KR20120001016A

KR20120001016A - Method of manufacturing 1000mpa grade having bore expanding characteristics hot rolled steel sheet

Info

Publication number: KR20120001016A
Application number: KR1020100061577A
Authority: KR
Inventors: 도형협; 이승하; 한성경
Original assignee: 현대제철 주식회사
Priority date: 2010-06-29
Filing date: 2010-06-29
Publication date: 2012-01-04

Abstract

The present invention relates to a method of manufacturing a 1000 MPa grade hot rolled steel sheet having a hole expansion property, and more specifically, to control the content of Si, Ti, Nb, and adding a small amount of Cr, B to increase the strength of the hot rolled steel sheet, The present invention relates to a 1000 MPa class hot rolled steel sheet having a hole expansion property that can bring a reduction.
Looking at the configuration of the present invention for achieving the above object,
By weight percent based on 100% by weight, C: 0.06 to 0.15%, Si: 0.1 to 0.3%, Mn: 2.5% or less, P: 0.01 to 0.03%, S: 0.002 to 0.004%, Al: 0.02 to 0.04% , Nb: 0.03 to 0.05%, Ti: 0.1 to 0.15%, Mo: 0.3% or less, Cr: 0.2 to 0.6%, B: 25 ppm or less, Cu: 0.2 to 0.5%, Ni: 0.2 to 0.4% and the rest of Fe A heating step of reheating the steel slab composed of other unavoidable impurities; A rolling step of hot rolling the steel slab reheated in the heating step; A cooling step of cooling the steel material 30 which is hot-rolled steel slab in the rolling step; And a winding-up step of winding up the steel material cooled in the cooling step.

Description

METHOD OF MANUFACTURING 1000MPa GRADE HAVING BORE EXPANDING CHARACTERISTICS HOT ROLLED STEEL SHEET}

The present invention relates to a method of manufacturing a 1000 MPa grade hot rolled steel sheet having a hole expansion property, and more specifically, to control the content of Si, Ti, Nb, and adding a small amount of Cr, B to increase the strength of the hot rolled steel sheet, The present invention relates to a 1000 MPa class hot rolled steel sheet having a hole expansion property that can bring a reduction.

High-strength hot rolled steel sheets for processing are widely known as ferrite, martensite structure, ferrite, mixed structure composed of bainite structure, or near single phase structure mainly composed of bainite and ferrite.

While it is difficult to satisfy all of the elongation associated with proper workability of such hot rolled steel sheet, especially when using high strength steel of low carbon steel series, the elongation must be secured up to 30% at 800MPa or more to apply to complex shaped parts. It was easy to do.

It is known that life-expansion and ductility tend to be opposed to each other, but a method of reducing the hardness difference between ferrite and bainite is used as a means of improving the hole expandability of ferrite and bainite structure.

1000MPa grade hot rolled steel sheet manufacturing method having a hole expansion according to the present invention, while securing a tensile strength of 1000 ~ 1080MPa class, and 60-75% hole expandability (molding), while reducing the manufacturing cost The purpose of the present invention is to provide a 1000 MPa grade hot rolled steel sheet manufacturing method.

1000MPa grade hot rolled steel sheet manufacturing method having a hole expansion according to the present invention, while ensuring the tensile strength and hole expandability of the steel produced by the manufacturing method, the object to provide a steel material that can minimize the center segregation zone have.

Looking at the configuration of the present invention for achieving the above object,

By weight percent based on 100% by weight, C: 0.06 to 0.15%, Si: 0.1 to 0.3%, Mn: 2.5% or less, P: 0.01 to 0.03%, S: 0.002 to 0.004%, Al: 0.02 to 0.04% , Nb: 0.03 to 0.05%, Ti: 0.1 to 0.15%, Mo: 0.3% or less, Cr: 0.2 to 0.6%, B: 25 ppm or less, Cu: 0.2 to 0.5%, Ni: 0.2 to 0.4% and the rest of Fe A heating step of reheating the steel slab composed of other unavoidable impurities; A rolling step of hot rolling the steel slab reheated in the heating step; A cooling step of cooling the steel material 30 which is hot-rolled steel slab in the rolling step; And a winding-up step of winding up the steel material cooled in the cooling step.

