KR101449137B1 - High strength hot-rolled steel having excellent weldability and hydroforming workability and method for manufacturing thereof - Google Patents

Abstract

One aspect of the present invention is to provide a high strength hot-rolled steel sheet excellent in weldability and hydroforming workability superior to the conventional hot-rolled steel sheet by maintaining control of microstructure and high product of uniform tensile strength and uniform elongation (TSxU-EI) .

Description

TECHNICAL FIELD [0001] The present invention relates to a high-strength hot-rolled steel sheet excellent in weldability and hydroforming workability, and a method of manufacturing the same. BACKGROUND ART < RTI ID = 0.0 >

The present invention relates to a high-strength hot-rolled steel sheet excellent in weldability and hydroforming workability and a method for producing the same.

Hollow members having various cross-sectional shapes are used as structural members for automobiles. Such a hollow member is mainly manufactured by joining parts formed by press working of a steel sheet by spot welding.

In recent years, a hollow member for a structural member of an automobile is required to have a higher impact absorbing capability at the time of collision, and thus a material having a higher strength is required. However, with the conventional press forming method, it is difficult to manufacture a member having excellent shape and dimensional accuracy without forming defects.

Recently, hydroforming has attracted attention as a new molding method to solve the above problem. Hydroforming is a method of molding a steel tube into a desired shape by injecting a high pressure liquid into the steel tube. By changing the cross-sectional dimension of the steel tube to expanding work or the like, a member having a complicated shape can be integrally formed, It is one of the molding methods. The material to be applied to such hydroforming is required to have high moldability and processability.

Patent Document 1 discloses a technique for producing a steel sheet excellent in in-plane anisotropy. Patent Document 2 discloses a technique for forming a second phase or a coarse carbide and inclusions in a ferrite base structure so that a product of a tensile strength and a marginal expansion ratio of a steel sheet is excellent. And at the same time securing a high strength by utilizing fine precipitates. In addition, Patent Document 3 discloses a technique of ensuring high strength by utilizing the solid solution strengthening effect of alloy components such as Mn, Cr, Cu, Ni, and Mo in a single ferrite phase.

The above-mentioned Patent Documents 1 to 3 mainly utilize alloy components such as C, Si, Mn, Mo, Ni, Cu and the like. The alloy components are effective for improving the strength of the hot-rolled steel sheet, The resistance of the steel plate is increased due to an increase in electrical resistance during resistance welding, resulting in a splash. When the input current value is lowered, a cold collapse occurs. Particularly, Si, Mn, Al and the like have the problem of deteriorating the weldability by reducing the integrity of the welded portion by forming oxides during welding. Further, when the second phase exists, the steel material is different from the base material in the heat affected zone of the weld, resulting in fracture or durability in the secondary processing.

Japanese Patent Application Laid-Open No. 2003-342679 Japanese Patent Application Laid-Open No. 2006-257526 Japanese Patent Application Laid-Open No. 2001-032034

An aspect of the present invention is to provide a high strength hot-rolled steel sheet which can be used for automobile engine cradles, cross members and chassis parts, and is superior in weldability and hydroforming workability to conventional hot-rolled steel sheets, and a method for manufacturing the same.

A high strength hot-rolled steel sheet excellent in weldability and hydroforming processability, which is one aspect of the present invention, comprises 0.07 to 0.15% of C, 0.01 to 0.3% of Si, 1.0 to 2.0% of Mn, 0.03 to 0.3% of Al, 0.001 to 0.3% of Mo, 0.001 to 0.2% of Mo, 0.001 to 0.01% of N, 0.005 to 0.05% of P, 0.001 to 0.05% of S and the balance Fe and other unavoidable impurities, 1, and the product of the tensile strength and the uniform elongation is 7000 or more.

0.164 + 0.2Mo - 0.25Si + 0.3Mn - 0.1Cr + 0.55Al + 0.2Mo -4.23Ti - 2.5Nb - 2.9V? 11

In the above formula (1), C, Si, Mn, Cr, Al, Mo, Ti, Nb and V represent the contents (weight%) of the respective elements.

