so that the difference in rolling speed between the top of the steel sheet and the tail in the finishing rolling step does not exceed 10%.

The finishing rolling temperature is preferably Ar ₃ -30 캜 to Ar ₃ + 60 캜. When the finish rolling temperature is lower than Ar ₃ -30 ° C, the load of the roll during hot rolling is greatly increased to increase the energy consumption and the working speed, and since a sufficient austenite fraction can not be secured, the target microstructure and material are secured Can not. On the other hand, when the finish rolling temperature is higher than Ar ₃ + 60 ° C, the crystal grains can not cooperate with each other to obtain a high strength. In order to obtain a sufficient bainite and martensite structure, the cooling rate must be further increased. The finishing rolling temperature is more preferably Ar ₃ -20 캜 to Ar ₃ + 50 캜.

At this time, the finish rolling can be performed such that the sheet passing speed is 200 to 600 mpm and the thickness of the hot-rolled steel sheet is 3.0 mm or less. Preferably 2.0 mm or less, and more preferably 1.6 mm or less.

If the finish rolling speed exceeds 600 mPm, it is possible to cause an operation accident such as plate breakage, and because isothermal constant velocity rolling is difficult, uniform temperature can not be ensured and material deviation may occur. On the other hand, in the case of less than 200 mpm, the finish rolling speed is too slow to secure the finishing rolling temperature.

Cooling and winding steps

The finish-rolled hot-rolled steel sheet is, for example, air-cooled for 1 to 8 seconds in a run-out table, cooled to a martensitic transformation completion temperature (Mf) or less at a cooling rate of 150 ° C / sec or more and wound.

The cooled hot rolled steel sheet undergoes air cooling for 1 to 8 seconds on the runout table. If the time is less than 1 second, the C concentration in the retained austenite is insufficient and the time for ferrite transformation is insufficient, And if it exceeds 8 seconds, there is a problem in securing a desired tensile strength due to excessive transformation of ferrite, and also a problem that a length of the equipment is long or productivity is lowered. Therefore, the air- To 8 seconds. The air cooling time is more preferably 1.5 to 6.5 seconds, and still more preferably 2.0 to 5.0 seconds.

On the other hand, it is preferable that the cooling rate during the cooling after the air cooling is 150 ° C / sec or more, and the coiling temperature is preferably not higher than the martensitic transformation completion temperature (Mf).

If the cooling rate is slower than 150 ° C / sec, ferrite transformation is promoted and cementite is formed, making it difficult to obtain a desired material. Further, when the coiling temperature exceeds the martensitic transformation completion temperature (Mf), it is difficult to obtain a sufficient martensite structure and the martensite obtained by cooling can be auto tempered to obtain a desired tensile strength Can be difficult.

Meanwhile, in the present invention, it is possible to further include a step of pickling the hot-rolled steel sheet to obtain a pickled &

In the present invention, scales are sufficiently removed in the step of removing the slab and bar plate scale, 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.

18, which illustrates a preferred lay-out of a performance-rolling direct process, to which the present invention may be applied, and another preferred example of a performance-rolling direct process, to which the present invention may be applied, out of the hot-rolled steel sheet according to the present invention will be described with reference to Fig.

The high-strength hot-rolled steel sheet having a small material deviation of 980 MPa and excellent in surface quality according to one aspect of the present invention can be manufactured through the performance-rolling direct-joining process as shown in FIGS. 18 and 19.

As shown in FIG. 18, the performance-to-rolling direct process layout comprises a high-speed continuous casting machine 100 for producing slabs (a) of a first thickness and a high-speed continuous casting machine 100 for producing slabs of a second thickness A roughing mill 600 for rolling the bar of the second thickness to a hot-rolled steel sheet c of a third thickness, and a winder 900 for winding the hot-rolled steel sheet, .

