KR20140083787A - High-strength hot-rolled steel plate having execellent weldability and bending workbility and method for manufacturing tereof - Google Patents

Abstract

The present invention relates to a high-strength hot rolled steel sheet having excellent weldability and bending workability and a method for manufacturing the high-strength hot rolled steel sheet. The present invention is to provide a hot rolled steel sheet in which a bending process is easily performed and the integrity of a welding unit is ensured. According to the present invention, the ratio of tensile strength and bending workability is 330 or greater by optimizing steel components and hot-rolled structure, and the high-strength hot rolled steel sheet having excellent integrity of the welding unit in a welding process can be ensured.

Description

TECHNICAL FIELD [0001] The present invention relates to a high-strength hot-rolled steel sheet having excellent weldability and bending 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 bending workability and a method for producing the same.

Pipes used in automotive parts are the basic materials used in hydroforming (HF) processes. The hydraulic pressure forming step is one of the molding processes useful for reducing the weight of the vehicle and improving the collision property, and means a method of manufacturing parts by applying a uniform deformation to the pressure of the liquid after mainly manufacturing the pipe.

In order to apply such a hydroforming process, a pipe must first be manufactured. The pipe used in the hydroforming process is usually manufactured by cutting the plate to an appropriate width, molding it into a cylindrical shape, and welding the joint.

At this time, welding is mainly performed by electric resistance welding. Electric resistance welding is widely used as the most general process which is inexpensive and easy to produce. However, when the alloy element such as Mn, Si, Al added to improve the formability of high-strength steel is included, It is known not to have. Researches for improving the weldability to high strength steels have been continuously carried out.

On the other hand, hot-rolled steel sheets are mainly used for parts directly related to the safety of passengers in an emergency such as a vehicle collision. These steel sheets are high-strength steels and may not be suitable for electric resistance welding. Furthermore, merely having a high tensile strength merely lowers competitiveness, and a high elongation for better molding is also required.

For this purpose, there are Multi-Phase Steel Steel, Transformation Induced Plasticity (TRIP) and so on.

However, in the case of the above-described abnormal texture steel, the martensite-ferrite two-phase structure having an increase in strength as the ratio of martensite increases and an increase in ductility as the ferrite ratio increases increases. If it is too high, the ferrite ratio may be relatively decreased and ductility may be lowered, and since the cooling rate must be increased sharply in order to form martensite at a low temperature, there is a disadvantage that the process load is large during cooling. In addition, additional facilities such as water-cooling and pickling equipment are required, which causes the manufacturing cost to increase.

Furthermore, a composite structure steel in which strength and ductility are simultaneously improved by forming ferrite, martensite and a mixed phase of bainite and martensite / austenite at normal temperature is a steel grade having improved strength and ductility of a transformed organopolysil There is a problem in that the moldability is disadvantageous. It is important to maintain a certain percentage of ferrite over the composite structure steel. For this purpose, it is important to add Si, Mn, etc. and add a small amount of Nb, Ti, etc. to make fine grains finer. There is a problem that the resistance heat is increased by increasing the electric resistance, or when the input current value is lowered, cold junction occurs. Further, Si, Mn, Al and the like have the problem of deteriorating the integrity of the welded portion by forming oxides during welding.

Further, bainite steel produced from bainite single phase has excellent bending workability, but it is also difficult to apply it to a conventional continuous annealing plant because high temperature annealing and rapid cooling equipment are also required. In addition, there is a problem of delayed fracture in which a large amount of martensite-forming composite-structure steel and bainite steel is broken after a certain period of time.

One aspect of the present invention is to provide a hot-rolled steel sheet which is easy to bend while securing the integrity of a welded portion.

A high strength hot-rolled steel sheet excellent in weldability and bending workability, which is one aspect of the present invention, comprises 0.1 to 0.25% of C, 0.1 to 1.0% of Si, 1.0 to 3.0% of Mn, 0.03 to 0.055% of Al, 0.01 to 0.1%, P: 0.01 to 0.05%, S: 0.001 to 0.01%, N: 0.001 to 0.01%, and the balance Fe and other unavoidable impurities. And the ratio of the tensile strength to the bending workability is 330 or more.