A method for producing a high strength hot-rolled steel sheet excellent in weldability and hydroforming processability, which is another aspect of the present invention, comprises the steps of: 0.07 to 0.15% of C; 0.01 to 0.3% of Si; 1.0 to 2.0% of Mn; 0.001 to 0.05% of Cr, 0.001 to 0.3% of Cr, 0.001 to 0.2% of Mo, 0.001 to 0.01% of N, 0.001 to 0.05% of P, 0.001 to 0.05% of S and balance Fe and other unavoidable impurities, Preparing a steel slab satisfying the following formula 1, reheating the steel slab at 1200 to 1300 ° C, hot rolling the reheated steel slab to a finish rolling temperature of 850 to 1000 ° C to obtain a steel sheet , Cooling the hot-rolled steel sheet at a cooling rate of 10 to 100 ° C / second, and winding the cooled steel sheet.

0.164 + 0.2Mo - 0.25Si + 0.3Mn - 0.1Cr + 0.55Al + 0.2Mo -4.23Ti - 2.5Nb - 2.9V? 11

In the above formula (1), C, Si, Mn, Cr, Al, Mo, Ti, Nb and V represent the contents (weight%) of the respective elements.

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 will be more fully understood by reference to the following specific embodiments.

According to the present invention, by optimizing the steel component and the hot-rolled structure, the product of the tensile strength and the uniform elongation is 7000 or more, thereby improving the soundness of the welded portion at the time of welding, and securing a hot-rolled steel sheet that is easy to process at the time of hydroforming.

Fig. 1 is a graph showing the product of the tensile strength and uniform elongation of all the steels in the examples and the values when they are substituted into Equation 1. Fig.

The inventors of the present invention have conducted studies to develop a hot-rolled steel sheet in which the above-mentioned problems have not been solved. As a result, it has been found that by controlling steel composition components, microstructure and process conditions, Steel sheet can be produced, leading to the present invention.

Hereinafter, a high-strength hot-rolled steel sheet excellent in weldability and hydroforming workability which is one aspect of the present invention will be described in detail.

A high strength hot-rolled steel sheet excellent in weldability and hydroforming processability, which is one aspect of the present invention, comprises 0.07 to 0.15% of C, 0.01 to 0.3% of Si, 1.0 to 2.0% of Mn, 0.03 to 0.3% of Al, 0.001 to 0.3% of Mo, 0.001 to 0.2% of Mo, 0.001 to 0.01% of N, 0.005 to 0.05% of P, 0.001 to 0.05% of S and the balance Fe and other unavoidable impurities, 1, and the product of the tensile strength and the uniform elongation is 7000 or more.

0.164 + 0.2Mo - 0.25Si + 0.3Mn - 0.1Cr + 0.55Al + 0.2Mo -4.23Ti - 2.5Nb - 2.9V? 11

In the above formula (1), C, Si, Mn, Cr, Al, Mo, Ti, Nb and V represent the contents (weight%) of the respective elements.

Carbon (C): 0.07 to 0.15 wt%

C is the most economical and effective element to fortify the river. When the carbon content is less than 0.07 wt%, precipitation elements such as Ti, Nb and B are difficult to form fine precipitates. On the other hand, when the content of carbon exceeds 0.15% by weight, coarse carbide formation, local strength increase, weldability, formability and ductility decrease. Therefore, the carbon content is preferably 0.07-0.15 wt%.

Silicon (Si): 0.01-0.3 wt%

Si is an element added for deoxidizing molten steel and improving strength by solid solution strengthening. In addition, it is an element effective for forming a uniform ferrite structure because it has the effect of promoting ferrite transformation during cooling after hot rolling as a ferrite stabilizing element. In order to exhibit such an effect in the present invention, it is preferable that it is contained in an amount of 0.01 wt% or more. On the other hand, when the content of silicon exceeds 0.3% by weight, a red color scale due to Si is formed on the surface of the steel sheet during hot rolling, thereby deteriorating the surface quality of the steel sheet. Therefore, the content of the silicon is preferably 0.01 to 0.3% by weight.