A thin slab a having a thickness of 60 to 120 mm is manufactured in the continuous casting machine 100 and the bar plate b is further heated in the heater 200 to ensure a sufficient finish rolling temperature. A roughing scale breaker 300 (hereinafter, referred to as 'RSB') and a finishing mill scale breaker (FSB) 500 are placed in front of the finishing mill 600 in front of the roughing mill 400 (Pickled & Oiled), which is excellent in surface quality when picking hot-rolled steel sheets in a later process, is also possible. In addition, it is possible to perform isothermal constant-speed rolling by the performance-rolling direct process, so that the steel plate width and the longitudinal temperature deviation are remarkably low, so precise cooling control is possible in ROT [Run Out Table (700)] It is possible to produce excellent high strength hot rolled steel sheets. The hot-rolled steel sheet thus rolled and cooled is cut by a high-speed shear machine 800 and wound by a winder 900 to be produced as a product.

19, the layout of the performance-rolling direct process includes the slab addition heater 200 in front of the roughing mill 400 in addition to the one shown in FIG. 18, whereby the slab edge temperature can be easily secured and the occurrence of edge defects can be reduced It is advantageous for ensuring surface quality. In addition, a space of at least one slab length is secured before the roughing mill, and batch rolling is also possible.

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 1)

Molten steel having the composition shown in Table 1 below was prepared. Table 1 below shows the C content of the initiation of the apodization reaction and is a value calculated using Thermo-Calc-3.0.1 Console Mode (Database: TCFE6), a commercial thermodynamic software.

Using the molten steel, a hot-rolled steel sheet having a thickness of 1.6 mm was produced in the continuous rolling mode in the performance-to-rolling direct connection process according to the production conditions shown in Table 2 below (Inventive Example 1 -7 and Comparative Examples 1 to 11). In the case of Conventional Example 1, a hot-rolled steel sheet having a thickness of 3.2 mm was produced in the batch mode in the conventional mini-mill process by applying the manufacturing conditions shown in Table 2. [

In Table 2 below, the Ar ₃ temperature and the Mf temperature are described, and the Ar ₃ temperature and the Mf temperature are values calculated using a commercial thermodynamic software, JmatPro-v9.1.

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, finishing rolling scale brake) represents cooling water injection pressure after rough rolling.

On the other hand, after the result of the casting stability evaluation and the hot-rolled steel sheet produced as described above were pickled to obtain a PO material, the microstructure (phase fraction, ferrite grain size (FGS), yield strength (YS), tensile strength Table 3 shows the results of measuring elongation (EL), deviation (material deviation) (? TS) of tensile strength, occurrence of edge cracks and PO surface quality.

The area fraction of ferrite (F), martensite (M) and bainite (B) was determined by scanning electron microscope (SEM) (FGS, Ferrite Grain Size) of 10 areas randomly photographed at a magnification of 3,000 times using Scanning Electron Microscope, image area ratio measurement using Image-Plus Pro software, Shows an average value measured at a circle-equivalent diameter using Image-Plus Pro software after randomly photographing 10 points at 3,000 times magnification using Electron Backscatter Diffraction (EBSD).

The tensile strength is a value measured by taking a JIS No. 5 specimen in a direction perpendicular to the rolling direction at a width W / 4, and a tensile strength deviation (material deviation) [? TS (MPa)] is a tensile strength It represents the value obtained by subtracting the minimum value from the maximum value among the intensity values.

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

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

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

Steel grade	Alloy element (% by weight)										Apoptotic response threshold C (%)	Equation (1)	Equation (2)
	C	Mn	Si	P	S	Ti	Al	Cr	B	N
Inventive Steel A	0.068	2.20	0.050	0.015	0.0010	0.023	0.035	0.80	0.0015	0.0070	0.085	0.0008	0.21
Invention steel B	0.069	2.23	0.045	0.015	0.0011	0.025	0.038	0.79	0.0013	0.0069	0.087	0.0009	0.21
Inventive Steel C	0.066	2.21	0.040	0.014	0.0009	0.026	0.039	0.82	0.0016	0.0076	0.086	0.0009	0.22
Comparative Steel D	0.121	2.19	0.045	0.011	0.0011	0.024	0.032	0.89	0.0014	0.0081	0.107	0.0005	0.26
Comparative Steel E	0.069	1.75	0.052	0.016	0.0011	0.025	0.036	0.81	0.0015	0.0071	0.095	0.0008	0.19
Comparative Steel F	0.070	2.22	0.061	0.015	0.0013	0.002	0.035	0.79	0.0014	0.0080	0.089	0.0023	0.22
Conventional steel G	0.050	1.45	0.680	0.020	0.0022	0.001	0.041	-	-	0.0070	0.053	0.0027	0.23

In Table 1, the formula (1) is represented by 1.9Al-3.4Ti, and each symbol represents a value (weight (%) / atomic weight of each element) obtained by dividing the weight (%) of each element by the atomic weight, The formula (2) is Ceq = C + Si / 30 + Mn / 20 + 2P + 3S, and each symbol represents the content of each element in weight%.

