KR101449130B1 - High strength hot rolled steel having excellent weldability and bake hardenability and method for manufacturing thereof - Google Patents

Abstract

High-strength hot-rolled steel sheet excellent in weldability and hot-hardening ability that can be processed with low yield strength during part forming and light weight of parts due to increase of yield strength in heat treatment after forming part Method.

Description

TECHNICAL FIELD [0001] The present invention relates to a high strength hot-rolled steel sheet excellent in weldability and sintering hardenability, 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 sintering ability, and a method for producing the same.

In the automobile industry, the application of high tensile strength steel to automobile bodies has been expanding in recent years in order to combine the weight reduction for reduction of CO ₂ emissions and the reinforcement of the body for improving the collision safety. High-strength steel is a cost-effective material that meets these conflicting demands, and we believe that the application of this material will increase in order to respond to stricter regulations in the future. Particularly, as the interest in collision safety increases, its role is increasing.

Accordingly, various techniques for increasing the strength of the steel sheet have been proposed. In this case, when the strength is further increased after the processing of the steel sheet, it is advantageous in that high strength can be ensured without deteriorating the formability of the steel sheet. The bake hardenability of the steel sheet refers to the property that the steel sheet can be further cured in the process of forming the steel sheet into a component shape and then coating and heat-treating the steel sheet. The steel sheet having excellent bake hardenability can be advantageously used as a high strength material.

Patent Documents 1 and 2 disclose a technique of controlling the content of dissolved nitrogen to a certain level or more and securing a superior hardening ability by reaction of solid nitrogen and dislocation during heat treatment after coating or a composite structure steel having a ferrite structure as a base Discloses a technique for a hot-rolled steel sheet that secures a high hardenability by sintering by a movable potential introduced at the time of transformation of the second phase and a solid carbon nitride present in the second phase.

In Patent Document 3, the microstructure ratio of needle-like ferrite having a high hardness value and polygonal ferrite having a low hardness value is controlled to improve elongation flange formability, and the ratio of solid-solution system and solid N existing in the crystal grain is controlled, And a technique for improving the hardening ability of the strength of the strength.

In order to produce a high strength steel, the solid solution strengthening effect of various alloying elements such as Si, Mn, Cr, and Mo and the strengthening effect of Ti, Nb, V and the like. In addition, the hot-rolled steel sheets manufactured by Patent Documents 1 to 3 are manufactured by welding joint portions when manufacturing automobile parts or machine parts. 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 elements such as Si, Mn, Cr, and Mo added to produce high strength steel are included, It is known not to have.

To solve this problem, Multi-Phase Steel Steel, TRIP (Transformation Induced Plasticity) and DP (Dual Phase) have been shown.

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.

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 the ferrite in the composite structure steel. For this purpose, it is important to add Si, Mn, Cr, Mo or the like and add a small amount of Ti, Nb, There is a problem that when the electric resistance welding is performed, the electric resistance increases to increase the resistance heat generation, or when the input current value is lowered, cold rolling occurs. In addition, Si, Mn, Cr, Mo, and the like have the problem of deteriorating the integrity of the welded portion by forming oxides during welding.

Japanese Patent Application Laid-Open No. 2006-199979 Japanese Patent Application Laid-Open No. 2002-053933 Japanese Patent Laid-Open Publication No. 2011-144425

High-strength hot-rolled steel sheet excellent in weldability and hot-hardening ability that can be processed with low yield strength during part forming and light weight of parts due to increase of yield strength in heat treatment after forming part Method.

A high strength hot-rolled steel sheet excellent in weldability and sintering capability, which is one aspect of the present invention, comprises 0.05 to 0.12% of C, 0.01 to 1.8% of Si, 1.2 to 2.0% of Mn, 0.03 to 0.5% of Al, 0.005 to 0.5% of Mo, 0.01 to 0.15% of Mo, 0.008 to 0.012% of N, 0.005 to 0.05% of P, 0.001 to 0.05% of S and the balance of Fe and other unavoidable impurities, 1, and the ratio of hardenability to cure and yield strength ratio is 0.06 to 0.09.