0.25 [Si] +0.3 [Mn] - 0.1 [Cr] + 0.55 [Al] +0.2 [Mo] -4.23 [Ti] -2.5 [Nb] - 2.9 [V] ≤ 11.3

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 of manufacturing a high strength hot-rolled steel sheet excellent in weldability and bending workability which is another aspect of the present invention is characterized by comprising 0.1 to 0.25% of C, 0.1 to 1.0% of Si, 1.0 to 3.0% of Mn, 0.03 to 0.055% of Al, 0.001 to 0.75% of Cr, 0.01 to 0.1% of Cr, 0.01 to 0.05% of P, 0.001 to 0.01% of S and 0.001 to 0.01% of N. The balance contains Fe and other unavoidable impurities, Preparing a steel slab satisfying the following formula 1, reheating the steel slab at 1200 to 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 to a temperature of 600 to 750 ° C at a cooling rate of 10 to 100 ° C / sec, air-cooling the primary-cooled steel sheet for 4 to 8 seconds and cooling the air- Lt; RTI ID = 0.0 > C, < / RTI >

0.25 [Si] +0.3 [Mn] - 0.1 [Cr] + 0.55 [Al] +0.2 [Mo] -4.23 [Ti] -2.5 [Nb] - 2.9 [V] ≤ 11.3

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 hot-rolled structure, it is possible to secure a high-strength hot-rolled steel sheet having a ratio of tensile strength to bending workability of 330 or more and excellent weldability at the time of welding.

1 is a graph showing the ratios of tensile strength and bending workability of all steels in the examples.

The present inventors have conducted studies to develop a hot-rolled steel sheet in which the above-mentioned problems have not been solved, and as a result, they have succeeded in producing a high-strength hot-rolled steel sheet excellent in bending workability and weldability by controlling steel composition components, microstructure and process conditions And the present invention.

Hereinafter, a high-strength hot-rolled steel sheet excellent in weldability and bending 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 bending workability, which is one aspect of the present invention, comprises 0.1 to 0.25% of C, 0.1 to 1.0% of Si, 1.0 to 3.0% of Mn, 0.03 to 0.055% of Al, 0.01 to 0.1%, P: 0.01 to 0.05%, S: 0.001 to 0.01%, N: 0.001 to 0.01%, and the balance Fe and other unavoidable impurities. And the ratio of the tensile strength to the bending workability is 330 or more.

0.25 [Si] +0.3 [Mn] - 0.1 [Cr] + 0.55 [Al] +0.2 [Mo] -4.23 [Ti] -2.5 [Nb] - 2.9 [V] ≤ 11.3

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): 0.1 to 0.25 wt%

C is the most economical and effective element for strengthening the steel and is essential for increasing the hardening ability. In order to exhibit such an effect in the present invention, it is preferable that 0.1 wt% or more is included. However, as the addition amount increases, a bainite phase and a martensite phase are formed, thereby increasing the tensile strength. On the other hand, there is a problem that the weldability, formability and ductility are reduced. Therefore, the upper limit is preferably limited to 0.25 wt% . Therefore, it is preferable that the carbon content is 0.1 to 0.25 wt%.

Silicon (Si): 0.1 to 1.0 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 0.1 wt% or more is included. On the other hand, when the content of silicon exceeds 1.0% 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, it is preferable that the silicon content is 0.1 to 1.0 wt%.

Manganese (Mn): 1.0 to 3.0 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 3.0%, the ferrite transformation is delayed, which makes it difficult to secure a proper fraction of ferrite as a matrix 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. Accordingly, the content of manganese is preferably 1.0 to 3.0% by weight.

Aluminum (Al): 0.03 to 0.055 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 in the steel during cooling after hot rolling, and forms AlN to contribute to stabilizing the solid nitrogen. 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 0.055% 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 0.055% by weight.

Cr (Cr): 0.005 to 0.75 wt%

Cr strengthens the steel and accelerates the formation of martensite by delaying the phase transformation of bainite during cooling. When the chromium content is less than 0.005 wt%, the solid solution strengthening effect can not be ensured. When the chromium content is more than 0.75 wt%, the ferrite transformation is delayed and the elongation is decreased due to an increase in the martensite fraction. Therefore, the content of Cr is preferably 0.005 to 0.75% by weight.

Molybdenum (Mo): 0.01 to 0.1 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, if the content exceeds 0.1% 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 0.1 wt%.

Phosphorus (P): 0.01 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 effects in the present invention, it is preferable to contain 0.01 wt% 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 P content is preferably 0.01 to 0.05 wt%.

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.

Sulfur (S): 0.001 to 0.01 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 the steelmaking and productivity is deteriorated. When the content of sulfur exceeds 0.01%, it forms a nonmetallic inclusion by binding with Mn and the like, Therefore, the content of sulfur is preferably 0.001 to 0.01% 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.