Example No. 2.	Steel grade	Rough rolling out bar plate edge temperature (캜)	Equation (3) (%)	RSB (Bar)	FSB (Bar)	Finishing rolling temperature (캜)	Ar 3 (° C)	Ar 3 -30 캜 (캜)	Mf (占 폚)	Air cooling time (sec)	ROT cooling rate (° C / sec)	Coiling temperature (캜)
Inventory 1	Inventive Steel A	880	<8.3X10 -7	189	216	811	760	730	282	3.4	225	165
Inventory 2		890	<8.3X10 -7	200	230	815				3.5	215	170
Inventory 3		895	<8.3X10 -7	181	210	814				3.2	210	169
Honorable 4	Invention steel B	900	<8.3X10 -7	205	200	815	761	731	299	3.1	195	175
Inventory 5		882	<8.3X10 -7	210	195	820				3.5	200	180
Inventory 6		892	<8.3X10 -7	195	210	816				3.7	215	168
Example 7	Inventive Steel C	885	<8.3X10 -7	190	220	809	759	729	281	3.2	219	175
Comparative Example 1		891	<8.3X10 -7	198	223	810				8.5	221	169
Comparative Example 2		889	<8.3X10 -7	200	219	808				3.1	95	172
Comparative Example 3		882	<8.3X10 -7	201	211	807				3.0	228	298
Comparative Example 4		895	<8.3X10 -7	199	395	710				2.9	230	169
Comparative Example 5		886	<8.3X10 -7	40	220	810				3.2	223	166
Comparative Example 6		890	<8.3X10 -7	195	30	812				3.1	230	174
Comparative Example 7		770	8.31X10 -6	180	200	712				3.0	230	155
Comparative Example 8	Comparative Steel D	-	-	-	-	-	740	710	259	-	-	-
Comparative Example 9	Comparative Steel E	885	<8.3X10 -7	197	212	803	790	760	305	3.4	229	155
Comparative Example 10	Comparative Steel F	880	41. 05X10 -5	195	209	811	775	745	280	3.0	220	165
Comparative Example 11		1110	0	200	250	825				3.2	219	162
Conventional Example 1	Conventional steel G	885	51. 05X10 -5	35	25	780	870	840	341	5.1	70	200

In the above Table 2, the formula (3) shows the molar fraction (%) of the precipitated AlN precipitates at the rough rolling out side bar plate temperature

Example No. 2.	Steel grade	Casting stability (whether molten steel is spilled)	Microstructure (area%)			FGS (占 퐉)	Tensile Properties				Edge cracking	PO surface quality
			F	M	B		YS (MPa)	TS (MPa)	? TS (MPa)	EL (%)
Inventory 1	Inventive Steel A	X	49	38	13	1.6	769	1010	13	13	X	O
Inventory 2		X	48	40	12	1.7	759	1009	14	13	X	O
Inventory 3		X	49	39	12	1.6	760	1011	13	13	X	O
Honorable 4	Invention steel B	X	48	41	11	1.8	761	1008	14	13	X	O
Inventory 5		X	48	39	13	1.5	755	1009	13	13	X	O
Inventory 6		X	47	41	12	1.6	756	1005	13	14	X	O
Example 7	Inventive Steel C	X	49	39	12	1.6	771	1009	12	13	X	O
Comparative Example 1		X	62	30	8	1.8	687	895	11	15	X	O
Comparative Example 2		X	62	28	10	1.7	659	879	12	16	X	O
Comparative Example 3		X	62	29	9	2.0	620	838	10	17	X	O
Comparative Example 4		X	64	28	8	2.0	570	820	12	17	X	O
Comparative Example 5		X	50	38	12	1.6	760	1011	12	12	X	X
Comparative Example 6		X	50	39	11	1.6	765	1008	14	13	X	X
Comparative Example 7		X	65	27	8	1.9	568	816	13	17	O	O
Comparative Example 8	Comparative Steel D	O	Casting stopped due to molten steel outflow, unmeasured
Comparative Example 9	Comparative Steel E	X	59	29	12	1.8	721	910	14	14	X	O
Comparative Example 10	Comparative Steel F	X	52	37	11	2.0	750	1001	15	12	O	O
Comparative Example 11		X	50	39	11	1.9	770	1009	13	14	X	X
Conventional Example 1	Conventional steel G	X	75	25	-	5.2	451	611	35	29	O	X