Subsequently, in the heating step, the reheating temperature at which the steel slab is reheated is preferably 1210 to 1290 ° C.

And, in the rolling step, the finish hot rolling temperature of the rolled steel slab is preferably 850 to 930 ℃.

Subsequently, in the winding step, the coiling temperature of the steel 30, which is a steel slab, is preferably 400 to 550 ° C.

In addition, in the method for manufacturing a hot rolled steel sheet, the steel further includes the Ti, and the sum of the Nb and Ti may be 0.2% by weight or less based on 100% by weight in total.

Subsequently, in the hot rolled steel sheet manufacturing method, the tensile strength of the steel has a hole expandability, characterized in that 1000 to 1080 MPa.

On the other hand, in the hot rolled steel sheet manufacturing method, the steel material, it is preferable that the elongation is 15.1 to 16.5.

Subsequently, in the method for manufacturing a hot rolled steel sheet, the steel material has a hole expandability (formability) of 60 to 75%.

In addition, in the hot rolled steel sheet manufacturing method, the steel, the microstructure has a structure composed of a ferrite bainite phase (Ferrite).

1000MPa grade hot rolled steel sheet manufacturing method having a hole expandability according to the present invention, it is possible to provide a 1000MPa grade high burring steel that can be applied simultaneously to the tensile strength of 1000 ~ 1080MPa class, and 60-75% hole expandability (forming) There is a technical effect that can.

1000MPa grade hot rolled steel sheet manufacturing method having a hole expandability according to the present invention, while securing the hole expandability (molding) and tensile strength, there is an economic effect that can reduce the manufacturing cost by minimizing the content of expensive alloy elements .

1 is a flow chart schematically showing a method for manufacturing a 1000 MPa grade hot rolled steel sheet having a hole expandability according to an embodiment of the present invention.
Figure 2 is a conceptual diagram showing each step of the manufacturing method of 1000MPa class hot rolled steel sheet having a hole expandability according to the example of one time of the present invention.
FIG. 3 is a view showing a structure change according to time and temperature of the steel produced by the step-by-step process of FIG.

DETAILED DESCRIPTION The following detailed description of the invention refers to the accompanying drawings that show, by way of illustration, specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. It is to be understood that the various embodiments of the invention are different, but need not be mutually exclusive. For example, certain shapes, structures, and characteristics described herein may be embodied in other embodiments without departing from the spirit and scope of the invention with respect to one embodiment. It is also to be understood that the position or arrangement of the individual components within each disclosed embodiment may be varied without departing from the spirit and scope of the invention. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present invention, if properly described, is defined only by the appended claims, along with the full range of equivalents to which such claims are entitled. In the drawings, like reference numerals refer to the same or similar functions throughout the several aspects, and length, area, thickness, and the like may be exaggerated for convenience.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings, so that those skilled in the art can easily carry out the present invention.

In the following detailed description, for example, by weight% to 100% by weight, Si (silicon) is limited to 0.1 to 0.3% to enable plating, and the total of Nb (niobium) and Ti (titanium) total 100 weight % By weight, to 0.20% by weight or less, to minimize expensive Mo (molybdenum) to 0.3% or less, 0.2 to 0.6% Cr (chromium), quenchable element, 20ppm or less Due to the addition, 1000MPa class hot rolled steel sheet manufacturing method having the hole expansion property of the present invention to improve the formability and reduce the manufacturing cost while increasing the strength of the steel material 30 [particularly, 0.2 ~ 0.6% Cr (chromium), B (boron) 20ppm or less added to the technical configuration of the steel 30] will be of course applicable.

Steel material (30) using the method of manufacturing a 1000 MPa class hot rolled steel sheet having a hole expandability according to the present invention, in a weight% to 100% by weight, C: 0.06 ~ 0.15%, Si: 0.1 ~ 0.3%, Mn: 2.5% or less, P: 0.01 to 0.03%, S: 0.002 to 0.004%, Al: 0.02 to 0.04%, Nb: 0.03 to 0.05%, Ti: 0.1 to 0.15%, Mo: 0.3% or less, Cr: 0.2 to 0.6 %, B: 20ppm or less, Cu: 0.2-0.5%, Ni: 0.2-0.4% and steel slab, which is a steel slab composed of the remaining Fe and other unavoidable impurities.