Manganese (Mn): 1.0 to 2.0 wt%

Mn, like Si, is an effective element for strengthening the steel. In order to exhibit such an effect in the present invention, it is preferable that 1.0 wt% or more is included. On the other hand, when it exceeds 2.0%, the ferrite transformation is delayed, which makes it difficult to secure a proper fraction of ferrite as a base structure of the present invention. In addition, when the slab casting is performed in the casting process, the segregation portion is greatly developed in the center segregation at the center of thickness, which may deteriorate the weldability of the final product. Therefore, the content of manganese is preferably 1.0 to 2.0 wt%.

Aluminum (Al): 0.03 to 0.3 wt%

Aluminum is added as a deoxidizer in combination with Si during steelmaking, and has a solid solution strengthening effect. In addition, as a ferrite stabilizing element, it helps to form a ferrite phase on steel during cooling after hot rolling. In order to exhibit such an effect in the present invention, it is preferable that it is contained in an amount of 0.03% by weight or more. On the other hand, when the content exceeds 0.3% by weight, defects tend to occur in the slab during continuous casting, and surface quality is deteriorated due to occurrence of surface defects after hot rolling. Therefore, the content of aluminum is preferably 0.03 to 0.3% by weight.

Cr (Cr): 0.005-0.3 wt%

Cr is an effective element for strengthening hardening of steel. In order to exhibit such an effect in the present invention, it is preferable that it is contained in an amount of 0.005% by weight or more. On the other hand, when the content of chromium exceeds 0.3% by weight, the ferrite transformation is delayed and the elongation is decreased due to an increase in the martensite fraction. Therefore, it is preferable that the Cr content is contained in an amount of 0.005 to 0.3% by weight.

Molybdenum (Mo): 0.01 to 0.2 wt%

Mo increases the hardenability of the steel to facilitate formation of the second phase texture. In addition, when added together with Ti, (TiMo) C is formed, which greatly contributes to precipitation strengthening and has an effect of controlling solid solution C. In order to exhibit such effects in the present invention, it is preferable to contain 0.01 wt% or more. On the other hand, when the content exceeds 0.2% by weight, the weldability is deteriorated due to an increase in the extrudability, which is economically disadvantageous. Therefore, the Mo content is preferably 0.01 to 0.2% by weight.

Nitrogen (N): 0.001 to 0.01 wt%

N is a typical solid solution strengthening element together with C, and coarse precipitates are formed together with Ti, Al and the like. In general, the solid solution strengthening effect of N is superior to carbon and is advantageous for obtaining a high hardening ability, but there is a problem that the toughness is greatly deteriorated as the amount of N increases in the steel. When the content of nitrogen is less than 0.001% by weight, the effect of strengthening solubility is not expected. When the content of nitrogen is more than 0.01% by weight, the ductility and toughness are deteriorated. Therefore, the content of nitrogen is preferably 0.001 to 0.01% by weight.

Phosphorus (P): 0.005 to 0.05 wt%

P, like Si, is an important element for securing the ferrite fraction because it has the effects of strengthening solids and promoting ferrite transformation. In order to exhibit such an effect in the present invention, it is preferable to contain 0.005% by weight or more. On the other hand, if it exceeds 0.05%, there is a problem that ductility is lowered due to band organization due to micro segregation. On the other hand, the content of P is preferably 0.005 to 0.05% by weight.

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. The son impurities are not specifically mentioned in this specification, as they are known to anyone skilled in the art of manufacturing.

However, since sulfur is generally referred to as an impurity, a brief description thereof is as follows.

Sulfur (S): 0.001 to 0.05 wt%

Sulfur is an impurity inevitably contained and inhibits ductility and weldability of the steel sheet, and therefore it is preferable to control the sulfur as low as possible. Theoretically, it is advantageous to control the sulfur content to 0%, but it is inevitably contained inevitably in the manufacturing process. If the content of sulfur is less than 0.001% by weight, it takes a long time to perform steelmaking, resulting in poor productivity. When the content of sulfur exceeds 0.05%, it forms a nonmetallic inclusion by bonding with Mn or the like. Accordingly, the content of sulfur is preferably 0.001-0.05 wt%.