In Table 3, F is Ferrite (ferrite), M is Martensite (martensite), B is Bainite (bainite) structure, and FGS is Ferrite Grain Size. YS represents Yield Strength, TS represents Tensile Strength, and EL Elongation. As shown in Tables 1, 2 and 3, Examples 1 to 7 satisfying the range of the present invention satisfy the target tensile strength (980 MPa or more) and elongation (10% or more) without slip of the molten steel in the production of the slab, and the edge surface quality and the surface quality Are also excellent. It is also understood that tensile strength and yield strength of Examples 1 to 7 are higher than those of Conventional Example 1. [

On the other hand, the structure of Inventive Example 7 was photographed using an optical microscope and a scanning electron microscope (SEM), the photographs of the optical microscope were shown in Fig. 1, and the photographs of the scanning electron microscope were shown in Fig. 1 and 2, it can be seen that the microstructure of the inventive example 7 is composed of ferrite (F) and M (martensite) in a phase and some bainite (B) is uniformly present .

The ferrite grain size distribution of Inventive Example 7 was measured using EBSD (Electron Backscatter Diffraction), and the results are shown in FIG. From Fig. 3, it can be seen that the crystal grains of 5 mu m or less are finely dispersed.

The precipitate of Example 7 was photographed with a transmission electron microscope (TEM), and the photograph is shown in Fig. 4. The left side of Fig. 4 is a 50,000 magnification photograph, and the right side is a 300,000 magnification photograph. From FIG. 4, it can be seen that the TiN, Ti (C, N) and the rounded TiC precipitates are uniformly distributed in the matrix.

Observing the invention example 7 per unit area (㎛ ²⁾ distribution of the precipitate the number of, and as described in the showed the results in Figure 5, can be seen in Figure 5, the deposit number is 5-30 / ㎛ ² mainly distributed in the range , And the average number of precipitates was 15 / 탆 ² . Here, the number of precipitates is obtained by quantifying the number of precipitates existing within a square of 1 mu m x 1 mu m in a tissue photograph obtained by making a sample by the carbon (carbon) replica method and photographing 50 places randomly at a magnification of 100,000 times with TEM .

The distribution of the precipitate size in Inventive Example 7 was observed. The results are shown in Fig. 6. As can be seen from Fig. 6, the precipitate size was mainly distributed in the range of 5 to 50 nm, and the average precipitate size was 20 nm. Here, the precipitate size was obtained by measuring the precipitate size using Image-Plus Pro software after preparing a sample by the carbon re-firing method and photographing 5 sheets of 50,000 times, 20 sheets of 100,000 sheets and 5 sheets of 300,000 sheets by TEM. TEM images were taken at different magnifications to precisely measure fine (less than 50 nm) precipitates and coarse (greater than 50 nm) precipitates.

FIG. 7 shows a surface photograph of the PO material obtained by pickling the hot-rolled steel sheet of Inventive Example 7, and FIG. 7 shows that the surface quality is excellent.

On the other hand, Comparative Examples 1, 2 and 3 do not satisfy the air cooling time, cooling rate, and coiling temperature proposed in the present invention, and do not satisfy the target microstructure and tensile strength.