In addition, by weight% to 100% by weight, Si (silicon) is limited to 0.1 to 0.3% to enable plating.

In particular, Ti (titanium) is limited to 0.1 ~ 0.15%, Mo (molybdenum) to minimize to 0.3% or less.

Hereinafter, the role and the amount of each component constituting the steel 30, which is a high strength hot rolled steel sheet according to the present invention, will be described.

Carbon (C): 0.06 to 0.15 wt%

Carbon (C) is an element added to improve the hardenability and secure the strength of the steel.

Subsequently, it plays a role of securing strength through precipitation or solid solution strengthening. Such carbon is preferably added at 0.06 to 0.15% by weight of the total weight of the steel 30.

The amount of carbon added based on 0.06 to 0.15% by weight is based on the pore expandability (HER), and the hole expandability or ductility compared to the plus factor that increases the strength when the amount of carbon exceeds 0.15% by weight. Since this deteriorating characteristic appears more rapidly, the significance for the further addition of carbon may be lowered.

If the carbon is added in less than 0.06% by weight, the hardenability is less and the difficulty in securing the strength of the steel 30 is different.

In addition, it is uneconomical to add a relatively large amount of other alloy elements in order to exhibit the same strength.

silicon( Si ): 0.1 to 0.3% by weight

Silicon increases the strength of steel and is an element necessary for deoxidation.

Such silicon serves to improve the hole expandability of the hot rolled steel sheet by delaying cementite formation.

In particular, in the present invention, silicon is added as a deoxidizer for removing oxygen from the steel sheet, and is also an effective element for cementite spheroidization.

Such silicon is preferably added at 0.1 to 0.3% by weight within the carbon content range according to the present invention.

If the silicon content exceeds 0.3% by weight, the weldability of the steel is lowered and red scale is generated during slab reheating and hot rolling, which may cause a problem in surface quality and also may cause a problem of deterioration of plating property after welding. have.

On the other hand, if the silicon content is added less than 0.1% by weight, the degree of contribution to the solid solution strengthening material of the steel 30 is insufficient, the effect of improving the strength is insignificant, the deoxidation of the molten steel is insufficient to clean steel Difficult to get.

manganese( Mn ): 2.5 wt% or less

Manganese is an effective element for increasing strength without deteriorating toughness.

In the present invention, manganese is very effective as a solid solution strengthening element, and is an effective element for securing strength by improving hardening of steel. Manganese also contributes to grain refinement of ferrite by delaying ferrite and pearlite transformation as an austenite stabilizing element.

Such manganese is preferably added in a content ratio of 2.5% by weight or less in the carbon addition range according to the present invention.

When the amount of manganese exceeds 2.5% by weight, the weldability is greatly reduced, causing inclusions and segregation, thereby acting as a factor that inhibits the toughness of the manufactured hot rolled steel sheet.

In addition, manganese has a problem in that the manufacturing cost is increased as it is added as an expensive element.

Therefore, in the present invention, it is preferable to adjust the amount of manganese to 2.5% by weight or less in order to reduce manufacturing cost and minimize the center segregation zone.

Phosphorus (P): 0.01 to 0.03 wt%

Phosphorus (P) is an impurity element present in steel, and it is advantageous to keep it as low as possible because it can greatly inhibit components and impact toughness that are advantageous for improving strength and corrosion resistance.

In addition, phosphorus (P) plays a role of ensuring the strength without lowering the desired workability or formability.

Such phosphorus (P) is preferably added in an amount ratio of 0.01 to 0.03% by weight of the total weight of the steel (30).

However, when the addition amount of phosphorus (P) exceeds 0.03% by weight, since it may adversely affect the weldability, it is preferable to limit the addition range.

Sulfur (S): 0.002 to 0.004 wt%

Sulfur (S) is an impurity element present in steel similarly to phosphorus (P).