In addition, the steel of the present invention can further improve the effect of the present invention when one or more elements selected from the group consisting of niobium (Nb), titanium (Ti) and vanadium (V) are additionally added. More preferably 0.001 to 0.15 wt% of the total of one or more elements selected from the group.

Ti exists as TiN in the steel and has an effect of inhibiting the growth of the crystal grains in the heating process for hot rolling. Ti is also useful for improving the strength and yield ratio (YS / TS) of the steel by forming TiC precipitate by reacting with nitrogen and solidifying the remaining Ti in the steel and bonding with carbon.

Nb and V form carbides in the steel, which are effective in grain refinement and form fine precipitates to improve strength and toughness of steel. It stabilizes the employment elements such as C and N which increase the electrical resistivity, so that there is an effect of relieving the phenomenon of local flame generation in electric resistance welding.

By additionally adding Ti, Nb and V, it is possible to form a fine precipitate, and the fine precipitate suppresses coarsening of the microstructure of the welded portion during welding, thereby suppressing the softening of the welded portion.

In order to improve the weldability, it is preferable that C, Si, Mn, Cr, Al, Mo, Ti, Nb and V satisfies the following formula (1) The electrical resistivity at high temperature is increased and the weldability is remarkably lowered. The lower limit is not particularly limited, but the lower limit may be controlled to 10.7 in consideration of the strength or elongation to be obtained. If the amount is less than the above range, there may arise a problem that the strength or elongation rate is drastically reduced.

0.164 + 0.2Mo - 0.25Si + 0.3Mn - 0.1Cr + 0.55Al + 0.2Mo -4.23Ti - 2.5Nb - 2.9V? 11

In the above formula (1), C, Si, Mn, Cr, Al, Mo, Ti, Nb and V represent the contents (weight%) of the respective elements.

The microstructure of the hot-rolled steel sheet preferably includes ferrite and a second phase. By including such microstructure, the steel has sufficient ductility to improve the weldability. In addition, it is preferable that the ferrite has an area percentage of 90% or more. When the ferrite content is less than 90%, bainite and coarse carbonitride are formed to deteriorate ductility and toughness of steel.

The hot-rolled steel sheet preferably has a product of a tensile strength (TS) and a uniform elongation (U-EI) of 7000 or more. When the product of the tensile strength and the uniform elongation is less than 7000, the non-uniform deformation increases during hydroforming, resulting in fracture during machining or the thickness of the molded part is not uniform, resulting in a disadvantage in endorphic properties.

The hot-rolled steel sheet has a tensile strength of 540 MPa or more, and more preferably a tensile strength of 540 to 850 MPa.

Hereinafter, a method for manufacturing a high-strength hot-rolled steel sheet excellent in weldability and hydroforming workability which is another aspect of the present invention will be described in detail.

A method for producing a high strength hot-rolled steel sheet excellent in weldability and hydroforming processability, which is another aspect of the present invention, comprises the steps of: 0.07 to 0.15% of C; 0.01 to 0.3% of Si; 1.0 to 2.0% of Mn; 0.001 to 0.05% of Cr, 0.001 to 0.3% of Cr, 0.001 to 0.2% of Mo, 0.001 to 0.01% of N, 0.001 to 0.05% of P, 0.001 to 0.05% of S and balance Fe and other unavoidable impurities, Preparing a steel slab satisfying the following formula 1, reheating the steel slab at 1200 to 1300 ° C, hot rolling the reheated steel slab to a finish rolling temperature of 850 to 1000 ° C to obtain a steel sheet , Cooling the hot-rolled steel sheet at a cooling rate of 10 to 100 ° C / second, and winding the cooled steel sheet.

0.164 + 0.2Mo - 0.25Si + 0.3Mn - 0.1Cr + 0.55Al + 0.2Mo -4.23Ti - 2.5Nb - 2.9V? 11

In the above formula (1), C, Si, Mn, Cr, Al, Mo, Ti, Nb and V represent the contents (weight%) of the respective elements.

Reheat step

It is preferable to reheat the slab satisfying the above-mentioned component system at 1200 to 1300 占 폚. If the reheating temperature is lower than 1200 ° C, precipitates are not sufficiently reused, and precipitates such as NbC and TiC are reduced in the process after hot rolling. On the other hand, if the temperature exceeds 1300 DEG C, the strength is lowered due to abnormal grain growth of the austenite grains. Therefore, the reheating temperature of the slab is preferably limited to 1200 to 1300 占 폚.