Comparative Example 4 shows that the surface of the steel strip is hardly cooled because the surface of the steel strip is higher than the pressure of the FSB 1 and 2 columns proposed in the present invention and the finish rolling temperature is lowered rapidly so that the desired microstructure and tensile strength are not satisfied .

Comparative Examples 5 and 6 did not satisfy the RSB or FSB pressure suggested in the present invention, indicating that the surface quality is favorable.

As can be seen from FIG. 8 showing a slab photograph of Comparative Example 8 where molten steel outflow occurred, Comparative Example 8 did not satisfy the C content shown in the present invention, and molten steel leakage occurred in the slab, Respectively. The cause of casting interruption can be explained by the phase transformation behavior.

FIGS. 9 and 10 show state diagrams of Example 7 and Comparative Example 8 calculated using the Thermo-Calc-3.0.1 Console Mode (Database: TCFE6). As can be seen from the state diagram of Inventive Example 7, since the low-temperature C component is lower than the initiation reaction critical temperature C, the apodization reaction does not occur and high-speed casting is possible without molten steel leakage.

However, in the case of Comparative Example 8, since the low-temperature C component is higher than the apoptosis initiation threshold C, apodization reaction occurs, and it is presumed that molten steel outflow occurs due to the formation of a non-uniform solidification shell.

The comparative example 7 did not satisfy the formula (3) proposed in the present invention, and the comparative example 10 did not satisfy the formula (1) and the formula (3) . These edge cracks are closely related to the precipitation behavior of AlN.

11 and 12 show the precipitation behavior according to the temperature of the inventive example 7 and the comparative example 10 calculated using Thermo-Calc-3.0.1 (Database: TCFE6), respectively. Fig. 13 shows the conventional example 1. Fig. As can be seen from this result, in the case of Inventive Example 7 which satisfies the formula (1) and the Ti content proposed in the present invention, TiN precipitates near the solidification temperature of the molten steel and the amount of AlN precipitates is remarkably decreased, . However, in the case of Comparative Example 10 and Comparative Example 1 which do not satisfy the formula (1) and Ti content, AlN precipitates at a high temperature and a large amount of AlN is precipitated in the edge temperature range (780 to 1100 ° C) . Therefore, in order to secure surface and edge quality while ensuring high strength, it is preferable to precisely control it so as to satisfy equations (1) and (3).

Comparative Example 9 did not satisfy the target tensile strength because the Mn content was not satisfied in the present invention and Comparative Example 11 did not satisfy the edge temperature range of the rough rolling out side bar plate proposed in the present invention It can be seen that the arithmetic scale is generated in large quantity and the surface quality of the PO material is heat.

(Example 2)

In the process of applying the steel sheet according to the present invention to an automobile body and producing a final product, most welding processes are performed, and resistance spot welding (RSW) is most frequently performed. Therefore, it is necessary to evaluate the RSW welded part of the steel sheet. Thus, the steel sheet of Inventive Example 7 of Example 1 was subjected to the RSW weld portion evaluation, and the results are shown in Figs. 14 to 17. Fig.

At this time, the evaluation of the RSW welded part was carried out according to the ISO 18728-2 shown in Table 4, and the welding current having the greatest influence on the nugget diameter and strength was changed at intervals of 0.2 (kA) Respectively.

Welding conditions (Single phase AC, 60Hz)						Electrode Specifications
Pressing force (kN)	Welding time (cycle)	Holding time (cycle)	Squeeze time (cycle)	Number of cooling water (l / min)	Welding current (kA)
4	17	17	40	4	5.0 to 9.0	Cr-Cu alloy, R type, 6 mm

14 shows a cross-sectional structural change of a nugget according to the welding current with respect to the inventive example 7. Fig. The nugget diameter was defined as the straight line distance between the bond line (BL) and the bond line, as in the cross-section after cutting the nugget at 1/2. As can be seen from FIG. 14, the nugget diameter increases with the increase of the welding current, and it is found that the nugget has a healthy weld portion without defects such as cracks and pores. FIG. 15 shows the hardness distribution of the resistance spot weld portion . The hardness was measured with a Vickers hardness gauge at 200 탆 intervals with 200 g load on the nugget diagonal. The weld is divided into nose (Fusion, FZ), heat affected zone (HAZ), softening zone (SZ) near the base material and base material (BM). 15, it can be seen that the hardness of the molten portion has a value of 370 to 400 Hv, and the difference between the hardness of the base material and the minimum hardness of the softened portion is 100 Hv or less. The softened part is a typical phenomenon observed in the AHSS weld part having a high martensite fraction. When the hardness of the softened part is too low, the softened part may be judged and the strength of the welded part may be low. Therefore, it is preferable that the difference between the hardness of the base material and the minimum hardness of the softened portion is 100 Hv or less.