Subsequently, sulfur (S) corresponds to impurities which are precipitated in the form of MnS or the like to increase the amount of precipitates.

In particular, it may inhibit the workability and weldability of the hot rolled steel sheet and cause cracks during processing. Therefore, in the present invention, the amount of addition is preferably 0.002 to 0.004% by weight, but preferably limited to the lowest amount of addition.

aluminum( Al ): 0.02 to 0.04 wt%

Aluminum (Al) generally contributes to the deoxidation of steel, and also forms an carbide and is an effective element for refining the microstructure of steel.

The aluminum (Al) to remove the oxygen present in the hot rolled steel sheet according to the present invention in the form of Al ₂ O ₃ to prevent the formation of non-metallic material, and brings the ferrite stabilization effect with the silicon (Si).

In particular, in the present invention, aluminum (Al) has an excellent deoxidation ability compared to silicon (Si) or manganese (Mn), and is an effective element for removing oxygen in molten steel during the steelmaking process.

Therefore, aluminum (Al) is preferably added in a content ratio of 0.04% by weight or less within the carbon addition range according to the present invention. When aluminum (Al) is added in excess of 0.04% by weight, it inhibits the spheroidization of cementite during perlite transformation, thereby reducing the machinability of the produced hot rolled steel sheet.

Niobium ( Nb ): 0.03 to 0.05% by weight

Niobium (Nb) is added to promote grain refining of the rolled microstructure of the steel and is a very effective element for increasing the strength of the material even with a small amount of addition.

Such niobium (Nb) is added for the purpose of securing inaging properties and improving moldability.

Subsequently, precipitation of hot rolled copper niobium carbonitride delays recrystallization and inhibits grain growth, leading to further strength increase.

Particularly, the solid solution carbon (C) during hot rolling is precipitated as an NbC composite precipitate, thereby increasing the strength of the refined ferrite and improving moldability, such as rimability and elongation.

However, when niobium exceeds 0.05%, the ductility of the hot rolled steel sheet is drastically reduced, so niobium is preferably limited to the addition amount ratio of 0.03 to 0.05% by weight.

titanium( Ti ): 0.1 to 0.15 wt%

Titanium (Ti) serves to improve the strength while maintaining excellent stretch and stretch flangeability.

In particular, titanium (Ti) may combine with niobium (Nb) to form carbides, nitrides, and sulfides, thereby contributing to the improvement of strength and toughness of the hot rolled steel sheet.

Although these effects are recognized as containing 0.001% or more, when it exceeds 0.1%, the addition amount of carbon (C) which contributes to plastic hardenability can be reduced.

Therefore, in this invention, it is preferable to adjust so that it may become 0.20 weight% or less with the total amount of Ti + Nb. At this time, since the content of niobium (Nb) in the present invention is limited to 0.03 to 0.05% by weight, the addition amount of titanium may be 0.1 to 0.15% by weight, the maximum addition amount should be 0.20% by weight or less.

If the addition amount of titanium is less than 0.1% by weight, the desired strength improvement effect cannot be obtained. On the contrary, if the addition amount is more than 0.15% by weight, a large amount of coarse titanium carbide or nitride is produced during heating before hot rolling. May occur and inhibit ductility or processability.

molybdenum( Mo ): 0.3 wt% or less

Molybdenum (Mo) is a very effective element to increase the strength of the material even with a small amount of addition.

Such molybdenum (Mo) can form a fine composite precipitate with titanium (Ti), and can strengthen the steel while maintaining excellent stretch and stretch flangeability.

However, when molybdenum is excessively added, a hard phase may be formed, and thus the stretch flangeability may be lowered. If the amount is 0.3 wt% or less, the strength increase effect required by the present invention cannot be obtained.

Here, 0.3% by weight belongs to a large amount of addition compared to other elements, the cost of molybdenum (Mo) may be a cause of the increase in manufacturing cost.

Therefore, in the present invention, molybdenum is minimally added in consideration of economical efficiency, and chromium (Cr) and boron (B) are added to obtain strength improvement and cost reduction effect.

chrome( Cr ): 0.2 to 0.6 wt%

Chromium (Cr) is an effective element added to ensure sufficient strength even at low carbon (C) content.