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 0.1 wt% of the total of one or more elements selected from the group.

The Ti is present in the steel as TiN and has the effect of inhibiting the growth of the crystal grains during the heating process for hot rolling. In addition, Ti reacts with nitrogen and is dissolved in the steel to bond with carbon to form TiC precipitate, which is a useful component for improving the strength and yield ratio (YS / TS) of steel.

Nb and V form carbides in the steel, which are effective in grain refinement and form fine precipitates to improve the strength and toughness of the 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 when considering the strength or elongation to be obtained, the lower limit can be controlled to 10.5. If it is added at a level lower than the above range, there may arise a problem that the strength is rapidly lowered.

0.164 + 0.2Mo - 4.23Ti - 2.5Nb - 2.9V? 11.3

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

It is also preferable that the bending workability (R / t) is 3.6 or less. Since the bending workability is 3.6 or less, cracks do not occur during roll forming.

The microstructure of the hot-rolled steel sheet preferably contains 10% or more of bainite, martensite, and ferrite, and the sum of the bainite, martensite, and ferrite is 95% or more.

By including such microstructure, the steel can secure sufficient strength, and the difference in hardness between the phases produced at low temperature transformation is small, and excellent bending workability can be ensured.

The hot-rolled steel sheet has a tensile strength of 900 MPa or more .

In addition, the ratio TS / R / t of the tensile strength TS to the bending workability R / t is preferably 330 or more. It is possible to secure a hot-rolled steel sheet having high strength and excellent bending workability by ensuring that the ratio TS / R / t of the tensile strength TS to the R / t ratio TS / (R / t)

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

A method of manufacturing a high strength hot-rolled steel sheet excellent in weldability and bending workability which is another aspect of the present invention is characterized by comprising 0.1 to 0.25% of C, 0.1 to 1.0% of Si, 1.0 to 3.0% of Mn, 0.03 to 0.055% of Al, 0.001 to 0.75% of Cr, 0.01 to 0.1% of Cr, 0.01 to 0.05% of P, 0.001 to 0.01% of S and 0.001 to 0.01% of N. The balance contains Fe and other unavoidable impurities, Preparing a steel slab satisfying the following formula 1, reheating the steel slab at 1200 to 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 to a temperature of 600 to 750 ° C at a cooling rate of 10 to 100 ° C / sec, air-cooling the primary-cooled steel sheet for 4 to 8 seconds and cooling the air- Lt; RTI ID = 0.0 > C, < / RTI >

0.25 [Si] +0.3 [Mn] - 0.1 [Cr] + 0.55 [Al] +0.2 [Mo] -4.23 [Ti] -2.5 [Nb] - 2.9 [V] ≤ 11.3

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, it becomes the hot finish rolling temperature is the case, the dialog organization of the steel bath in excess of 1000 ℃ steel is susceptible, the scale is thicker, and the surface quality degradation, such as hot rolling, the scale defect sex. 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 it reaches 600 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 600 占 폚, most of the microstructure in the steel is formed into bainite, and the elongation rate 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 heating temperature is less than 10 ° C / second, the sum of the volume fraction of the bainite phase and the martensite phase is less than 30% in the ferrite base structure, so that the strength to secure the present invention can not be secured. On the other hand, when it exceeds 100 ° C / second, the martensite fraction increases and the bending workability deteriorates.

Air cooling step

It is preferable that the cooled steel sheet is air-cooled. By air-cooling the hot-rolled steel sheet as described above, the ferrite structure is sufficiently formed to obtain the strength to secure the present invention. Air cooling is performed for 4 seconds or more in order to secure a superior effect. More preferably 4 to 8 seconds.

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 room temperature to 400 ° C. When the coiling temperature exceeds 400 ° C, coarse ferrite and pearlite are formed and the strength of the steel is reduced.

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)

A steel slab satisfying the component system described in Table 1 was heated at 1250 占 폚 and subjected to hot rolling at the temperature (FDT) shown in Table 2 below. After cooling to 40 占 폚 / sec and air cooling for 4 seconds up to the coiling temperature (CT) shown in Table 2 below, the coils were wound up at the coiling temperature (CT) shown in Table 2 below.

The yield strength (YS), tensile strength (TS), elongation (El), bending workability (R / t) and weldability of the final hot-rolled steel sheet obtained by completing the winding process are shown in Table 2 below. In addition, fractions of bainite and martensite were measured and 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 area ratio of bainite and martensite was measured by an optical microscope at a magnification of 500, after etching the rolled plate specimens with a Nital etchant and a Lepera etchant, and analyzed with an image analyzer. Table 2 shows the area fraction.