0.25 Si + 0.3 Mn - 0.1 Cr + 0.55 Al + 0.2 Mo - 4.23 Ti - 2.5 Nb - 2.9 V? 11.2

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

A high strength hot-rolled steel sheet excellent in weldability and sintering ability, which is another aspect of the present invention, comprises 0.05 to 0.12% of C, 0.01 to 1.8% of Si, 1.2 to 2.0% of Mn, 0.03 to 0.5% of Al, 0.03 to 0.5% of Al, , 0.005 to 0.5% of Mo, 0.01 to 0.15% of Mo, 0.008 to 0.012% of N, 0.005 to 0.05% of P, 0.001 to 0.05% of S and the balance Fe and other unavoidable impurities, Preparing a steel slab satisfying the formula 1, reheating the steel slab at 1200 to 1300 캜, hot-rolling the reheated steel slab to a finish rolling temperature of 850 to 1000 캜 to obtain a steel sheet, Cooling the rolled steel sheet at a cooling rate of 10 to 100 캜 / second, and winding the cooled steel sheet.

0.25 Si + 0.3 Mn - 0.1 Cr + 0.55 Al + 0.2 Mo - 4.23 Ti - 2.5 Nb - 2.9 V? 11.2

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 ratio of the hardenability to the hardenability to the yield strength is 0.06 to 0.09, and the soundness of the welded portion at the time of welding is improved, thereby securing a high strength hot-rolled steel sheet excellent in weldability and bake hardenability .

FIG. 1 is a graph showing the ratio of the hardening ability of the bake hardenability to the yield strength of all the steels in the embodiment and the value obtained by substituting in Equation 1. FIG.

The present inventors have conducted research to develop a hot-rolled steel sheet in which the above-mentioned problems have not been solved, and as a result, it is possible to produce a high-strength hot-rolled steel sheet having excellent weldability and hardening hardenability by controlling composition components, microstructure, And reached the present invention.

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

A high strength hot-rolled steel sheet excellent in weldability and sintering capability, which is one aspect of the present invention, comprises 0.05 to 0.12% of C, 0.01 to 1.8% of Si, 1.2 to 2.0% of Mn, 0.03 to 0.5% of Al, 0.005 to 0.5% of Mo, 0.01 to 0.15% of Mo, 0.008 to 0.012% of N, 0.005 to 0.05% of P, 0.001 to 0.05% of S and the balance of Fe and other unavoidable impurities, 1, and the ratio of hardenability to cure and yield strength ratio is 0.06 to 0.09.

0.25 Si + 0.3 Mn - 0.1 Cr + 0.55 Al + 0.2 Mo - 4.23 Ti - 2.5 Nb - 2.9 V? 11.2

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): 0.05 to 0.12 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 it is contained in an amount of 0.05 wt% or more. However, as the addition amount increases, a bainite phase and a martensite phase are formed, which causes a problem that weldability, formability and ductility decrease while the tensile strength increases. Therefore, the upper limit is preferably limited to 0.12 wt% . Therefore, it is preferable that the carbon content is 0.05 to 0.12 wt%.

Silicon (Si): 0.01 to 1.8 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 1.8% by weight, not only the ductility and weldability are deteriorated due to the solid solution strengthening effect, but also the problem that the surface quality of the steel sheet is lowered due to the formation of a red color scale due to Si on the surface of the steel sheet during hot rolling have. Therefore, the content of the silicon is preferably 0.01 to 1.8% by weight.

Manganese (Mn): 1.2 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 it is contained in an amount of 1.2 wt% or more. 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, during slab casting in the casting process, micro-segregation is developed in the center segregation and in the thickness direction at the center of the thickness to deteriorate the low-temperature impact toughness and weldability. Accordingly, the content of manganese is preferably 1.2 to 2.0% by weight.

Aluminum (Al): 0.03-0.5 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, if the content exceeds 0.5% by weight, defects are likely to occur in the slab during continuous casting, and the surface quality is deteriorated due to occurrence of surface defects after hot rolling. Therefore, the content of aluminum is preferably 0.03-0.5 wt%.