16 shows the change in ductility ratio (%) according to the welding current with respect to the inventive example 7. The ductility ratio (CTS / TSS) is defined as the ratio of Tensile Shear Strength (TSS) to Cross Tensile Strength (CTS) and is used as a comprehensive index to judge the characteristics of the weld in the AHSS RSW . In FIG. 16, the minimum nugget diameter 4t1 ^{/ 2} (t = thickness of the base material) and 5t1 ^{/ 2} for transition to the pull out fracture (POF) in the interface fracture (IF) The lower limit current is indicated.

The reason why 5t1 ^{/ 2} is used is that AHSS with a tensile strength of 980 MPa or higher has a high carbon equivalent (Ceq) and a rapid cooling rate specific to RSW, so that a martensite structure is formed in the molten portion and the fracture shear sensitivity to the interface , So it is indicated as 4t ^{1/2 that} is based on the existing standard is not suitable. The upper limit current is defined as a value obtained by subtracting 0.2 (kA) from the current generated by scattering. As can be seen from this result, IF break occurred at current satisfying 4t ^1/2 , and POF break occurred at current more than this.

On the other hand, the appropriate welding range in RSW is a key index for evaluating weldability and can be defined as the interval between the upper limit current and the lower limit current. The optimum welding range between the lower current limit and the upper current limit satisfying the minimum nugget diameter of 4t ^1/2 is 2.6 (kA), and the minimum nugget diameter is 5t ^1/2 , the optimum welding range is 1.8kA. It can be seen that the ductility ratio has more than 45% in the given appropriate welding current range regardless of the fracture.

17 shows a SEM micrograph of a molten portion with respect to Example 7. It can be seen that a full lath martensite structure is uniformly present, which has a high carbon equivalent (Ceq) It is judged to be due to rapid cooling rate.

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.

(Explanation of Symbols)

a: Slab b: Bar plate

c: Hot-rolled steel sheet

100: continuous casting machine 200: heater

300: RSB (Roughing Mill Scale Breaker, rough rolling scale brake)

400: rough rolling mill

500: FSB (Fishing Mill Scale Breaker, Finishing Rolled Scale Breaker)

600: finishing mill 700: run-out table

800: High speed shear machine 900: Winder

Claims (29)

1. The steel sheet according to any one of claims 1 to 3, which is manufactured by a casting and rolling direct rolling process, wherein the steel sheet comprises 0.030 to 0.085% of C, 1.8 to 3.0% of Cr, 0.03 to 1.0% of Si, 0.005 to 0.05% of P, 0.001 to 0.050% of N, 0.001 to 0.010% of N, the balance of Fe and other unavoidable impurities, and an area fraction of 40 to 60% % Of ferrite, 30 to 50% of martensite, and 10 to 20% of bainite.
2. The super-high strength hot-rolled steel sheet according to claim 1, wherein Al and Ti satisfy the following relational expression (1).

   [Relation 1]

   1.9Al - 3.4 Ti <0.002

   1.9 and 3.4 represent values of Al / N atomic ratio and Ti (atomic weight), respectively. In the formula (1), each symbol represents the weight (%) of each element divided by the atomic weight / N atomic ratio]
3. The ultra-high strength hot-rolled steel sheet according to claim 1, wherein Ceq expressed by the following formula (2) is 0.18 to 0.28.