Since chromium (Cr) has an effect of increasing hardenability and producing martensite or bainite in the ferrite structure, it plays a role of improving strength while maintaining excellent stretching and stretching flangeability.

However, when added in a large amount of more than 0.6% by weight within the range of the amount of carbon according to the present invention, it is preferable that the content does not exceed 0.6% by weight because the production of ferrite can be rather suppressed.

In addition, a minimum may be an impurity level, and in this invention, even if it adds by 0.2 weight%, a preferable strength improvement effect can be acquired.

Boron (B): 20 ppm Below

Boron (B) is to increase the hardenability of the steel material 30 by delaying the phase transformation during continuous cooling transformation, and to secure the strength after quenching as a strong hardenable element.

Such boron (B) is an element capable of suppressing the formation of saltpeter ferrite during cooling after hot rolling, improving the elongation flangeability, and increasing the hardenability.

Therefore, in the present invention plays an important role in improving the strength of the steel 30 in place of molybdenum (Mo) together with chromium (Cr).

However, when the addition amount exceeds 20 ppm, the effect is saturated, and the load of hot rolling may become high, and operability may fall.

Therefore, it is preferable to limit the addition amount of boron (B) to 20 ppm or less.

Copper( Cu ): 0.2 to 0.5 wt%

Copper (Cu) is an element effective for improving toughness and increasing strength.

Such copper (Cu) plays a role in contributing to the solid solution strengthening effect of the steel material 30 by controlling a certain amount together with the above-described silicon (Si) and manganese (Mn) and helps to secure high-strength mechanical properties of structural steel Gives.

However, when the content of copper (Cu) is added in excess of 0.5% by weight may cause surface defects, in the present invention, it is preferable to limit the content of copper (Cu) to 0.2 to 0.5% by weight.

nickel( Ni ): 0.2 to 0.4 wt%

Nickel (Ni) is a graphitization promoting element and an element that improves delayed fracture resistance.

In addition, it may be added as an element for preventing the high temperature brittleness generated when the addition of copper (Cu) to increase the strength and to improve the corrosion resistance.

However, as an expensive gate, it is uneconomical if it contains a large amount of additives, and if it exceeds 0.4% by weight, the effect may be saturated. Therefore, in the present invention, the content of nickel (Ni) is limited to 0.2 to 0.4% by weight. desirable.

The rest other than the above substantially consists of iron (Fe).

The fact that the remainder is substantially made of iron (Fe) means that inclusion of other trace elements, including unavoidable impurities, may be included in the scope of the present invention, as long as it does not interfere with the effects of the present invention.

The steel material 30 having such a composition may have a target hole expandability (molding) of 60 to 75%, a tensile strength of 1000 to 1080 MPa, and a firing rate of 15.1 to 16.5. have.

Particularly, the sum of Nb (niobium) and Ti (titanium) is 0.2% by weight or less, based on 100% by weight, and minimizes expensive Mo (molybdenum) to 0.3% or less, and hardenable elements. By adding 0.2 to 0.6% of phosphorus Cr (chromium) and 20 ppm or less of B (boron), it is possible to improve the formability and reduce the manufacturing cost while increasing the strength of the steel material 30.

1000MPa grade hot rolled steel sheet manufacturing method having a hole expandability according to the present invention is prepared by the above-described composition, the specific manufacturing method will be described as follows.

1 is a flow chart schematically showing a method for manufacturing a 1000 MPa grade hot rolled steel sheet having a hole expandability according to an embodiment of the present invention.

Referring to Figure 1, the 1000MPa class hot rolled steel sheet manufacturing method having a hole expandability is large heating step (S100), re-heating the steel 30, hot rolling the steel 30 heated in the heating step (S100) Rolling step (S200), the cooling step (S300) for cooling the steel 30 rolled in the rolling step (S200), the winding step of winding the steel material 30 cooled in the cooling step (S300) ( S400).

In addition, Figure 2 is a conceptual diagram showing each step of the manufacturing method 1000MPa grade hot rolled steel sheet having a hole expandability according to the example of one time of the present invention.