Cr (Cr): 0.005-0.5 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 secured. When the chromium content is more than 0.5 wt%, the ferrite transformation is delayed and the elongation percentage is decreased due to an increase in the martensite fraction. Therefore, it is preferable that the content of Cr is in the range of 0.005 to 0.5% by weight.

Molybdenum (Mo): 0.01 to 0.15 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 it exceeds 0.15% 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 0.15% by weight.

Nitrogen (N): 0.008 to 0.012 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 0.008 wt%, the effect of strengthening solubility is not expected to be expected. When the content of nitrogen is more than 0.012 wt%, the ductility and toughness are deteriorated. Therefore, the content of nitrogen is preferably 0.008-0.012 wt%.

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. In addition, Ti, which reacts with nitrogen and is solved in the steel and bonds with carbon, forms a TiC precipitate, which is a useful component for improving the strength of the steel, and is effective in controlling the hardening of the hardening.

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. C and N which increase the electrical resistivity are stabilized, so that the phenomenon of local flame generation is alleviated during electric resistance welding, and the softening of the welded portion is also suppressed.

It is preferable that C, Si, Mn, Cr, Al, Mo, Ti, Nb and V satisfies the following formula (1) in order to improve the hardenability after baking and weldability. , The electrical resistivity at a 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 the amount is less than the above range, there may arise a problem that the strength or elongation rate is drastically reduced.

0.25 Si + 0.3 Mn - 0.1 Cr + 0.55 Al + 0.2 Mo - 4.23 Ti - 2.5 Nb - 2.9 V? 11.2

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

The hot-rolled steel sheet preferably has a ratio of hardenability to sintering to a yield strength of 0.06 to 0.09. When the ratio is less than 0.06, the hardening ability of the bake hardeness is low as compared with the strength of the hot-rolled steel sheet, and the effect of increasing the strength after machining and heat treatment is insignificant. On the other hand, if it exceeds 0.09, there is a problem that the resistance to aging greatly deteriorates and the yield strength and material of the steel change during long-term storage.

The microstructure of the hot-rolled steel sheet preferably includes ferrite. The inclusion of such microstructure improves the weldability and sintering ability of steel. In addition, it is preferable that the ferrite has an area percentage of 90% or more. When the ferrite area fraction is less than 90%, bainite and coarse carbonitride are formed to deteriorate ductility and toughness of steel.

The hot-rolled steel sheet has a tensile strength of 500 MPa or more, and more preferably a tensile strength of 540 to 780 MPa.

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

A high strength hot-rolled steel sheet excellent in weldability and sintering ability, which is another aspect of the present invention, comprises 0.05 to 0.12% of C, 0.01 to 1.8% of Si, 1.2 to 2.0% of Mn, 0.03 to 0.5% of Al, 0.03 to 0.5% of Al, , 0.005 to 0.5% of Mo, 0.01 to 0.15% of Mo, 0.008 to 0.012% of N, 0.005 to 0.05% of P, 0.001 to 0.05% of S and the balance Fe and other unavoidable impurities, Preparing a steel slab satisfying the formula 1, reheating the steel slab at 1200 to 1300 캜, hot-rolling the reheated steel slab to a finish rolling temperature of 850 to 1000 캜 to obtain a steel sheet, Cooling the rolled steel sheet at a cooling rate of 10 to 100 캜 / second, and winding the cooled steel sheet.

0.25 Si + 0.3 Mn - 0.1 Cr + 0.55 Al + 0.2 Mo - 4.23 Ti - 2.5 Nb - 2.9 V? 11.2

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 into bainite, which causes a problem that 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, There is a problem of deterioration.

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), fracture elongation (T-El), and bake hardenability (BH) of the final hot-rolled steel sheet obtained by completing the winding process are shown in Table 2 below. In addition, BH / YS means the ratio of hardening ability to yield strength.