   [Relation 2]

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

    [Each symbol of the element in the formula (2) is a value indicating the content of each element in weight%
4. The super-high strength hot-rolled steel sheet according to claim 1, wherein the hot-rolled steel sheet contains at least one of Cu, Ni, Mo, Sn and Pb as a tramp element and the total content thereof is 0.2 wt% or less.
5. The ultra-high strength hot-rolled steel sheet according to claim 1, wherein the bainite is formed at an interface between ferrite and martensite.
6. The ultra-high strength hot-rolled steel sheet according to claim 1, wherein an average size of the ferrite grain measured by the circle equivalent diameter is 5 탆 or less.
7. The hot-rolled steel sheet according to claim 1, wherein the hot-rolled steel sheet contains 5 to 1000 Ti / ² N precipitates, and the average size of the Ti (C, N) precipitates is 50 nm or less in circle- .
8. 3. The method according to claim 1 or 2, wherein the rolling process comprises a rough rolling process for producing a bar plate, wherein a molar fraction (%) of the AlN precipitate at the edge portion of the rough- Super high strength hot-rolled steel sheet.

   [Relation 3]

   Molar fraction (%) of AlN precipitate at the edge of the bar plate < 8.3X10 ^-6
9. The super-high strength hot-rolled steel sheet according to claim 1, wherein the hot-rolled steel sheet has a tensile strength of 980 MPa or more, an elongation of 10% or more, and a tensile strength variation of 30 MPa or less.
10. The ultra-high strength hot-rolled steel sheet according to claim 1, wherein the thickness of the hot-rolled steel sheet is 3.0 mm or less.
11. The steel sheet according to any one of claims 1 to 3, which is manufactured by a casting and rolling direct rolling process, wherein the steel sheet comprises 0.030 to 0.085% of C, 1.8 to 3.0% of Cr, 0.03 to 1.0% of Si, 0.005 to 0.05% of P, And the balance of Fe and other unavoidable impurities, in an area fraction of from 40 to 60%, preferably from 0.2 to 2.0%, from 0.01 to 0.07% of Al, from 0.005 to 0.10% of Ti, from 0.0005 to 0.0050% Of ferrites, 30 to 50% of martensite and 10 to 20% of bainite, and having a minimum nugget diameter of 4 t ^1/2 [t = The optimum welding range between the lower current satisfying the base metal thickness (mm) and the upper current limit causing the scattering is 1.5 (kA) or more and the minimum nugget diameter is 5t ^1/2 [t = base metal thickness (mm)] The optimum welding range between the lower limit current and the upper limit current is 1.0 (KA) or more, and the ultrahigh-strength hot-rolled steel sheet having a ductility ratio of 35% or more, which is a ratio of Tensile Shear Strength (TSS) to Cross Tensile Strength (CTS) River .
12. The ultra-high strength hot-rolled steel sheet according to claim 11, wherein Al and Ti satisfy the following relational expression (1).

    [Relation 1]

    1.9Al - 3.4 Ti <0.002

    1.9 and 3.4 represent values of Al / N atomic ratio and Ti (atomic weight), respectively. In the formula (1), each symbol represents the weight (%) of each element divided by the atomic weight / N atomic ratio]
13. The ultra-high strength hot-rolled steel sheet according to claim 11, wherein Ceq expressed by the following formula (2) is 0.18 to 0.28.

    [Relation 2]

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

     [Each symbol of the element in the formula (2) is a value indicating the content of each element in weight%
14. The method according to claim 11 or 12, wherein the rolling process comprises a rough rolling process for producing a bar plate, wherein a molar fraction (%) of the AlN precipitate at the edge of the roughly rolled bar plate satisfies the following formula (3) Super high strength hot-rolled steel sheet.

    [Relation 3]

    Molar fraction (%) of AlN precipitate at the edge of the bar plate < 8.3X10 ^-6
15. The ultrahigh-strength hot-rolled steel sheet according to claim 11, wherein a Vickers hardness of the melted portion corresponding to the nugget is 350 to 450 Hv and a difference between the hardness of the base material and the minimum hardness of the softened portion is 100 Hv or less.
16. The ultrahigh-strength hot-rolled steel sheet according to claim 11, wherein the microstructure of the molten portion corresponding to the nugget comprises martensite structure in an area ratio of 95% or more.
17. A method for producing a hot-rolled steel sheet by a performance-rolling direct process, comprising the steps of: C: 0.030 to 0.085%, Mn: 1.8 to 3.0%, Si: 0.03 to 1.0%, P: 0.005 to 0.05% The molten steel containing 0.2 to 2.0% of Cr, 0.01 to 0.07% of Al, 0.005 to 0.10% of Ti, 0.0005 to 0.0050% of B, 0.001 to 0.010% of N and the balance of Fe and other unavoidable impurities in a thickness of 60 Continuous casting in a slab of ~ 120 mm;