Hot rolling step

As described above, the hot-rolled slab can be subjected to hot rolling. At this time, the finish rolling is preferably performed at 850 to 1000 캜. If the hot finish rolling temperature is lower than 850 占 폚, the rolling load greatly increases. On the other hand, when the hot finish rolling temperature exceeds 1000 캜, the structure of the steel sheet becomes coarse, the steel becomes weak, the scale becomes thick, and the surface quality deteriorates such as a high-temperature rolling-ability scale defect. Therefore, the hot finish rolling is preferably limited to 850 to 1000 ° C.

Cooling step

It is preferable to cool the hot-rolled steel sheet as described above. Further, it is preferable to cool the hot-rolled steel sheet until the temperature reaches 500 to 750 ° C from the finish hot-rolling temperature of the hot-rolled steel sheet. When the temperature at which the cooling is terminated is less than 500 ° C, most of the microstructure in the steel is formed of bainite, and the elongation is greatly reduced. On the other hand, when the temperature is higher than 750 ° C, coarse ferrite and pearlite are formed and the strength of the steel is reduced. It is also preferable to cool at a cooling rate of 10 to 100 ° C / second. When the temperature is less than 10 ° C / sec, coarsening of the ferrite grains occurs and the precipitates are coarse. Thus, there is a problem in securing the strength to be obtained by the present invention. When the temperature is higher than 100 ° C / sec, Is lowered.

Winding step

It is preferable that the cooled steel sheet is wound to facilitate storage and movement. At this time, it is more preferable that the winding is performed in a temperature range of 500 to 750 ° C. When the coiling temperature is lower than 500 캜, most of the microstructure in the steel is formed of bainite and the elongation rate is greatly reduced. When the coiling temperature exceeds 750 캜, coarse ferrite and pearlite are formed, .

The steel sheet produced by the above method is naturally cooled and pickled to remove the scale of the surface layer, whereby a pickled steel sheet can be manufactured.

Further, after the pickling steel sheet is reheated to 450 to 480 캜, a step of passing the hot-dip galvanizing bath is further included, whereby a hot-dip galvanized steel sheet can be produced.

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)

The steel slabs satisfying the component systems described in Table 1 were heated to 1250 占 폚 and hot-finished at a temperature (FDT) shown in Table 2 below. Thereafter, cooling was carried out at a cooling rate of 40 DEG C / sec up to the coiling temperature (CT) described in Table 2, and then the coiling was carried out at the coiling temperature (CT) shown in Table 2 below.

The yield strength (YS), tensile strength (TS), uniform elongation (U-EI) and fracture elongation (T-El) of the final hot-rolled steel sheet obtained by completing the winding process are shown in Table 2 below.

Yield strength (YS) means 0.2% off-set yield strength or lower yield point.

The tensile test was carried out on specimens taken in accordance with JIS No. 5 standard with respect to the rolling direction of the rolled sheet material in the direction of 90 °.

The ferrite area fraction was obtained by etching each of the rolled plate specimens with a Nital etchant and a Lepera etchant, observing the specimens at 500 magnifications using an optical microscope, analyzing them with an image analyzer, .

In addition, the weldability of the final hot-rolled steel sheet is also shown in Table 2 below. The weldability was evaluated by measuring the strength of the welded portion of the steel sheet by the uniaxial tensile test after welding under the conditions shown in Table 3 below. At this time, a test piece of JIS No. 13 standard was prepared and the tensile test was carried out so that the welded portion came to the center of the test piece. The weldability was evaluated as heat when the welds were broken.