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 bake hardenability (BH) is a result obtained by making a tensile test piece (JIS No. 5) of JIS standard in the direction perpendicular to the rolling direction. The tensile test specimens were heat treated at 170 ° C for 20 minutes after 2% tensile deformation, and then subjected to tensile test. Sinter hardenability is the difference between the lower yield strength or 0.2% offset yield strength of the heat treated and tensile specimens and the strength value measured at 2% tensile strain before heat treatment.

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 0.05 0.08 1.6 0.05 0.5 0.01 0.003 0.008 0.01 0.05 0.01 0.005 Comparative Example 2 0.08 0.2 1.6 0.05 0.2 0.01 0.003 0.009 0.01 0.12 0.01 0.005 Comparative Example 3 0.09 0.5 1.8 0.01 0.05 0.01 0.003 0.008 0.02 0.1 0.02 0.005 Comparative Example 4 0.07 0.05 1.5 0.01 0.1 0.01 0.003 0.011 0.02 0.05 0.04 0.005 Comparative Example 5 0.09 0.7 1.8 0.1 0.5 0.01 0.003 0.012 0.06 0.15 0.005 0.025 Comparative Example 6 0.12 One 1.6 0.1 0.03 0.01 0.01 0.008 0.06 0.1 0.02 0.01 Comparative Example 7 0.1 1.2 1.8 0.01 0.5 0.01 0.005 0.009 0.04 0.1 0.03 0.005 Comparative Example 8 0.11 1.5 2 0.01 0.03 0.01 0.003 0.01 0.06 0.05 0.025 0.005 Comparative Example 9 0.09 1.5 1.8 0.01 0.03 0.01 0.005 0.01 0.06 0.1 0.015 0.02 Comparative Example 10 0.1 1.8 1.9 0.01 0.03 0.01 0.003 0.01 0.07 0.15 0.02 0.02 Inventory 1 0.06 0.05 1.6 0.01 0.03 0.01 0.003 0.008 0.01 0.01 0.01 0.005 Inventory 2 0.08 0.12 1.7 0.3 0.03 0.01 0.003 0.009 0.02 0.01 0.015 0.005 Inventory 3 0.08 0.05 1.5 0.5 0.3 0.01 0.003 0.01 0.02 0.01 0.02 0.005 Honorable 4 0.1 0.05 1.5 0.01 0.1 0.01 0.003 0.01 0.03 0.02 0.03 0.02 Inventory 5 0.1 0.5 1.6 0.5 0.03 0.01 0.005 0.01 0.03 0.01 0.03 0.005 Inventory 6 0.09 0.5 1.7 0.2 0.1 0.01 0.005 0.009 0.03 0.01 0.03 0.005 Honorable 7 0.1 0.5 1.8 0.2 0.03 0.01 0.005 0.01 0.03 0.05 0.02 0.03 Honors 8 0.11 One 1.8 0.3 0.03 0.01 0.005 0.011 0.03 0.07 0.03 0.04

Psalter FDT
(° C) CT
(° C) Equation 1 YS
(MPa) TS
(MPa) T-El
(%) BH BH / YS ferrite
Phase fraction
(%) Weldability Comparative Example 1 882 605 11.354 466 504 27 32 0.069 91 X Comparative Example 2 878 611 11.239 488 525 26 34 0.07 93 X Comparative Example 3 892 585 11.226 521 563 26 43 0.083 94 X Comparative Example 4 888 588 10.987 554 593 25 31 0.056 93 O Comparative Example 5 894 592 11.335 610 642 23 26 0.043 90 X Comparative Example 6 875 605 11.094 633 677 21 32 0.051 87 O Comparative Example 7 901 597 11.541 672 724 20 38 0.057 90 X Comparative Example 8 898 592 11.338 698 755 18 41 0.059 85 X Comparative Example 9 904 604 11.265 783 843 17 35 0.045 91 X Comparative Example 10 891 593 11.327 876 912 14 43 0.049 82 X Inventory 1 899 599 11.086 554 585 26 48 0.087 92 O Inventory 2 895 598 11.054 562 599 24 46 0.082 93 O Inventory 3 902 609 11.092 589 620 23 42 0.071 94 O Honorable 4 908 592 10.927 653 704 22 41 0.063 92 O Inventory 5 896 595 11.023 662 683 21 52 0.079 90 O Inventory 6 908 602 11.120 702 736 20 45 0.064 92 O Honorable 7 893 604 11.074 724 755 19 55 0.076 91 O Honors 8 895 616 11.141 743 771 18 56 0.075 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 can confirm that the high-strength hot-rolled steel sheet excellent in weldability and excellent in bake hardenability can be obtained by satisfying the composition range and manufacturing conditions proposed by the present invention .