    Spraying the slab with cooling water at a pressure of 50 to 350 bar to remove the surface scale;

    Rolling the scaled slab to obtain a bar plate;

    Spraying cooling water onto the bar plate at a pressure of 50 to 350 bar to remove surface scale;

    Finishing rolling the bar plate from which the scale has been removed in a temperature range of Ar ₃ -30 캜 to Ar ₃ + 60 캜 to obtain a hot-rolled steel sheet;

    Cooling the hot-rolled steel sheet in a run-out table for 1 to 8 seconds and cooling to a martensitic transformation completion temperature (Mf) or less at a cooling rate of 150 ° C / sec or more; And

    And winding the cooled hot-rolled steel sheet at a martensitic transformation completion temperature (Mf) or lower.
18. 18. The method of producing an ultra-high strength hot-rolled steel sheet according to claim 17, wherein Al and Ti satisfy the following relational expression (1).

    [Relation 1]

    1.9Al - 3.4 Ti <0.002

    1.9 and 3.4 represent values of Al / N atomic ratio and Ti (atomic weight), respectively. In the formula (1), each symbol represents the weight (%) of each element divided by the atomic weight / N atomic ratio]
19. The method for producing an ultra-high strength hot-rolled steel sheet according to claim 17, wherein Ceq expressed by the following formula (2) is 0.18 to 0.28

    [Relation 2]

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

     [Each symbol of the element in the formula (2) is a value indicating the content of each element in weight%
20. 18. The method of producing an ultra-high strength hot-rolled steel sheet according to claim 17, wherein the molten steel contains at least one of Cu, Ni, Mo, Sn, and Pb as a tram element and the total content thereof is 0.2 wt% or less.
21. The method of manufacturing an ultra-high strength hot-rolled steel sheet according to claim 17, wherein the casting speed during continuous casting is 4 to 8 mpm.
22. 18. The method of manufacturing an ultra-high strength hot-rolled steel sheet according to claim 17, wherein the slab is heated before the surface-scaling step.
23. The method of manufacturing an ultra-high strength hot-rolled steel sheet according to claim 22, wherein the heating temperature of the slab is 900 to 1200 占 폚.
24. The method of manufacturing an ultra-high strength hot-rolled steel sheet according to claim 17 or 18, wherein the surface temperature of the rough-rolled ingot slab at the time of rough rolling is 900 to 1200 占 폚 and the edge temperature of the rough-rolled bar bar plate is 780 to 1100 占 폚.
25. The method of manufacturing an ultra-high strength hot-rolled steel sheet according to claim 24, wherein the rough rolling-out side bar plate edge temperature is controlled so that the molar fraction (%) of AlN precipitates precipitated at the bar plate edge temperature satisfies the following formula .

    [Relation 3]

    Molar fraction (%) of AlN precipitate at the edge of the bar plate < 8.3X10 ^-6
26. 18. The method of manufacturing an ultra-high strength hot-rolled steel sheet according to claim 17, wherein the cumulative rolling reduction during rough rolling is 60 to 90%.
27. 18. The method of producing an ultra-high strength hot-rolled steel sheet according to claim 17, wherein the steel sheet has a thickness of not more than 3.0 mm, and the steel sheet has a through-hole speed of 200 to 600 mpm during finish rolling.
28. 18. The method of manufacturing an ultra-high strength hot-rolled steel sheet according to claim 17, wherein the rolling speed difference between the top and the tail of the steel sheet during the finish rolling is 10% or less.
29. 18. The method of manufacturing an ultra-high strength hot-rolled steel sheet according to claim 17, further comprising the step of pickling the rolled hot-rolled steel sheet to obtain a pickled &

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