As shown in Figures 1 and 2, a method of manufacturing a 1000 MPa grade hot rolled steel sheet having a hole expandability according to the present invention will be described below.

Heating stage S100 )

The heating step (S100) for reheating the steel slab is in weight% based on 100% by weight, C: 0.06 to 0.15%, Si: 0.1 to 0.3%, Mn: 2.5% or less, P: 0.01 to 0.03%, S : 0.002 to 0.004%, Al: 0.02 to 0.04%, Nb: 0.03 to 0.05%, Ti: 0.1 to 0.15%, Mo: 0.3% or less, Cr: 0.2 to 0.6%, B: 25 ppm or less, Cu: 0.2 to 0.5 %, Ni: 0.2 ~ 0.4% and the heating step to reheat the steel slab composed of the remaining Fe and other unavoidable impurities.

At this time, the steel slab formed as described above is reheated at a temperature of 1210 to 1290 ℃.

If the reheating temperature is less than 1210 ° C., the precipitates are not sufficiently reusable, and precipitates such as niobium (Nb) are reduced in the process after hot rolling.

Therefore, by maintaining such a reheating temperature of 1210 ℃ or more, it is possible to control the re-use of the precipitate, to improve the strength of the material and to ensure a uniform microstructure in the longitudinal direction of the material.

On the contrary, when such a reheating temperature exceeds 1290 ° C., abnormal grain growth of grains may occur. As a result, this may act as a factor against the increase in strength.

Therefore, the reheating temperature of the steel slab in the heating step (S100) is preferably limited to 1210 to 1290 ℃.

Rolling stage ( S200 )

In the rolling step (S200) of hot rolling the steel slab reheated in the heating step (S100), the final hot rolling of the reheated steel slab is performed.

At this time, it is preferable that FDT which is finishing hot rolling temperature is 850-930 degreeC.

As such, when the finish hot rolling temperature exceeds 930 ° C., brittleness of the steel sheet may increase with a large amount of cooling, and when the finish hot rolling temperature is less than 850 ° C., workability may be deteriorated depending on microstructure unevenness.

Therefore, the finish hot rolling temperature in the rolling step (S200) is preferably limited to 850 to 930 ℃.

Cooling stage ( S300 )

After holding the steel slab 30, which is the hot slab finished hot rolling in the rolling step S200, for a predetermined time, cooling is started at a specific cooling start temperature, and cooling is completed at a specific cooling end temperature.

Referring to Figure 2, it can be seen that shear quenching is performed by the cooling water in the ROT section.

In the present invention by performing such a shear quenching process, it is preferable to limit the cooling end temperature of the steel 30 to a temperature of 400 to 550 ℃.

As a result, the cooling start temperature in the cooling step (S300) is 820 to 920 ℃, the cooling end temperature is 400 to 550 ℃.

Here, the cooling start temperature is limited as described above because cooling of the rolled steel slab directly above the phase transformation temperature can suppress generation of a supercooled structure.

Likewise, the cooling end temperature is limited as described above in order to effectively control the tensile strength, the elongation, and the hole expandability (formability) of the steel material 30.

If the cooling end temperature is less than 400 ℃, the phenomenon that the tensile strength, elongation and hole expandability of the steel 30 does not reach the reference value occurs, if the cooling end temperature exceeds 550 ℃ the steel The elongation of (30) becomes poor.

Therefore, the cooling start temperature in the cooling step (S300) is preferably 820 to 920 ℃, the cooling end temperature is preferably limited to 400 to 550 ℃.

Winding stage ( S400 )

It is a step of winding the steel material 30 cooled in the rolling step (S200).

At this time, the winding temperature which can wind the cooled steel material 30 is preferably 400 to 550 ° C.

This may be similar to the cooling end temperature of the steel 30 in the cooling step (S300).

The final microstructure of the steel 30 forms a structure composed of bainite phase, and also has a ferrite phase structure.

As a result, the microstructure of the steel 30 in the winding step (S400), forms a structure composed of a ferrite bainite (Bainite) phase.

In order to obtain bainite, which is a low-temperature structure, winding in the bainite region, rather than general cooling, requires an appropriate amount of cooling in the cooling line.