The bending workability (R / t) of the hot-rolled plate having a thickness (t) of 2.0 mm to 4 mm was measured by using a V-bending tester to determine the minimum And the radius of curvature (R).

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.

division C Si Mn Cr Al P S N Ti Mo Nb V Comparative Example 1 0.13 0.1 3.15 0.65 0.03 0.01 0.004 0.004 0.015 0.017 0.03 0.001 Comparative Example 2 0.13 0.1 2.97 0.42 0.035 0.01 0.003 0.005 0.01 0.015 0.04 0.001 Comparative Example 3 0.15 0.35 2.35 0.2 0.035 0.006 0.004 0.003 0.02 0.01 0.015 0.001 Comparative Example 4 0.08 0.18 2.4 0.5 0.035 0.015 0.003 0.004 0.015 0.1 0.01 0.001 Comparative Example 5 0.18 0.15 2.62 0.42 0.03 0.012 0.003 0.005 0.02 0.01 0.03 0.002 Comparative Example 6 0.18 0.28 2.88 0.75 0.027 0.009 0.003 0.004 0.015 0.01 0.04 0.003 Comparative Example 7 0.25 0.38 2.25 0.8 0.045 0.01 0.003 0.005 0.02 0.1 0.03 0.003 Comparative Example 8 0.17 0.24 2.48 0.25 0.03 0.011 0.003 0.005 0.04 0.01 0.02 0.004 Comparative Example 9 0.14 0.25 1.85 0.4 0.05 0.02 0.006 0.005 0.028 0.1 0.03 0.001 Inventory 1 0.14 0.18 2.65 0.45 0.025 0.01 0.002 0.004 0.035 0.01 0.025 0.001 Inventory 2 0.18 0.3 2.5 0.7 0.027 0.008 0.003 0.003 0.025 0.01 0.035 0.002 Inventory 3 0.13 0.15 2.6 0.6 0.025 0.006 0.003 0.005 0.04 0.1 0.025 0.001 Honorable 4 0.14 0.16 2.3 0.55 0.025 0.01 0.003 0.004 0.02 0.1 0.01 0.003 Inventory 5 0.13 0.2 2.8 0.7 0.028 0.006 0.003 0.003 0.045 0.01 0.05 0.003 Inventory 6 0.16 0.2 2.7 0.25 0.032 0.01 0.002 0.004 0.03 0.01 0.05 0.004 Honorable 7 0.14 0.15 2.2 0.35 0.025 0.006 0.004 0.003 0.03 0.01 0.025 0.001 Honors 8 0.12 0.14 2.45 0.5 0.022 0.01 0.003 0.004 0.02 0.01 0.05 0.006 Proposition 9 0.11 0.1 1.85 0.6 0.025 0.01 0.002 0.004 0.035 0.1 0.025 0.001

Psalter FDT
(° C) CT
(° C) Equation 1 YS
(MPa) TS
(MPa) EL
(%) R / t TS / (R / t) Bainite fraction
(%) Martensite fraction
(%) Ferrite fraction
(%) Weldability Comparative Example 1 878 253 11.456 726 1109 6.6 4.5 246.4 38 45 11 Bad Comparative Example 2 896 273 11.423 762 1028 8.4 3.9 277.8 38 25 32 Bad Comparative Example 3 875 276 11.345 618 785 15.6 2.2 356.8 48 16 32 Bad Comparative Example 4 884 316 11.325 559 624 14.1 1.5 416 48 12 35 Bad Comparative Example 5 872 249 11.317 925 1052 7.4 3.6 292.2 34 34 26 Bad Comparative Example 6 887 248 11.386 1112 1179 4.5 4.2 280.7 27 52 15 Good Comparative Example 7 898 239 11.262 1064 1184 6.3 4.1 288.8 32 56 11 Bad Comparative Example 8 876 289 11.247 625 847 12.4 3.5 242 44 19 29 Good Comparative Example 9 888 308 11.103 746 794 13.4 3.2 248.1 47 18 28 Good Inventory 1 897 226 11.271 1149 1242 7.4 3.4 365.3 36 47 12 Good Inventory 2 904 283 11.255 1106 1169 8.2 2.8 417.5 53 34 11 Good Inventory 3 890 227 11.229 1124 1214 11.7 3.5 346.9 42 46 10 Good Honorable 4 885 268 11.264 1057 1165 10.5 3 395.7 44 36 15 Good Inventory 5 899 249 11.185 1128 1224 7.6 3.2 382.5 38 46 11 Good Inventory 6 879 219 11.269 1138 1221 8.4 3.6 339.2 40 44 11 Good Honorable 7 893 292 11.160 911 987 8.8 2.8 401.4 48 22 26 Good Honors 8 892 250 11.177 1128 1204 7.4 3.2 376.3 48 41 11 Good Proposition 9 872 320 11.008 863 924 9.3 2.7 403.3 58 22 15 Good