Psalter C Si Mn Cr Al P S N Ti Mo Nb V Comparative Example 1 0.08 0.3 1.5 0.01 0.1 0.01 0.003 0.004 0.01 0.05 0.03 0.005 Comparative Example 2 0.08 0.1 1.5 0.01 0.2 0.01 0.003 0.004 0.02 0.1 0.01 0.02 Comparative Example 3 0.1 0.3 1.5 0.05 0.1 0.01 0.003 0.004 0.03 0.04 0.02 0.005 Comparative Example 4 0.1 0.2 1.6 0.01 0.1 0.01 0.003 0.004 0.02 0.1 0.02 0.005 Comparative Example 5 0.11 0.2 1.7 0.05 0.1 0.01 0.003 0.004 0.02 0.1 0.02 0.025 Comparative Example 6 0.09 0.1 1.5 0.2 0.05 0.01 0.003 0.004 0.04 0.02 0.03 0.03 Comparative Example 7 0.11 0.05 1.8 0.01 0.2 0.01 0.003 0.004 0.04 0.06 0.04 0.005 Comparative Example 8 0.13 0.1 2 0.01 0.2 0.01 0.003 0.004 0.03 0.08 0.025 0.005 Comparative Example 9 0.14 0.25 1.8 0.1 0.1 0.01 0.003 0.004 0.01 0.18 0.04 0.02 Comparative Example 10 0.15 0.3 1.7 0.01 0.1 0.01 0.003 0.004 0.01 0.2 0.05 0.02 Inventory 1 0.07 0.2 1.2 0.01 0.03 0.01 0.003 0.004 0.01 0.01 0.02 0.005 Inventory 2 0.08 0.15 1.4 0.01 0.03 0.01 0.003 0.004 0.02 0.01 0.02 0.005 Inventory 3 0.08 0.03 1.5 0.1 0.03 0.01 0.003 0.004 0.03 0.05 0.02 0.005 Honorable 4 0.1 0.1 1.4 0.2 0.1 0.01 0.003 0.004 0.03 0.02 0.01 0.005 Inventory 5 0.12 0.05 1.6 0.1 0.2 0.01 0.003 0.004 0.02 0.01 0.02 0.05 Inventory 6 0.07 0.1 1.5 0.01 0.03 0.01 0.003 0.004 0.03 0.03 0.02 0.01 Honorable 7 0.12 0.15 1.5 0.01 0.1 0.01 0.003 0.004 0.03 0.08 0.02 0.03 Honors 8 0.11 0.2 1.6 0.1 0.1 0.01 0.003 0.004 0.1 0.1 0.02 0.01

Psalter FDT
(° C) CT
(° C) Equation 1 YS
(MPa) TS
(MPa) U-El
(%) T-El
(%) TSxU-El Ferrite
Phase fraction
(%) Weldability Comparative Example 1 886 629 11.119 527 612 12 22 7344 94 X Comparative Example 2 891 622 11.098 540 608 12 22 7296 95 X Comparative Example 3 877 627 11.058 487 623 12 21 7476 92 X Comparative Example 4 878 618 11.121 534 644 11 20 7084 93 X Comparative Example 5 869 625 11.091 557 630 11 21 6930 85 X Comparative Example 6 902 621 10.819 703 745 9 17 6705 94 O Comparative Example 7 911 614 11.058 748 792 8 16 6336 92 O Comparative Example 8 905 619 11.218 643 693 11 19 7623 90 X Comparative Example 9 910 645 11.157 708 735 9 18 6615 84 X Comparative Example 10 922 631 11.130 735 805 9 18 7245 82 X Inventory 1 922 643 10.981 455 550 13 23 7150 95 O Inventory 2 928 638 10.988 528 580 13 22 7540 96 O Inventory 3 926 635 10.945 562 640 12 21 7680 94 O Honorable 4 931 642 10.984 583 660 11 20 7260 92 O Inventory 5 926 633 10.985 702 759 10 20 7590 91 O Inventory 6 925 645 10.951 685 721 11 20 7931 95 O Honorable 7 927 641 10.964 754 798 9 19 7182 92 O Honors 8 935 640 10.761 768 812 9 17 7308 91 O

Welding condition Initial clearance (mm) Final clearance (mm) Upset length (mm) Weld time (sec) Upset time (seconds) Upset Current (%) Upset pressure
(kgf / cm2) 13 13 2.0 4 0.5 60 60

As shown in Tables 1 and 2, Inventive Examples 1 to 8 are excellent in weldability and high in tensile strength and uniform elongation by satisfying the composition range and manufacturing conditions proposed by the present invention, It can be confirmed that a high-strength hot-rolled steel sheet can be secured.