When the cooled steel material 30 is viewed from the point of winding, the steel material 30 is formed of a composite structure of ferrite and bainite, thereby exhibiting excellent hole expandability.

At this time, the hole expandability (molding) of the steel 30 will have 60 to 75%.

FIG. 3 is a view showing a structure change according to time and temperature of the steel produced by the step-by-step process of FIG.

3, F represents a ferrite region, P represents a pearlite region, B represents a bainite region, and M represents a martensite region.

By the above process, the 1000 MPa class hot rolled steel sheet having a hole expandability according to the present invention has an excellent hole expansion value.

As described above, the excellent hole expandability limits the addition amount of silicon (Si) to 0.1% by weight to 0.3% by weight, and the sum of Nb and Ti to be% by weight, 0.20% by weight or less, and the content of Mo. Minimize to 0.3% or less by this weight%, and add 0.2 to 0.6% by weight of Cr as a hardenable element, and add B or less to 25 ppm to increase porosity (molding) and to increase tensile strength. It was.

On the other hand, the bainite transformation temperature slightly varies depending on the composition of the steel sheet, the bainite transformation temperature of the 1000MPa class hot rolled steel sheet having a hole expandability according to the present invention may be approximately 500 ℃, in consideration of the error range The winding temperature at which the steel 30 is wound is preferably limited to 400 ~ 550 ℃.

As a result, the steel 30 in the winding step (S400) is wound in the bainite region temperature range corresponding to a temperature higher than the martensite transformation temperature.

As such, since the winding step is carried out at the bainite region temperature, the final structure of the steel material 30 produced by the manufacturing method according to the present invention consists of a two-phase composite structure in which ferrite tissue is formed on bainite.

Thus, the steel 30 produced by the manufacturing method according to the present invention, the tensile strength (TS) has a tensile strength corresponding to the range of 1000 to 1080 MPa.

In addition, the elongation EL of the steel material 30 has 15.1 to 16.5.

As can be seen from the experimental results below, Mo (molybdenum) is not added or specifically, 0.3% or less by weight, Cr (chromium) by weight 0.2 ~ 0.6% and B (boron) by weight When added to less than 25ppm, the tensile strength (TS) and elongation (EL) in the hole than the existing hot-rolled steel sheet exhibits excellent properties such as the hole expandability (formability) is also increased.

In addition, the hole expansion rate (HER) of the steel material 30 produced by the manufacturing method according to the present invention will have 60 to 75%.

Hereinafter, the present invention will be described in more detail with reference to the following examples.

Example

Hereinafter, the configuration and operation of the present invention through the preferred embodiment of the present invention will be described in more detail.

However, this is presented as a preferred example of the present invention, it should not be construed that the present invention is limited by this in any sense.

Details that are not described herein will be omitted since those skilled in the art can sufficiently infer technically.

1. Manufacturing of Expandable Steels

In accordance with the compositions shown in Table 1 and the process conditions described in Table 2, the hole-expandable steels according to Examples 1 to 2 and Comparative Examples 1 to 2 of the present invention were prepared.

In Table 1, the addition amount ratio of the component is weight%, the addition amount of boron is ppm, and the temperature unit of FDT and CT is ° C.

division C Si Mn P S Al Nb Ti Mo Cu Cr Ni B (ppm) Example 1 0.06 0.3 1.8 0.02 0.003 0.03 0.04 0.1 - 0.5 0.4 0.2 20 Example 2 0.06 0.3 1.8 0.02 0.003 0.03 0.04 0.1 0.2 - 0.2 0.2 20 Comparative Example 1 0.07 1.3 2.4 0.015 0.005 0.02 0.04 0.22 - - - - - Comparative Example 2 0.05 1.3 2.4 0.015 0.005 0.02 0.035 0.15 0.32 0.32 - - -

2. Characterization

Table 2 shows the rolling end temperature (° C.), winding temperature (° C.), hole expandability (%), and tensile strength of the hole-expandable steels prepared according to Examples 1 to 2 and Comparative Examples 1 to 2 of the present invention. (MPa) and elongation (EL) are shown.

division FDT (℃) CT (℃) HER (%) TS (MPa) EL (%) Example 1
850-930
400-550 70 1047 16.3 Example 2 66 1062 15.2 Comparative Example 1
900
350-600 3 998 13.2 Comparative Example 2 10 1021 15.0

Referring to Tables 1 and 2, Mo (molybdenum) is not added or a small amount is added, Ti and Nb (niobium) added amount of 0.14% by weight, Cr (chromium) added amount of 0.2 to 0.4% by weight The hole expandability (HER) of Examples 1 and 2 shows a value significantly higher than that of Comparative Examples 1 and 2.