0.25 [Si] +0.3 [Mn] - 0.1 [Cr] + 0.55 [Al] +0.2 [Mo] -4.23 [Ti] -2.5 [Nb] - 2.9 [V] C, Si, Mn, Cr, Al, Mo, Ti, Nb and V in the above formula (1)

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 Table 2, Inventive Examples 1 to 9 satisfy the composition range and manufacturing conditions proposed by the present invention, and have a bending workability (R / t) of 3.6 or less, a tensile strength of 900 MPa or more, an elongation of 7% It can be confirmed that the weldability and the bending workability are excellent despite the high strength steel.

On the other hand, Comparative Examples 1, 2, 5 and 6 do not satisfy the formula (1) proposed by the present invention, and the ratio of tensile strength / bending workability is unsatisfactory, thereby confirming that weldability and bending workability are improved .

It can be confirmed that Comparative Examples 3 and 4 do not satisfy the formula (1) proposed by the present invention and thus weldability is improved.

In Comparative Examples 7 to 9, it was confirmed that the bending workability was improved because the ratio of the tensile strength to the bending workability was unsatisfactory, although it satisfied the formula (1) proposed by the present invention.

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 (7)

The steel sheet according to any one of claims 1 to 3, which comprises 0.1 to 0.25% of C, 0.1 to 1.0 of Si, 1.0 to 3.0% of Mn, 0.03 to 0.055% of Al, 0.005 to 0.75% of Cr, 0.01 to 0.1% 0.001 to 0.01% of S, 0.001 to 0.01% of the remainder Fe, and the balance of Fe and other unavoidable impurities. Each of the above-mentioned components satisfies the following formula (1), and the tensile strength TS and the bending workability R / (TS / (R / t)) of 330 or more and excellent in weldability and bending workability.

0.25 [Si] +0.3 [Mn] - 0.1 [Cr] + 0.55 [Al] +0.2 [Mo] -4.23 [Ti] -2.5 [Nb] - 2.9 [V] ≤ 11.3
(Wherein C, Si, Mn, Cr, Al, Mo, Ti, Nb and V are the contents (weight%
The method according to claim 1,
The hot-rolled steel sheet is excellent in weldability and bending workability of 0.001 to 0.1% in total of at least one selected from Nb, Ti and V.
The method according to claim 1,
Wherein the microstructure of the hot-rolled steel sheet contains 10% or more of bainite, martensite, and ferrite, and the sum of the microstructures is 95% or more.
The steel sheet according to any one of claims 1 to 3, which comprises 0.1 to 0.25% of C, 0.1 to 1.0 of Si, 1.0 to 3.0% of Mn, 0.03 to 0.055% of Al, 0.005 to 0.75% of Cr, 0.01 to 0.1% 0.001 to 0.01% of S, 0.001 to 0.01% of N, the balance Fe and other unavoidable impurities, and 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 to a temperature of 600 to 750 ° C at a cooling rate of 10 to 100 ° C / sec;
Air cooling the primary cooled steel sheet for 4 to 8 seconds; And
And winding the air-cooled steel sheet in a temperature range of room temperature to 400 ° C.

0.25 [Si] +0.3 [Mn] - 0.1 [Cr] + 0.55 [Al] +0.2 [Mo] -4.23 [Ti] -2.5 [Nb] - 2.9 [V] ≤ 11.3
(Wherein C, Si, Mn, Cr, Al, Mo, Ti, Nb and V are the contents (weight%
5. The method of claim 4,
Wherein the slab is 0.001 to 0.1% in total of at least one of Nb, Ti and V, and is excellent in weldability and bending workability.
5. The method of claim 4,
Further comprising a step of reheating the steel sheet at a temperature of 450 to 480 占 폚 after pickling treatment and performing hot dip galvanizing, thereby producing a high strength hot-rolled steel sheet having excellent weldability and bending workability.
5. The method of claim 4,
Wherein the microstructure of the hot-rolled steel sheet comprises 10% or more of bainite, martensite, and ferrite, and the sum of the microstructures is 95% or more.

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