On the other hand, the comparative examples 1 to 5 and 8 to 10 do not satisfy the formula (1), confirming that the weldability is improved.

In addition, although the weldability was good in Comparative Examples 6 and 7, it was confirmed that the strength was greatly increased by increasing the content of the alloying element and the precipitate-forming element, but the uniform elongation and the fracture elongation were decreased, and the workability was improved.

In addition, it can be confirmed that the ferrite fraction of Comparative Examples 5, 9, and 10 is less than 90%, which is outside the range proposed by the present invention, and in particular, the uniform elongation ratios of Comparative Examples 5 and 9 are also confirmed.

FIG. 1 shows the yield ratios of Inventive Examples 1 to 8 and Comparative Examples 1 to 10 and the values when they are substituted into Equation 1. It can be confirmed that a high-strength hot-rolled steel sheet excellent in weldability and hydroforming processability can be produced when the steel sheet is included in the region as shown in Fig.

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.

Claims (8)

0.001 to 0.2% of Cr, 0.001 to 0.2% of Cr, 0.001 to 0.2% of Mo, 0.001 to 0.01% of N, 0.01 to 0.3% of Cr, 0.005 to 0.05% of P, 0.001 to 0.05% of S, balance Fe and other unavoidable impurities, each of the components satisfying the following formula (1), the product of the tensile strength and the uniform elongation of 7000 or more, High-strength hot-rolled steel sheet excellent in forming workability.
0.164 + 0.2Mo - 0.25Si + 0.3Mn - 0.1Cr + 0.55Al + 0.2Mo -4.23Ti - 2.5Nb - 2.9V? 11
(Wherein C, Si, Mn, Cr, Al, Mo, Ti, Nb and V are the contents (weight%
The method according to claim 1,
And 0.001 to 0.15% in total of at least one selected from the group consisting of Nb, Ti and V, and having excellent weldability and hydroforming processability.
The method according to claim 1,
Wherein the microstructure of the hot-rolled steel sheet includes ferrite and a second phase, and the ferrite has an aspect ratio of 90% or more, and excellent hydroforming workability.
0.001 to 0.2% of Cr, 0.001 to 0.2% of Cr, 0.001 to 0.2% of Mo, 0.001 to 0.01% of N, 0.01 to 0.3% of Cr, %, P: 0.005 to 0.05%, S: 0.001 to 0.05%, balance Fe and other unavoidable impurities, each of the components satisfying the following formula (1):
Reheating the steel slab at 1200-1300 占 폚;
Hot-rolling the reheated steel slab to a finish rolling temperature of 850 to 1000 占 폚 to obtain a steel sheet;
Cooling the hot-rolled steel sheet at a cooling rate of 10 to 100 DEG C / sec; And
And a step of winding the cooled steel sheet. The method of manufacturing a high-strength hot-rolled steel sheet excellent in weldability and hydroforming processability.
0.164 + 0.2Mo - 0.25Si + 0.3Mn - 0.1Cr + 0.55Al + 0.2Mo -4.23Ti - 2.5Nb - 2.9V? 11
(Wherein C, Si, Mn, Cr, Al, Mo, Ti, Nb and V are the contents (weight%
5. The method of claim 4,
And 0.001 to 0.15% in total of at least one selected from the group consisting of Nb, Ti, and V. The method of manufacturing a high strength hot-
5. The method of claim 4,
Wherein the product of the tensile strength and the uniform elongation of the hot-rolled steel sheet is 7000 or more, and the hydroforming processability is excellent.
5. The method of claim 4,
The method of manufacturing a high strength hot-rolled steel sheet excellent in weldability and hydroforming workability performed at 500 to 700 ° C in the step of winding in the winding step.
5. The method of claim 4,
Further comprising a step of reheating the hot-rolled steel sheet at a temperature of 450 to 480 ° C after the pickling treatment and performing hot-dip galvanizing, thereby producing a hot-dip galvanized steel sheet having excellent weldability and hydroforming processability.

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