In addition, it can be seen that the tensile strength (TS) and the elongation (EL) of Examples 1 and 2 and Comparative Examples 1 and 2 also have a difference.

Therefore, the 1000 MPa grade hot rolled steel sheet manufactured according to the present invention can improve the tensile strength and elongation while reducing the hole expandability (formability), and reduce the manufacturing cost.

It has been described above a preferred embodiment of the hole expandable steel produced according to the 1000MPa class hot rolled steel sheet manufacturing method having a hole expandability according to the present invention.

The foregoing embodiments are to be understood in all respects as illustrative and not restrictive, the scope of the invention being indicated by the following claims rather than the foregoing description, and the meaning and scope of the claims and All changes or modifications derived from the equivalent concept should be interpreted as being included in the scope of the present invention.

30: steel
S100: heating stage
S200: rolling step
S300: Cooling Step
S400: winding step

Claims

By weight percent based on 100% by weight, C: 0.06 to 0.15%, Si: 0.1 to 0.3%, Mn: 2.5% or less, P: 0.01 to 0.03%, S: 0.002 to 0.004%, Al: 0.02 to 0.04% , Nb: 0.03 to 0.05%, Ti: 0.1 to 0.15%, Mo: 0.3% or less, Cr: 0.2 to 0.6%, B: 25 ppm or less, Cu: 0.2 to 0.5%, Ni: 0.2 to 0.4% and the rest of Fe A heating step of reheating the steel slab composed of other unavoidable impurities;
A rolling step of hot rolling the steel slab reheated in the heating step;
A cooling step of cooling the steel, the steel slab hot rolled in the rolling step; And
1000MPa class hot rolled steel sheet manufacturing method having a hole expandability, including; winding step of winding the steel material cooled in the cooling step.

The method of claim 1,
In the heating step,
The reheating temperature of the steel slab is reheated is a manufacturing method of 1000MPa grade hot rolled steel sheet having a hole expandability, characterized in that 1210 to 1290 ℃.

The method of claim 1,
In the rolling step,
Finishing hot rolling temperature of the rolled steel slab 1000MPa class hot rolled steel sheet manufacturing method characterized in that the 850 to 930 ℃.

The method of claim 1,
In the winding step,
Winding temperature of the steel slab is a steel slab 1000MPa class hot rolled steel sheet manufacturing method characterized in that the expansion.

The method of claim 1,
In the hot rolled steel sheet manufacturing method,
The steel further includes the Ti, and the sum of the Nb and Ti is 100% by weight, based on 100% by weight, characterized in that the 1000MPa class hot rolled steel sheet having a hole expandability, characterized in that less than 0.20% by weight .

The method of claim 1,
In the hot rolled steel sheet manufacturing method,
The tensile strength of the steel is 1000 to 1080 MPa characterized in that the 1000MPa class hot rolled steel sheet manufacturing method having a hole expandability.

The method of claim 1,
In the hot rolled steel sheet manufacturing method,
The steel,
A method of manufacturing a 1000 MPa grade hot rolled steel sheet having a hole expansion property, characterized in that the elongation is 15.1 to 16.5.

The method of claim 1,
In the hot rolled steel sheet manufacturing method,
The steel,
A method of manufacturing a 1000 MPa grade hot rolled steel sheet having a hole expandability, characterized in that the hole expandability (forming) is 60 to 75%.

The method of claim 1,
In the hot rolled steel sheet manufacturing method,
The steel,
Method for producing a 1000 MPa-class hot rolled steel sheet having a hole expandability, characterized in that the microstructure is a structure composed of a ferrite bainite phase.