On the other hand, Comparative Examples 1 to 3, 5 and 7 to 10 do not satisfy the formula (1) and the BH / YS ratio, and it is confirmed that the weldability is improved.

In Comparative Examples 4 and 6, although the weldability was good, the formula (1) did not satisfy the range proposed by the present invention, and it exhibited a low sintering hardenability as compared with a high initial yield strength and a ratio It can be confirmed that the present invention is out of the range proposed by the present invention.

In addition, in Comparative Examples 6, 8, and 10, it can be seen that the ferrite area fraction deviates from the range proposed by the present invention and the elongation rate is increased.

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 (9)

The steel sheet according to any one of claims 1 to 3, wherein the steel contains 0.05 to 0.12% of C, 0.01 to 1.8% of Si, 1.2 to 2.0% of Mn, 0.03 to 0.5% of Al, 0.005 to 0.5% of Cr, 0.01 to 0.15% 0.02 to 0.09%, P: 0.005 to 0.05%, S: 0.001 to 0.05%, balance Fe and other unavoidable impurities, wherein each of the components satisfies the following formula (1) High strength hot-rolled steel sheet excellent in weldability and curing hardenability.
10.5? 10.646 + 0.2C + 0.25Si + 0.3Mn - 0.1Cr + 0.55Al + 0.2Mo - 4.23Ti - 2.5Nb - 2.9V? 11.2
(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% of the total of at least one selected from the group consisting of Nb, Ti and V. The high strength hot-rolled steel sheet has excellent weldability and sintering ability.
The method according to claim 1,
Wherein the microstructure of the hot-rolled steel sheet comprises ferrite, and the ferrite is excellent in weldability and curing hardening ability of 90% or more in area fraction.
The method according to claim 1,
Wherein the tensile strength of the hot-rolled steel sheet is 500 MPa or more and excellent weldability and curing hardenability.
The steel sheet according to any one of claims 1 to 3, wherein the steel contains 0.05 to 0.12% of C, 0.01 to 1.8% of Si, 1.2 to 2.0% of Mn, 0.03 to 0.5% of Al, 0.005 to 0.5% of Cr, 0.01 to 0.15% 0.012%, P: 0.005 to 0.05%, S: 0.001 to 0.05%, balance Fe and other unavoidable impurities, each of which satisfies 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 for manufacturing a high-strength hot-rolled steel sheet according to claim 1,
10.5? 10.646 + 0.2C + 0.25Si + 0.3Mn - 0.1Cr + 0.55Al + 0.2Mo - 4.23Ti - 2.5Nb - 2.9V? 11.2
(Wherein C, Si, Mn, Cr, Al, Mo, Ti, Nb and V are the contents (weight%
6. The method of claim 5,
Wherein the steel sheet has a weldability and a curing hardening ability which further comprises 0.001 to 0.15% of at least one selected from the group consisting of Nb, Ti and V.
6. The method of claim 5,
Wherein the hot-rolled steel sheet has excellent weldability and bake hardenability with a ratio of hardening of hardening and a yield strength of 0.06 to 0.09.
6. The method of claim 5,
The method of manufacturing a high strength hot-rolled steel sheet excellent in weldability and bake hardenability performed at 500 to 700 占 폚 in the step of winding in the winding step.
6. The method of claim 5,
Further comprising a step of reheating the steel sheet at a temperature of 450 to 480 캜 after the pickling treatment and performing hot dip galvanizing, thereby producing a hot-dip galvanized steel sheet having excellent weldability and bake hardenability.

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